JPH02132143A - Rubber composition for tire tread - Google Patents

Abstract

PURPOSE:To provide the subject rubber composition for tire tread excellent in gripping properties in a wide range of temperature and composed of a rubber component containing a rubber consisting of two specified styrene/butadiene blocks, carbon black and a petrolem-based softening agent. CONSTITUTION:To 100 pts. wt. rubber component composed of (A) 90-50 pts.wt. block copolymer rubber with >=20X10<4> weight-average molecular weight and 20-45wt.% total bonded styrene consisting of (A1) a styrene/butadiene block with 15-25wt.% bonded styrene, -80--60 deg.C glass transition temperature (herein after represented as Tg) and <=12 deg.C range thereof and (A2) a styrenebutadiene 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) 20-90 pts.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 frictional force with the road surface by increasing the hysteresis loss of the tresod 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 William Sue Landeruferry's temperature-time conversion law, it depends on hysteresis loss measured at a temperature lower than the temperature at which the tire is used. There is. In fact, there is a good correlation between tan δ (#i4 lapse coefficient), which is a measure of hysteresis loss, and the tire friction coefficient. is involved. Conventionally, in order to increase the hysteresis loss of a rubber composition, a method has been used to compound a rubber with a high glass transition temperature (Tg) such as a styrene-butadiene copolymer rubber containing a high styrene content. This is because the tan δ of rubber has a peak around 'rg, so the tan δ of the rubber has a peak around 'rg, so the temperature range that is involved in friction performance (corresponding to 0 °C if the tire running temperature is 30 °C)
The purpose is to utilize a high tan δ by bringing the n δ peak closer together.

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 greatly 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 a high-cis BR polymer, is used, the grip performance at low temperatures will improve, but on the other hand, the tan δ will decrease around 0°C, and the gripping ability will be insufficient at high temperatures. This creates a contradiction of putting it away. Therefore, attempts have been made to subside 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 Fricti
on of Pneumatic Tires,
Elasevter Scientific Pub
Lisbing Company: 1975, U
．． S. Patent No. 4,748,168, JP-A-62-12932, etc.).

However, since such rubber exhibits properties intermediate to those of the blended polymers, it cannot take full advantage of the inherent strengths of the constituent polymers, and has the disadvantage that neither high-temperature nor low-temperature grips are 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 such antinomies, Japanese Patent Application Laid-open No. 1983-
No. 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 an improvement in low l1 Grisob is certainly observed, At high temperatures, the elastic modulus of the rubber decreases significantly, and the compatibility between handling stability at high temperatures and Grisop 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, little temperature change, and a broad tan δ peak. A block 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 block polymer consisting of two types of SBR blocks with different amounts of vinyl bonds in the styrene and butadiene moieties. These block polymers have a monumental cost compared to conventional polymers.
Although the anδ peak has broadened, it is not enough to cover the temperature range of several tens of degrees as mentioned above, and the peak temperature position shifts depending on how the two types of blocks are combined, making it difficult to improve Grisov performance. Sometimes you can't hope for it.

In addition, ice skid resistance has been taken into consideration.
Not at a satisfactory level.

Furthermore, Japanese Patent Application Laid-Open No. 54-62248 discloses a technique for simultaneously improving wet grip performance and reducing transfer resistance by increasing the amount of 1,2-vinyl bonds in butadiene. These polymers have a high Tg and inevitably have poor grip ability at low temperatures.

Regarding the composition of the binary Prosoc copolymer, JP-A-57-1
This method is disclosed in Japanese Patent Application Laid-open No. 09817 and Japanese Patent Application Laid-open No. 108142/1983. These have a content of 20 to 50 weight ■
%, vinyl total 40-75% by weight, high Tg SBR block, srene content 10% by weight or less, vinyl content 20-5
It is copolymerized with SBR block of 0 weight percent and low T'g, and is said to improve wet grip performance and reduce rolling resistance.

JP-A-57402912 discloses a diblock copolymer containing blocks containing styrene and vinyl of 10 to 50% by weight and 20 to 50% by weight, and 1 to 30% by weight, respectively.
, 60 weight percent or more blocks, they have different solvability parameters, so when unvulcanized each block shows l゛g corresponding to the block, but when vulcanized] two blocks are used, they are different from each other. It has been shown that they are compatible and have a single Tg, and as a result, the peak of the tan δ -temperature curve becomes extremely C broad.
In JP 0-192739, when block copolymerizing butadiene containing 160% by weight or less of vinyl into an SBR block containing 80% by weight of styrene and 30% to 70% by weight of vinyl, the difference in Tg 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 wet skid physical properties, ice skid physical properties, transfer resistance, and fracture properties. It is said that a balance will be achieved. In JP-A-61-55135, the ratio of weight average molecule N/number-average molecular weight of the block copolymer disclosed in JP-A-60-192739 is 1.8 to 5.
．． It is said that setting it to 0 will further improve the performance balance. However, these techniques
Although the peak of 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 the performance, handling stability, and ice skid performance, it is necessary to maintain each performance at a very high level. I am still dissatisfied with the goal of achieving this goal. Unexamined Japanese Patent Publication 1986
Publication No. 231016 also discloses a block polymerer consisting of three SBR blocks, which has relatively good ice scythode resistance and well-balanced wesotogrisop resistance, but still does not meet the required level. has not yet been reached.

