JP2024148178A - Rubber composition for undertread - Google Patents

Abstract

To provide a rubber composition for undertreads, which provides improved maneuvering stability and high-speed durability while ensuring good low-heat-generating properties and allows for a high level of balance among these properties.SOLUTION: In the present invention, 3-60 pts.mass of silica having a CTAB adsorption specific surface area of 60-100 m2/g is added to 100 pts.mass of a rubber component containing 60-90 mass% of an isoprene rubber and 10-40 mass% of an unmodified butadiene rubber. As the unmodified butadiene rubber, one having a molecular weight distribution (Mw/Mn), as determined from the weight average molecular weight (Mw) and the number average molecular weight (Mn), of at most 2.8 is used.SELECTED DRAWING: Figure 1

Description

The present invention relates to a rubber composition for undertreads intended for use primarily in the undertread layer of tires.

In pneumatic tires, there is a demand for improved fuel efficiency during driving in order to reduce the environmental impact. For this reason, efforts have been made to suppress heat generation in the rubber compositions that constitute the various parts of pneumatic tires. In recent years, in order to further improve fuel efficiency, for example, studies have been conducted on suppressing heat generation in the rubber composition that constitutes the undertread rubber layer of pneumatic tires (rubber composition for undertread).

As an index of heat generation of a rubber composition, tan δ at 60°C (hereinafter referred to as "tan δ (60°C)") obtained by dynamic viscoelasticity measurement is generally used, and the smaller the tan δ (60°C) of the rubber composition, the smaller the heat generation. Methods for reducing the tan δ (60°C) of a rubber composition include, for example, increasing the amount of vulcanizing agent or reducing the amount of filler such as carbon black (see, for example, Patent Document 1). However, these methods do not sufficiently ensure the breaking elongation required for a rubber composition for an undertread, and there is a risk that the steering stability and high-speed durability of the tire are not necessarily sufficient. Therefore, further measures are required for improving steering stability and high-speed durability while ensuring good low heat generation (tan δ (60°C)) in a rubber composition for an undertread.

JP 2013-177113 A

The object of the present invention is to provide a rubber composition for undertreads that improves steering stability and high-speed durability while maintaining good low heat generation properties, thereby achieving a good balance between these performance characteristics.

The rubber composition for undertread of the present invention that achieves the above-mentioned object is characterized in that 100 parts by mass of a rubber component containing 60% by mass to 90% by mass of isoprene-based rubber and 10% by mass to 40% by mass of unmodified butadiene rubber is blended with 3 parts by mass to 60 parts by mass of silica having a CTAB adsorption specific surface area of 60 ^m2 /g to 100 ^m2 /g, and the molecular weight distribution (Mw/Mn) calculated from the weight average molecular weight (Mw) and number average molecular weight (Mn) of the unmodified butadiene rubber is 2.8 or less.

The rubber composition for undertread of the present invention is composed of the above-mentioned blending, and can improve the driving stability and high-speed durability while ensuring good low heat generation. Specifically, by using isoprene-based rubber and unmodified butadiene rubber in the above-mentioned blending ratio as the rubber component, it is possible to improve the low heat generation and high-speed durability in a well-balanced manner. In addition, since a suitable amount of large particle size silica (CTAB adsorption specific surface area of 60 m ² /g to 100 m ² /g) is blended with this rubber component, it is possible to improve the low heat generation. Furthermore, since the unmodified butadiene rubber used has a molecular weight distribution (Mw/Mn) of 2.8 or less, it is possible to effectively improve the driving stability and high-speed durability when made into a tire while maintaining good low heat generation. Through the cooperation of these, it is possible to improve the driving stability and high-speed durability while ensuring good low heat generation, and to achieve a good balance between these performances.

In the present invention, it is preferable to compound 20 to 70 parts by mass of carbon black having a CTAB adsorption specific surface area of 40 ^m2 /g to 120 ^m2 /g per 100 parts by mass of the rubber component. By compounding an appropriate amount of carbon black of a specific particle size as a filler in addition to the above-mentioned large particle size silica, processability (rubber cohesion, adhesion to other members, etc.) can be improved.

In the present invention, the cis-1,4 bond content of the unmodified butadiene rubber is preferably 95% or more. Furthermore, the unmodified butadiene rubber preferably satisfies a relationship of 2.0 to 7.0 in terms of the ratio Tcp/ML _{1+ 4 of the viscosity Tcp (unit: cps) of a 5% by mass toluene solution at 25°C to the Mooney viscosity ML 1+4} at 100°C. In addition, the unmodified butadiene rubber is preferably synthesized using a neodymium-based catalyst. By using an unmodified butadiene rubber that satisfies the specific conditions in this way, the effect of improving the low heat generation property and high-speed durability described above can be enhanced, which is advantageous for improving the steering stability and high-speed durability while ensuring good low heat generation property.

In the present invention, a silane coupling agent can be blended, and in that case, it is preferable to set the ratio of the amount of silane coupling agent to the amount of silica blended to 3.0% by mass to 10.0% by mass. This allows for good dispersion of the silica, which is advantageous for improving low heat generation and high-speed durability.

The rubber composition for undertread of the present invention can be suitably used in the undertread layer of a tire. Specifically, a tire using the rubber composition for undertread of the present invention has a tread portion extending in the tire circumferential direction to form an annular shape, and a pair of sidewall portions arranged on both sides of the tread portion, and the tread portion is preferably constructed by laminating a cap tread layer constituting the tire tread surface and an undertread layer arranged on the inner circumferential side of the cap tread layer. In such a tire, it is preferable that the undertread layer is constructed from the rubber composition for undertread of the present invention. In this way, a tire having an undertread layer made of the rubber composition for undertread of the present invention can improve steering stability and high-speed durability while ensuring good low heat generation due to the excellent physical properties of the rubber composition for undertread of the present invention.

In this case, it is preferable that the ratio _{M100UT/M100SW of the tensile stress M100UT} [unit: MPa] of the undertread rubber constituting the undertread layer at 100% elongation at 23° C. to the tensile stress _M100SW [unit: MPa] of the side rubber constituting the sidewall portion at 100% elongation at 23° _C. is 1.5 to 4.0. By optimizing the relationship of the tensile stress (M100 ₎ between the undertread layer using the rubber composition for undertread of the present invention and the sidewall portion (side rubber) adjacent thereto in this way, it is advantageous to improve low heat build-up properties and high-speed durability.

In the present invention, the "CTAB adsorption specific surface area" is measured in accordance with ISO 5794. The "weight average molecular weight Mw" and the "number average molecular weight Mn" are measured in terms of standard polystyrene by gel permeation chromatography (GPC). The "cis-1,4 bond content" is the proportion of cis-1,4-bonds among the cis-1,4-bonds, trans-1,4-bonds, and 1,2-vinyl bonds, which are the bonding modes of butadiene, and is measured by infrared spectroscopy (Hampton method). The "Mooney viscosity at 100°C ML ₁₊₄ " is measured in accordance with JIS K6300-1:2001 using an L-type rotor in a Mooney viscometer under the conditions of a preheating time of 1 minute, a rotor rotation time of 4 minutes, and a temperature of 100°C. "Viscosity Tcp of a 5% by mass toluene solution at 25° C." is the viscosity of a toluene solution containing 5% by weight of the target butadiene rubber, and is measured using a Cannon-Fenske viscometer at 25° C. "Tensile stress at 100% elongation at 23° C." is a value measured using a No. 3 dumbbell test piece in accordance with JIS K6251 at a tensile speed of 500 mm/min and a temperature of 23° C.

1 is a meridian cross-sectional view showing an example of a pneumatic tire using the rubber composition for an undertread of the present invention.

The configuration of the present invention will be described in detail below with reference to the attached drawings.

As shown in FIG. 1, a pneumatic tire in which the rubber composition for undertread of the present invention is used includes a tread portion 1, a pair of sidewall portions 2 arranged on both sides of the tread portion 1, and a pair of bead portions 3 arranged on the tire radial inner side of the sidewall portions 2. In FIG. 1, the symbol CL indicates the tire equator. Although not depicted because FIG. 1 is a meridian cross section, the tread portion 1, the sidewall portions 2, and the bead portions 3 each extend in the tire circumferential direction to form an annular shape, which constitutes the basic toroidal structure of a pneumatic tire. The following explanation using FIG. 1 is basically based on the meridian cross section shown in the figure, but each tire component extends in the tire circumferential direction to form an annular shape.