[Problem to be solved by the invention]

An object of the present invention is to provide a rubber silk composition for tire treads 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 extensive research 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. The present inventors have discovered that a styrene-butadiene copolymer block having a styrene content and a glass transition of 1 degree and having a narrow Tg transition temperature range can achieve the above object, and have arrived at the present invention. . The key point of the present invention is to combine the SDR, which has a high 1g, and therefore has high glyph performance, and the SBR, which has a low Tg, and therefore has excellent snow and ice performance, to create a single product that has only the advantages of both. The purpose is to create a polymer, and in order to do so, contrary to the conventional technology, the Tg of the block polymers that make up the diblock copolymerization must be at least 60°C apart, and the transition temperature range must be 12°C or less. The present invention is motivated by this discovery.

[Means to solve the problem]

The present invention is a styrene-butadiene copolymer block containing 15 to 25% by weight of bound styrene, having a glass transition temperature of -80°C to -60°C, and a transition temperature range of 12°C or less.
(A), contains 20 to 50% by weight of bound styrene, has a glass transition temperature of -20°C to +15°C, and has a transition temperature range of 1
Styrene-butadiene copolymer Prosoc (2℃ or less)
B), the difference in glass transition temperature between the block (A) and the block (B) is 60°C or more,
20-35% by weight of total bound styrene in the entire copolymer
, 90 to 50 parts by weight of a styrene-butadiene block copolymer rubber having a vinyl bond content of 20 to 45% by weight in all butadiene parts and a weighted average molecular weight of 20 x 10' or more, the balance being natural rubber or impregnated rubber. 80 to 130 parts by weight of carbon black having a nitrogen specific surface area of 100 i/g or more, and 20 to 9 parts by weight of a petroleum softener having a viscosity specific gravity constant of 0.90 to 0.98, to a total of 100 parts by weight of the rubber content.
The gist of the present invention is a rubber composition for a tire tread, which contains 0 parts by weight and has a shear storage modulus at -30''c of 500 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 contains 15 to 25% by weight of bound styrene. Tg is -80℃~
At -60°C, a styrene-butadiene copolymer block (A) with a transition temperature range of 12°C or less and two bound styrene
It consists of a styrene-butadiene copolymer block (B) containing 0 to 50% by weight, a Tg of -20°C to +15°C, and a transition temperature width of 12°C or less, block (A), block (B), and The difference in glass transition temperature of
35% by weight, the amount of vinyl bonds in the total butadiene part is 20-4
5% by weight, and the weight average molecular weight is 20 x 10' or more.

T of one block (A). is -80°C to -60°C, and the Tg of the other block (B) is 20°C to +1
It is essential that the temperature is 5°C, and the difference is 60°C.
It needs to be more than that. That is, if the difference in Tg between the blocks is less than 60°C, the styrene-butadiene copolymers constituting each block will be compatible with each other, and the tan δ peak will not be sufficiently broad. Furthermore, each transition temperature width must be 12°C or less. This is because even if the Tg is 60°C or more apart, if the transition temperature width is 12°C or more, the two will partially dissolve, and the resulting tan δ - temperature curve will be somewhere between the two. This is not desirable because it indicates the character of the person.

In JP-A-60-192739 and JP-A-61-55135, the Tg difference between each block of the diblock copolymer is 30°C or more and the transition temperature range is wide, so that ta
It has been pointed out that the nδ-temperature curve becomes broad. Such a block copolymer has 2
Although they have two 'rg points, they are compatible and integrated by vulcanization, resulting in one broad peak. However, according to the inventors' opinion, the Tg points are combined. In this case, it is not possible to take advantage of the strengths of each block as a single unit, and as a result, the physical properties of the block become the same as a blend of the constituent polymers.

In other words, although it is improved in terms of physical property balance compared to the prior art SBR/B R polymer blend, it does not exhibit exceptionally good physical properties. Therefore, in order to preserve the properties of the binary Prosoc polymer even after copolymerization, we thought that it would be possible to proactively make the Prosox polymers incompatible with each other, and after considering the polymer composition for this purpose, we found the following. Under such conditions, it was possible to obtain an improved binary prosodic polymer that retained the unique properties of each component polymer.