A carcass layer 4 is installed between a pair of left and right bead portions 3. This carcass layer 4 includes a plurality of reinforcing cords extending in the tire radial direction, and is folded back from the inner side to the outer side in the tire width direction around the bead cores 5 arranged in each bead portion 3. In addition, a bead filler 6 is arranged on the outer periphery of the bead cores 5, and this bead filler 6 is wrapped by the main body and folded back parts of the carcass layer 4. On the other hand, a plurality of belt layers 7 (two layers in FIG. 1) are embedded on the outer periphery of the carcass layer 4 in the tread portion 1. Each belt layer 7 includes a plurality of reinforcing cords inclined with respect to the tire circumferential direction, and is arranged so that the reinforcing cords cross each other between the layers. In these belt layers 7, the inclination angle of the reinforcing cords with respect to the tire circumferential direction is set to, for example, a range of 10° to 40°. Furthermore, a belt reinforcing layer 8 (two layers: a full cover 8a that covers the entire width of the belt layer 7 and an edge cover 8b that locally covers the end portions of the belt layer 7) is provided on the outer periphery of the belt layer 7. The belt reinforcement layer 8 includes organic fiber cords oriented in the tire circumferential direction. In the belt reinforcement layer 8, the organic fiber cords are set at an angle of, for example, 0° to 5° with respect to the tire circumferential direction.

A tread rubber layer 10 is arranged on the outer peripheral side of the carcass layer 4 in the tread portion 1, a side rubber layer 20 is arranged on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the sidewall portion 2, and a rim cushion rubber layer 30 is arranged on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the bead portion 3. The tread rubber layer 11 has a structure in which two types of rubber layers with different physical properties (a cap tread 11 that forms the tread surface of the tread portion 1 and an undertread 12 arranged on its inner peripheral side) are laminated in the tire radial direction.

The tire in which the rubber composition for undertread of the present invention is used is preferably a pneumatic tire (a tire filled with air, an inert gas such as nitrogen, or other gas) as described above, but may be a non-pneumatic tire. In the case of a non-pneumatic tire, the rubber composition for undertread of the present invention can be used in a rubber layer (a portion corresponding to the undertread layer 12 in a pneumatic tire) arranged on the inner periphery side of the portion that contacts the road surface (a portion corresponding to the cap tread layer 11 in a pneumatic tire). As long as the tread portion 1 (tread rubber layer 10) of the tire to which the present invention is applied is composed of the cap tread 11 and the undertread 12, the basic structure of the other portions is not limited to the above-mentioned structure.

In the rubber composition for undertread of the present invention, the rubber component is a diene rubber, and necessarily contains an isoprene rubber and an unmodified butadiene rubber. In this way, the use of an isoprene rubber and an unmodified butadiene rubber is advantageous in improving low heat generation while maintaining good elongation at break.

Examples of isoprene-based rubber include various natural rubbers, epoxidized natural rubbers, and various synthetic polyisoprene rubbers. Of these isoprene-based rubbers, natural rubber is particularly suitable. The content of isoprene-based rubber is 60% to 90% by mass, preferably 65% to 90% by mass, and more preferably 70% to 90% by mass, of 100% by mass of the rubber component. By including such an amount of isoprene-based rubber (particularly natural rubber), it is possible to improve the breaking elongation and low heat generation property in a well-balanced manner. If the amount of isoprene-based rubber is less than 60% by mass, the breaking elongation decreases. If the amount of isoprene-based rubber is more than 90% by mass, the low heat generation property deteriorates.

The unmodified butadiene rubber used has a molecular weight distribution (Mw/Mn) calculated from the weight average molecular weight (Mw) and number average molecular weight (Mn) of 2.8 or less, preferably 2.5 or less. By using unmodified butadiene rubber with a moderate molecular weight distribution in this way, the rubber properties become better, and the handling stability and high-speed durability of the tire when made into a tire can be effectively improved while maintaining good low heat generation. If the molecular weight distribution (Mw/Mn) of the unmodified butadiene rubber exceeds 2.8, the hysteresis loss increases, the heat generation of the rubber increases, and the compression set resistance decreases.

The content of unmodified butadiene rubber is 10% to 40% by mass, preferably 15% to 35% by mass, and more preferably 20% to 35% by mass, out of 100% by mass of the rubber component. Incorporating an appropriate amount of unmodified butadiene rubber in this way is advantageous for improving elongation at break and low heat buildup in a well-balanced manner. If the amount of unmodified butadiene rubber is less than 10% by mass, low heat buildup deteriorates. If the amount of unmodified butadiene rubber is more than 40% by mass, elongation at break decreases.

The cis-1,4 bond content of unmodified butadiene rubber is preferably 95% or more, and more preferably 96% to 99%. A high cis-1,4 bond content is advantageous for improving breaking strength and breaking elongation. If the cis-1,4 bond content is less than 95%, breaking strength and breaking elongation will decrease. The cis-1,4 bond content of butadiene rubber can be increased or decreased by the usual methods, such as by using a catalyst.

The unmodified butadiene rubber preferably satisfies the relationship of a ratio Tcp/ML _{1+ 4 of the viscosity Tcp (unit: cps) of a 5% by mass toluene solution at 25°C to the Mooney viscosity ML 1+} 4 at 100°C of 2.0 to 7.0, more preferably 2.5 to 6.8. By setting the ratio Tcp/ML ₁₊₄ in a specific range in this way, it is advantageous to achieve low heat buildup, breaking strength, and breaking elongation at the same time. If the ratio Tcp/ML ₁₊₄ is less than 2.0, the low heat buildup property deteriorates. If the ratio Tcp/ML ₁₊₄ exceeds 7.0, the processability deteriorates.

In setting the above-mentioned ratio Tcp/ML1 ₊₄ , the value of the 5 mass % toluene solution viscosity Tcp itself at 25° C. is not particularly limited, but can be set preferably to 50 cps to 200 cps, more preferably to 80 cps to 180 cps. Similarly, the value of the Mooney viscosity ML1 ₊₄ itself at 100° C. is not particularly limited, but can be set preferably to 45 to 52, more preferably to 45 to 50.

It is preferable that the unmodified butadiene rubber is a butadiene rubber synthesized using a neodymium catalyst (hereinafter, sometimes referred to as Nd-BR). Although the desired effect can be obtained by using butadiene rubber polymerized with other catalysts (e.g., nickel or cobalt), Nd-BR is preferably used from the viewpoint of achieving both low heat generation, breaking strength, and breaking elongation.

Butadiene rubber polymerized using a neodymium-based catalyst (Nd-BR) is a well-known material, and is butadiene rubber polymerized using a neodymium-based catalyst such as simple neodymium, compounds of neodymium with other metals, or organic neodymium compounds. Butadiene rubber synthesized using these neodymium-based catalysts has the characteristics of a high molecular weight and a sharp molecular weight distribution. Commercially available Nd-BR can be used.

The rubber components constituting the rubber composition for undertread of the present invention include at least the above-mentioned isoprene-based rubber (particularly natural rubber) and unmodified butadiene rubber, but may optionally contain other diene-based rubbers. As the other diene-based rubbers, rubbers that can generally be used in rubber compositions for tires can be used. For example, unmodified styrene-butadiene rubber can be exemplified. In addition, when natural rubber is used as the isoprene-based rubber, isoprene rubber can also be used in combination as the other diene-based rubber. These other diene-based rubbers can be used alone or as any blend.

In the present invention, silica is always blended as a filler with the above-mentioned rubber component. As the silica used in the present invention, silica normally used in rubber compositions for tires, such as wet-process silica, dry-process silica, or surface-treated silica, can be used. However, the CTAB adsorption specific surface area of the silica is limited to 60 m ² /g to 100 m ² /g, preferably 70 m ² /g to 90 m ² /g, and more preferably 75 m ² /g to 85 m ² /g. By using silica with such a large particle size, low heat generation and wear resistance can be improved. If the CTAB adsorption specific surface area of the silica is less than 60 m ² /g, low heat generation deteriorates. If the CTAB adsorption specific surface area of the silica is more than 100 m ² /g, wear resistance decreases. If the silica satisfies the above-mentioned conditions, it may be appropriately selected from commercially available products and used, and silica obtained by a normal manufacturing method may also be used.

The amount of silica blended is 3 to 60 parts by mass, preferably 5 to 50 parts by mass, and more preferably 10 to 50 parts by mass, per 100 parts by mass of the rubber component described above. By blending an appropriate amount of silica in this way, it is possible to improve the low heat generation properties, driving stability, and high-speed durability of the resulting tire in a well-balanced manner. If the amount of silica blended is less than 3 parts by mass, the elongation at break decreases. If the amount of silica blended exceeds 60 parts by mass, the low heat generation properties deteriorate.

In the present invention, carbon black may be blended as a filler in addition to the silica described above. The carbon black used in the present invention is preferably one having a CTAB adsorption specific surface area of 40 m ² /g to 120 m ² /g, more preferably 70 m ² /g to 100 m ² /g, among the carbon blacks usually used in rubber compositions for tires. The use of such carbon black is advantageous in improving low heat generation, driving stability, and high-speed durability in a well-balanced manner when the tire is made. If the CTAB adsorption specific surface area of the carbon black is less than 40 m ² /g, the abrasion resistance decreases. If the CTAB adsorption specific surface area of the carbon black is more than 120 m ² /g, the low heat generation property deteriorates.