The Tg of block (B) with Tg on the high temperature side is 20℃~
It needs to be +15°C. This is because if the temperature is below 20℃, it is not possible to increase tan δ in the temperature range that is related to Grisov performance, and if it is higher than +15℃, the heat generation property worsens and has a negative impact on the durability of the tire. At the same time, the snow and ice road surface grissop becomes irreparably worse.

Regarding the amount of styrene and the amount of 1,2 vinyl to obtain such a Tg, the styrene content of block (B) is preferably 20% by weight or more, and if the styrene content is less than 20% by weight, the amount of styrene having the required Tg is If a polymer cannot be obtained, and conversely, the styrene content is more than 50% by weight, styrene cracks tend to form, which is unfavorable in terms of wear resistance and fracture properties. The amount of vinyl is limited due to the relationship between Tg and styrene amount, but it is approximately 40 to 75% by weight.
is the desired range.

Also, the 'rg of block (A) is -80°C to -60°C.
It needs to be ℃. This is because if the temperature is higher than -60°C, the elastic modulus at low temperatures becomes high, and the grip performance on snowy and icy roads deteriorates. On the other hand, Tg lower than -80℃
In order to obtain a copolymer block having a styrene content of 15% by weight or less, the styrene content must be reduced to 15% by weight or less, but in this case, the rupture properties deteriorate, which poses a practical problem. If the styrene content exceeds 25% by weight, a copolymer having the required Tg cannot be obtained. The vinyl content is determined by Tg, but is approximately 10 to 20% by weight. Furthermore, Tg of block (A) and block (B)
If the difference is less than 60℃, are they in phase? Therefore, it has one broad tan δ-temperature curve. Such temperature dependence of the viscoelasticity of a compatible copolymer is appropriate for use in low rolling resistance tires, for example, but as for the physical properties of tresod rubber for all-season high performance tires, which is the object of the present invention, tan δ It is necessary that the temperature curve is not only broad, but also trapezoidal, and the absolute value of tan δ itself is large ("About the temperature dependence of hysteresis friction": Journal of the Japan Rubber Association, 1988, October issue). .

To do this, block (A) and block (B)
It is important that these are incompatible or semi-compatible. 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 greater than 12°C, TgO
Even if the difference is 60° C. or more, each block will partially dissolve, and the tan δ-temperature curve is not desirable for an all-season high-performance tire. The weight average molecular weight is 20X10' or more, preferably 1 for processing and fracture properties.
00 xlO' or less.

The purpose of the binary copolymer blocks found in conventionally published patent publications is to achieve both low rolling resistance and wet friction, and in this respect they differ from the all-season high-performance tires that are the object of the present invention.

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 in the conventional patent.

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

■． The amount of styrene constituting the flocs (A) and (B),
The amount of vinyl is different.

2. The difference in Tg 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 polymer base component, the more block (A
) is even better. This is because if the ratio of each block is constant regardless of the molecular age, 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 when Furthermore, when the temperature becomes higher and the molecular chains of the high-Tg side block begin to move, the temperature dependence of the overall mobility suddenly increases, resulting in a sudden decrease in tanδ and a broadening of the curve. This is because it is difficult to become At the same time, it is also desirable to apply known techniques for modifying copolymers, such as terminal modification or cambling, as long as it does not adversely affect the improvement of the temperature dependence of viscoelasticity, which is the goal of the present invention. It is.

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) It is important from a practical standpoint to blend natural rubber or IR with the styrene-butadiene block copolymer rubber. Toi? The reason is that 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 rubbing and cutting. This is because it is possible to reduce the 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, the essential properties of the styrene-butadiene block copolymer rubber will be diluted and the grip performance will deteriorate.
It is necessary to suppress the blending amount to 50 parts by weight or less.

(2) Carbon black.

The nitrogen specific surface area is 100 rrr/g or more.

Specifically, for example, [SAF, SAF, etc. In addition, the nitrogen specific surface area is 100n? If it is less than /g, the grip performance will be poor and the abrasion resistance will be poor.

(3) Petroleum-based softener.

It has a viscosity specific gravity constant of 0.90 to 0.98. Other than this, such as paraffin oil with a viscosity specific gravity constant of less than 0.90, grip performance cannot be achieved;
．． If it exceeds 98, there is no sufficient softening effect during mixing. Examples of the petroleum softener include aromatic process oil,
Examples include highly aromatic process 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, L7 must have sufficient performance in terms of wear resistance and handling stability. In order to increase steering stability to a level suitable for high-performance tires, the nitrogen specific surface area must be 100rr? /
It is necessary to blend 80 parts by weight or more of carbon black with a small particle size of 1.5 g or more. However, if it exceeds 130 parts by weight, the abrasion resistance and heat generation properties will be significantly inferior, 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 an aromatic extender oil with a viscosity specific gravity constant of 0.90 to 0.98 is preferable. It is necessary to adjust the tread rubber modulus by appropriately increasing or decreasing the blending amount of extender oil depending on the blending amount of carbon black.