When carbon black is used in addition to silica as described above, the total amount of silica and carbon black is preferably 40 to 95 parts by mass, more preferably 45 to 90 parts by mass, per 100 parts by mass of the rubber component described above. By blending appropriate amounts of silica and carbon black in this way, it is advantageous to improve the low heat generation, handling stability, and high-speed durability of the resulting tire in a well-balanced manner. If the total amount of silica and carbon black is less than 40 parts by mass, the breaking strength decreases and high-speed durability deteriorates. If the total amount of silica and carbon black is more than 95 parts by mass, fuel efficiency deteriorates and the breaking elongation decreases.

Furthermore, when silica and carbon black are used in combination, the ratio Ms/Mc of the amount of silica to the amount of carbon black to be mixed is preferably 0.05 to 0.7, more preferably 0.1 to 0.6. By mixing silica and carbon black in an appropriate balance in this way, and in particular by keeping the amount of silica in the filler low, it is possible to suppress the deterioration of tan δ (60°C) caused by mixing large particle size silica, and to improve the breaking elongation and low heat generation in a well-balanced manner. If the ratio Ms/Mc of the amount of silica to the amount of carbon black to be mixed exceeds 0.7, processability (rubber cohesion, adhesion to other components, etc.) will deteriorate.

The rubber composition of the present invention can contain fillers other than silica and carbon black. Examples of other fillers include materials commonly used in rubber compositions for tires, such as clay, talc, calcium carbonate, mica, and aluminum hydroxide.

In the rubber composition for undertread of the present invention, a silane coupling agent may be blended when blending the above-mentioned silica. By blending a silane coupling agent, the dispersibility of silica in the rubber component (diene rubber) can be improved. Examples of silane coupling agents include bis-(3-triethoxysilylpropyl) tetrasulfide, bis(3-triethoxysilylpropyl) disulfide, 3-trimethoxysilylpropyl benzothiazole tetrasulfide, γ-mercaptopropyl triethoxysilane, 3-octanoylthiopropyl triethoxysilane, and the like. Among these, those having a tetrasulfide bond in the molecule can be preferably used. The blending amount of the silane coupling agent is preferably 3.0% by mass to 10% by mass, more preferably 4.0% by mass to 9.0% by mass, relative to the blending amount of silica. If the blending amount of the silane coupling agent is less than 3.0% by mass of the blending amount of silica, the blending amount is so small that the effect of blending the silane coupling agent cannot be expected to be sufficient. If the amount of silane coupling agent exceeds 10% by mass of the amount of silica, the silane coupling agents will condense with each other, making it difficult to obtain the desired hardness and strength in the rubber composition.

Other compounding agents other than those mentioned above can be added to the rubber composition for undertread of the present invention. Examples of other compounding agents include various compounding agents generally used in rubber compositions for tires, such as vulcanizing or crosslinking agents, vulcanization accelerators, antiaging agents, and liquid polymers. The compounding amounts of these compounding agents can be conventional amounts as long as they do not go against the object of the present invention. In addition, as the kneading machine, a normal rubber kneading machine, such as a Banbury mixer, kneader, or roll, can be used.

The rubber composition for undertread of the present invention can be suitably used in the undertread layer of a tire as described above, and a tire having an undertread layer made of the rubber composition for undertread of the present invention can improve steering stability and high-speed durability while ensuring good low heat generation due to the excellent physical properties of the rubber composition for undertread of the present invention. When the rubber composition for undertread of the present invention is used in a tire, the ratio M100UT/M100SW of the tensile stress _M100UT [unit: MPa] at 100% elongation at 23°C of the undertread rubber constituting the undertread layer (i.e., the vulcanizate of the rubber composition for undertread of the present invention) to the tensile stress _M100SW [unit: MPa _] at 100% elongation at 23 _° C of the side rubber constituting the sidewall portion is preferably 1.5 to 4.0, more preferably 1.8 to 3.5. In this way, optimizing the relationship of tensile stress (M100) between the undertread layer using the rubber composition for undertread of the present invention and the adjacent sidewall portion (side rubber) is advantageous in improving driving stability and high-speed durability while ensuring low heat buildup. If the ratio _M100UT / _M100SW is less than 1.5, the effects of improving low heat buildup, driving stability, and high-speed durability are limited. If the ratio _M100UT / _M100SW exceeds 4.0, the effects of improving low heat buildup, driving stability, and high-speed durability are limited.

The present invention will be further explained below with reference to examples, but the scope of the present invention is not limited to these examples.

Tires (test _tires) of Standard Example 1, Comparative Examples 1 to 7, and Examples 1 to 8 were manufactured having a tire size of 245/45R18 and the basic structure shown in FIG. 1, with the formulation of the rubber composition for the undertread, and the tensile stresses ( _M100UT , M100SW) and ratios _M100UT / _M100SW of the undertread rubber and side rubber at 100% elongation at 23°C set as shown in Tables 1 and 2.

Regarding the compounding of the rubber composition for the undertread, the compounding common to all examples is summarized in Table 3. Tables 1 and 2 also list the type of side rubber used in each example. The type of side rubber is listed with the numbers of side rubbers A to B listed in Table 4, and the compounding of these side rubbers A to B is as shown in Table 4.

The tensile stresses ( _M100UT , _M100SW ) shown in Tables 1 to 2 and 4 are values (unit: MPa) measured using undertread rubber and side rubber, using No. 3 dumbbell test pieces prepared in accordance with JIS K6251, at a tensile speed of 500 mm/min and a temperature of 23°C.

Each test tire was evaluated for handling stability, low rolling resistance, and high-speed durability using the methods described below.

Steering stability: Each test tire was mounted on a wheel with a rim size of 18x7J, the air pressure was set to 240kPa, and the tire was mounted on a test vehicle with an engine displacement of 2000cc. The test driver performed a sensory evaluation of the steering stability on a test course consisting of a paved road surface. The evaluation results were evaluated on a 5-point scale, with the result of Standard Example 1 being 3 points (standard). The higher the score, the better the steering stability.

Low rolling resistance Each test tire was mounted on a wheel with a rim size of 18x7J, and the rolling resistance was measured using an indoor drum tester (drum diameter: 1707.6mm) in accordance with ISO28580 under the conditions of an air pressure of 210kPa, a load of 4.82kN, and a speed of 80km/h. The evaluation results were expressed as an index using the reciprocal of the measured value, with Standard Example 1 being 100. The higher the index value, the lower the rolling resistance and the better the low rolling resistance.

High-speed durability Each test tire was mounted on a wheel with a rim size of 18x7J, and a running test was carried out on an indoor drum test machine (drum diameter: 1707.6 mm) under the conditions of an air pressure of 340 kPa, a load of 5 kN, and a negative camber angle of 3°. Specifically, the initial speed was set to 250 km/h, and the speed was increased by 10 km/h every 20 minutes, and the tire was run until failure occurred, and the reached step (speed) was measured. The evaluation results were expressed as an index with the measured value (speed) of Standard Example 1 set to 100. The larger the index value, the larger the reached step (speed) until failure and the better the high-speed durability with camber angle.

The types of raw materials used in Tables 1 to 3 are shown below.
NR: Natural rubber, RSS#3
BR1: Nipol 1220 manufactured by Zeon Corporation (butadiene rubber synthesized using a cobalt catalyst, cis-1,4 bond content: 98 mol%, Mw/Mn=2.9, Tcp/ML ₁₊₄ =1.4)
BR2: Nipol 1250H manufactured by Zeon Corporation (butadiene rubber synthesized using a lithium catalyst, cis-1,4 bond content: 35 mol%, Mw/Mn=1.4)
BR3: UBEPOL BR360L manufactured by Ube Industries (butadiene rubber synthesized using a cobalt catalyst, cis-1,4 bond content: 98 mol%, Mw/Mn=2.2, Tcp/ML ₁₊₄ =2.8)
BR4: Buna CB22 manufactured by ARLANXEO (butadiene rubber synthesized using a neodymium catalyst, cis-1,4 bond content: 98 mol%, Mw/Mn = 2.3, Tcp/ML ₁₊₄ = 6.5)
BR5: Buna CB24 manufactured by ARLANXEO (butadiene rubber synthesized using a neodymium catalyst, cis-1,4 bond content: 96 mol%, Mw/Mn=2.0)
CB1: Carbon black (grade: ISAF), Seast 6 (CTAB adsorption specific surface area: 109 m ² /g) manufactured by Tokai Carbon Co., Ltd.
CB2: Carbon black (grade: HAF), manufactured by Tokai Carbon Co., Ltd., SEAT 3 (CTAB adsorption specific surface area: 82 ^m2 /g)
CB3: Carbon black (grade: FEF), manufactured by Tokai Carbon Co., Ltd., SEAT F (CTAB adsorption specific surface area: 47 ^m2 /g)
・Silica 1: ULTRASIL VN3GR manufactured by Evonik (CTAB adsorption specific surface area: 175 m ² /g)
Silica 2: ZEOSIL 115GR (CTAB adsorption specific surface area: 115 m ² /g) manufactured by Solvay
Silica 3: Zeosil 1185GR manufactured by Solvay (CTAB adsorption specific surface area: 80 m ² /g)
Silane coupling agent: Si69 manufactured by Evonik Industries AG
・Stearic acid: Stearic acid 50S manufactured by Nisshin Rika Co., Ltd.
・Zinc oxide: Three types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. ・Anti-aging agent: PILFLEX 13 manufactured by NOCIL LIMITED
- Tackifier: Hitachi Chemical's Hitanol 1502Z
Sulfur: Shikoku Chemical Industry Co., Ltd., Myucron OT-20
- Vulcanization accelerator: Sancerer NS-G manufactured by Sanshin Chemical Industry Co., Ltd.