However, if it is less than 20 parts by weight, the compounded rubber will not elongate, resulting in poor chipping and casing properties, and processability is also difficult, which is not preferable. On the other hand, if it exceeds 90 parts by weight, the strength will decrease and the abrasion resistance will be 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. Generally, rubber-like substances are in a glass state at temperatures below Tg, and the elastic modulus is 10 at room temperature.
It becomes more than 0 times as large, and becomes brittle, breaking with the slightest strain. The temperature at this time is called the low-temperature temperature, and is known as an indicator of the low-temperature performance of the rubber material. However, 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 Tg. Fig. 4 is a plot of the low-temperature embrittlement temperature and shear storage modulus (G°) at that temperature for various rubber compositions. From this, it can be seen that the G" value at the embrittlement temperature of all samples exceeds 500 MPa. Therefore, it can be said that the embrittlement temperature is not exceeded if it is less than 500 lpa. The storage modulus is used because tires are usually not used at temperatures lower than -30°C.

In addition, this shear modulus was determined using a dynamic torsion tester.
Distortion 0. 5%, measured at a frequency of 20 Hz.

Examples and comparative examples are shown below.

[Example, comparative example]

Diblock copolymers AG having the structures shown in Table 1 were prepared. Furthermore, these copolymers and contrast emulsion polymerization S
BR was vulcanized according to the formulation shown in Table 2. Numbers in the table are parts by weight unless otherwise specified. The vulcanization conditions were 160° C. for 20 minutes, and a rubber sheet with a thickness of 2 mm was obtained. The physical properties of this sheet 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 styrene content was determined by the Hambuton method using an infrared spectrometer. Tg and transition temperature range are DuPont's THER?
Measurement was carried out using IAL ANALYZER at a heating rate of 10° C./min, and the extrapolation starting temperature and the method shown in FIG. 2 were used to determine the temperature.

FIG. 2 shows a DSC curve, and the vertical axis shows the heat flow near the glass transition point. Also, the weight average molecule I
M. is expressed in terms of boristyrene by cpc. Tensile strength T and elongation at break E, are JIS K6301
Niyotta. Shear modulus G' (-
30°C) and tan δ at Q'C is I? II
A dynamic viscoelasticity measuring device manufactured by EOMETRICS was used at a frequency of 20 Hz and a shear strain of 0. Measured at 5%.

(Margins below this page) Comparative Example 5.6 is an example of a blend of emulsion polymerized SBR and natural rubber, but the blend of Comparative Example 5 has good low temperature performance, but low tan δ (0°C) and poor wet performance. Inferior. On the contrary, in Comparative Example 6, the tan δ is high but the low temperature performance is poor. In addition, Comparative Examples 2, 3, and 4 are examples in which styrene-butadiene copolymer rubbers were used in which the 1°g temperature difference between Furofuku (8) and Furuku (B) was 6θ°C or less. ,
Since both of the compounds are compatible with each other and exhibit a tan δ-temperature curve with a single peak, the low-temperature performance is good, but the wet performance is poor. Compared to these, Examples 1 to 3 have better balance in 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 trend part of a tire with a tire, especially the trend part of a high-performance all-season tire.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between low-temperature embrittlement temperature and shear modulus of rubber compounds, 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 illustration of emulsion polymerization with different styrene contents. FIG. 2 is an explanatory diagram showing a tan δ-temperature curve of SBR showing that the peak temperature is higher when the styrene content is higher.

Claims (1)

[Claims] A styrene-butadiene copolymer block (A) containing 15 to 25% by weight of bound styrene, a glass transition temperature of -80°C to -60°C, and a transition temperature width of 12°C or less, and 20 to 50% by weight of bound styrene. %, glass transition temperature is -2
It consists of a styrene-butadiene copolymer block (B) having a transition temperature range of 12°C or less at 0°C to +15°C, and the difference in glass transition temperature between the block (A) and the block (B) is 60°C. The total amount of bound styrene in the entire copolymer is 20 to 35% by weight, the amount of vinyl bonds in all butadiene parts is 20 to 45% by weight, and the weight average molecular weight is 2.
0x10^4 or more styrene-butadiene block copolymer rubber, 90 to 50 parts by weight, the balance is natural rubber or isoprene rubber, and the total rubber content is 100 parts by weight, nitrogen specific surface area 100 m^2/g 80 to 130 parts by weight of the above carbon black, viscosity specific gravity constant 0.90 to
Blending 20 to 90 parts by weight of a petroleum softener of 0.98, -
A rubber composition for a tire tread, characterized in that the shear storage modulus at 30°C is 500 MPa or less.