The types of raw materials used in Table 4 are shown below.
NR: Natural rubber, natural rubber, RSS#3
・BR: Nippon Zeon Co., Ltd. Nipol 1220
CB: Carbon black (grade: FEF), manufactured by Tokai Carbon Co., Ltd., SEAT F (CTAB adsorption specific surface area: 47 ^m2 /g)
- Tackifier: Hitachi Chemical's Hitanol 1502Z
・Zinc oxide: Three types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. ・Anti-aging agent: PILFLEX 13 manufactured by NOCIL LIMITED
・Stearic acid: Stearic acid 50S manufactured by Nisshin Rika Co., Ltd.
Aroma oil: Aromax T-DAE, manufactured by ENEOS
Sulfur: Sulfax 5 manufactured by Tsurumi Chemical Industry Co., Ltd.
- Vulcanization accelerator: Sancerer NS-G manufactured by Sanshin Chemical Industry Co., Ltd.

As is clear from Tables 1 and 2, Examples 1 to 8 improved steering stability, low rolling resistance, and high-speed durability compared to Standard Example 1, and achieved a good balance between these performances. On the other hand, Comparative Example 1 had a poor low rolling resistance due to the large CTAB specific surface area of silica. Comparative Example 2 had a poor low rolling resistance due to the large CTAB specific surface area of silica. Comparative Example 3 had a poor high-speed durability due to the use of modified butadiene rubber. Comparative Example 4 had a poor high-speed durability due to the small amount of silica blended. Comparative Example 5 had a poor steering stability due to the large amount of silica blended. Comparative Example 6 had a poor low rolling resistance due to the large amount of isoprene-based rubber (natural rubber) blended and a small amount of unmodified butadiene rubber blended. Comparative Example 7 had a poor steering stability and high-speed durability due to the small amount of isoprene-based rubber (natural rubber) blended and a large amount of unmodified butadiene rubber blended.

The present disclosure includes the following inventions.
Invention [1] A rubber composition for undertread, comprising 100 parts by mass of a rubber component containing 60% by mass to 90% by mass of isoprene-based rubber and 10% by mass to 40% by mass of unmodified butadiene rubber, and 3 to 60 parts by mass of silica having a CTAB adsorption specific surface area of 60 ^m2 /g to 100 ^m2 /g, wherein the molecular weight distribution (Mw/Mn) calculated from the weight average molecular weight (Mw) and number average molecular weight (Mn) of the unmodified butadiene rubber is 2.8 or less.
Invention [2] The rubber composition for undertread according to invention [1], characterized in that 20 to 70 parts by mass of carbon black having a CTAB adsorption specific surface area of 40 ^m2 /g to 120 ^m2 /g is blended with 100 parts by mass of the rubber component.
Invention [3] The rubber composition for undertread according to invention [1] or [2], characterized in that the unmodified butadiene rubber has a cis-1,4 bond content of 95% or more.
Invention [4] The unmodified butadiene rubber has a 5 mass% toluene solution viscosity Tcp (unit: cps) at 25°C and a Mooney viscosity ML1 ₊₄ at 100°C, and the ratio Tcp/ML1 ₊₄ satisfies a relationship of 2.0 to 7.0. A rubber composition for undertread according to any one of inventions [1] to [3].
Invention [5] The rubber composition for undertread according to any one of inventions [1] to [4], characterized in that the unmodified butadiene rubber is synthesized using a neodymium-based catalyst.
Invention [6] A silane coupling agent is blended, and the ratio of the blended amount of the silane coupling agent to the blended amount of the silica is 3.0 mass% to 10.0 mass%. The rubber composition for undertread according to any one of inventions [1] to [5].
Invention [7] A tire comprising a tread portion extending in a circumferential direction of the tire to form an annular shape, and a pair of sidewall portions disposed on both sides of the tread portion, the tread portion being constructed by laminating a cap tread layer constituting the tire tread surface and an undertread layer disposed on the inner side of the cap tread layer, the undertread layer being constructed from the rubber composition for undertread according to any one of inventions [1] to [6].
Invention [8] The tire according to invention [7], characterized in that the ratio M100UT _{/ M100SW} of the tensile stress _M100UT [unit: MPa] of the undertread rubber constituting the undertread layer at 100% elongation at 23°C of the undertread rubber constituting the sidewall portion at 100% elongation at 23°C of the side _rubber is 1.5 to 4.0.

Reference Signs List 1 Tread portion 2 Sidewall portion 3 Bead portion 4 Carcass layer 5 Bead core 6 Bead filler 7 Belt layer 8 Belt cover layer 10 Tread rubber layer 11 Cap tread layer 12 Under tread layer 20 Side rubber layer 30 Rim cushion rubber layer CL Tire equator

The rubber composition for undertread of the present invention that achieves the above-mentioned object is characterized in that 100 parts by mass of a rubber component containing 60% by mass to 90% by mass of isoprene-based rubber and 10% by mass to 40% by mass of unmodified butadiene rubber is blended with 25 parts by mass to 60 parts by mass of silica having a CTAB adsorption specific surface area of 60 ^m2 /g to 100 ^m2 /g, and the molecular weight distribution (Mw/Mn) calculated from the weight average molecular weight (Mw) and number average molecular weight (Mn) of the unmodified butadiene rubber is 2.8 or less.

Claims (8)

A rubber composition for undertread comprising 100 parts by mass of a rubber component containing 60 to 90% by mass of isoprene-based rubber and 10 to 40% by mass of unmodified butadiene rubber, and 3 to ^{60 parts by mass of silica having a CTAB adsorption specific surface area of 60 to 100 m2 /} g, and wherein the molecular weight distribution (Mw/Mn) calculated from the weight average molecular weight (Mw) and number average molecular weight (Mn) of the unmodified butadiene rubber is 2.8 or less. 2. The rubber composition for undertread according to claim 1, characterized in that 20 to 70 parts by mass of carbon black having a CTAB adsorption specific surface area of 40 to 120 ^{m 2 /} g is blended with respect to 100 parts by mass of the rubber component. The rubber composition for undertreads according to claim 1 or 2, characterized in that the unmodified butadiene rubber has a cis-1,4 bond content of 95% or more. The unmodified butadiene rubber satisfies a relationship of 2.0 to 7.0 in terms of a ratio Tcp/ML ₁₊₄ of a 5% by mass toluene solution viscosity Tcp (unit: cps) at 25°C to a Mooney viscosity ML ₁₊ 4 at 100°C. The rubber composition for undertreads according to claim 1 or 2, characterized in that the unmodified butadiene rubber is synthesized using a neodymium-based catalyst. The rubber composition for undertread according to claim 1 or 2, characterized in that a silane coupling agent is blended, and the ratio of the blended amount of the silane coupling agent to the blended amount of the silica is 3.0% by mass to 10.0% by mass. A tire having a tread portion extending in the circumferential direction of the tire to form an annular shape and a pair of sidewall portions disposed on both sides of the tread portion, the tread portion being constructed by laminating a cap tread layer constituting the tire tread surface and an undertread layer disposed on the inner circumferential side of the cap tread layer, the undertread layer being constructed from the rubber composition for undertreads according to claim 1 or 2. The tire according to claim 7, characterized in that the ratio M100UT/M100SW of the tensile stress _M100UT [unit: MPa] of the undertread rubber constituting the undertread layer at 100% elongation at 23°C to the tensile stress _M100SW [unit: MPa] of the side rubber constituting the sidewall portion at 100 _% elongation at 23° _C is 1.5 to 4.0.

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