JP5935243B2 - Rubber composition for run-flat tires - Google Patents

Description

  The present invention relates to a rubber composition for a run-flat tire, in which carbon black with controlled colloidal characteristics is blended to reduce the rolling resistance when made into a tire and to improve the steering stability to a conventional level or more.

  Conventionally, as a pneumatic tire with run-flat running performance, a side rubber reinforcement layer made of hard rubber with a crescent cross section is provided on the sidewall, and it is constant even if the air pressure decreases by puncturing based on the rigidity of this side rubber reinforcement layer It is performed to ensure the travel distance (see, for example, Patent Document 1). The side rubber reinforcing layer is required to have low heat generation and high rigidity in order to suppress heat generation during run-flat running and to ensure rigidity at a high level.

  In addition, with increasing interest in global environmental problems, there is a strong demand for improving fuel efficiency during normal driving. In order to improve the fuel efficiency, it is necessary to suppress heat generation during running and reduce the rolling resistance of the tire, and the heat generation of the rubber composition constituting the tire is reduced. Generally, 60 ° C. tan δ by dynamic viscoelasticity measurement is used as an index of exothermic property of the rubber composition. The smaller the tan δ (60 ° C.) of the rubber composition, the smaller the exothermic property.

  As a method for reducing the tan δ (60 ° C.) of the rubber composition, for example, the amount of carbon black is decreased or the particle size of the carbon black is increased. However, in such a method, there is a problem that rubber hardness and elastic modulus are lowered, and steering stability and running performance during run-flat running are lowered when the tire is formed.

JP 2004-268714 A

  An object of the present invention is to provide a rubber composition for run-flat tires, which contains carbon black with controlled colloidal characteristics and has improved rolling stability when made into a tire while improving steering stability to a conventional level or higher. It is to provide.

The rubber composition for a run-flat tire of the present invention that achieves the above-mentioned object is a nitrogen adsorption specific surface area N ₂ based on 100 parts by weight of a diene rubber containing 10 to 80% by weight of butadiene rubber and 10 to 40% by weight of natural rubber. 20 to 100 parts by weight of a reinforcing filler containing 50% by weight or more of carbon black having an SA of 55 to 81 m ² / g and a DBP absorption of 120 to 150 ml / 100 g, and Stokes of the carbon black aggregate The relationship between the mode diameter Dst (nm) in the mass distribution curve of the diameter and the N ₂ SA is expressed by the following equation (1).
Dst = α (N ₂ SA) ^−0.61 (1)
(However, Dst is the mode diameter (nm) in the mass distribution curve of the Stokes diameter of the aggregate, which is 152 nm or more , N ₂ SA is the nitrogen adsorption specific surface area (m ² / g), and α is a coefficient.)
The coefficient α is 1979 or more and 2215 or less.

The rubber composition for run flat tires of the present invention has a nitrogen adsorption specific surface area N ₂ SA of 55 to 81 with respect to 100 parts by weight of diene rubber containing 10 to 80% by weight of butadiene rubber and 10 to 40% by weight of natural rubber. A reinforcing filler containing 50% by weight or more of carbon black having a m ² / g, DBP absorption of 120 to 150 ml / 100 g, and a coefficient α of 1979 to 2215 when expressed by the relationship of the above formula (1). Since 20 to 100 parts by weight are blended, in order to ensure rubber hardness and elastic modulus while reducing tan δ (60 ° C.) of the rubber composition, steering stability is reduced while reducing rolling resistance when used as a tire. It can be improved beyond the conventional level.

The coefficient alpha is by the range of 1979 ≦ α ≦ 2215, it is possible to suppress the production cost while ensuring excellent properties described above.

  As the butadiene rubber, a butadiene rubber containing a syndiotactic 1,2-polybutadiene component is preferably contained in the diene rubber in an amount of 10 to 40% by weight, so that the reinforcing property of the rubber composition can be further increased.

  The run-flat tire with the rubber composition for the run-flat tire according to the present invention in which a rubber reinforcing layer having a crescent-shaped cross section is disposed on the left and right sidewall portions of the pneumatic tire reduces rolling resistance during normal driving and improves fuel efficiency. However, the steering stability can be improved to a level higher than the conventional level. Further, during run flat running, the run flat running performance is excellent, and the heat generation of the side rubber reinforcing layer having a crescent-shaped cross section of the side wall portion is suppressed, and the durability performance is also excellent.

It is sectional drawing of the tire meridian direction which shows an example of embodiment of the run flat tire using the rubber composition for run flat tires of this invention. It is a graph showing the relationship between Dst and N ₂ SA of the carbon black used in the run-flat tire rubber composition of the present invention.

  Examples of tires to which the rubber composition for run-flat tires of the present invention is applied include side-reinforced run-flat tires in which left and right side wall portions are provided with crescent-shaped rubber reinforcing layers. FIG. 1 is a half cross-sectional view in the tire meridian direction showing an example of a side reinforcing type run-flat tire.

  In FIG. 1, the run flat tire has a tread portion 1, a sidewall portion 2, and a bead portion 3, and a two-ply carcass 6 is interposed between a pair of left and right bead cores 4 and 4, and a bead core 4 and a bead filler 5. A radial structure in which a belt layer 7 composed of a plurality of plies (2 plies in the figure) is disposed over the circumference of the tire on the outer peripheral side of the carcass 6 of the tread portion 1 so as to be folded back from the inside to wrap the tire It has become. An inner liner layer 9 is disposed on the innermost peripheral side of the tire. In addition, rubber reinforcing layers 8 and 8 having a crescent cross section are inserted inside the carcass 6 in the left and right sidewall portions 2 and 2, respectively. In addition, the position where this side rubber reinforcing layer is inserted is not limited to the illustrated example, and may be inserted between the two carcass 6.

  The rubber composition for run-flat tires of the present invention can be suitably used for the rubber reinforcing layer 8 having a crescent-shaped cross section.

  In the rubber composition for a run-flat tire of the present invention, the diene rubber necessarily includes natural rubber and butadiene rubber. By containing natural rubber, the tensile strength at break and tensile elongation at break increase from the height of the molecular weight. Further, by containing butadiene rubber, the temperature dependence of hardness is lowered due to the low embrittlement temperature, and the durability performance is improved.

  The content of the natural rubber is 10 to 40% by weight, preferably 20 to 40% by weight, in 100% by weight of the diene rubber. If the content of the natural rubber is less than 10% by weight, the tensile strength at break becomes low, and it becomes unsuitable for a rubber reinforcing layer having a crescent-shaped cross section of a run flat tire. When the content of natural rubber exceeds 40% by weight, the tensile strength at break and tensile elongation at break are sufficient and the strength becomes a crescent-shaped rubber reinforcing layer, but the cyclic fatigue property becomes insufficient and the durability is inferior.

  The content of butadiene rubber is 10 to 80% by weight, preferably 20 to 80% by weight, in 100% by weight of diene rubber. When the content of butadiene rubber is less than 10% by weight, the repeated fatigue properties deteriorate. When the content of butadiene rubber exceeds 80% by weight, the rubber strength of the rubber composition is excessively lowered.

  In addition, as a butadiene rubber, it is preferable that the one part is a butadiene rubber containing a syndiotactic 1,2-polybutadiene component. The syndiotactic-1,2-polybutadiene component is a resin with high crystallinity and is finely dispersed in butadiene rubber. By blending the butadiene rubber containing the syndiotactic-1,2-polybutadiene component, the hardness and tensile stress of the rubber composition can be increased, and the deformation amount of the rubber can be improved to an optimum value.

  The content of the butadiene rubber containing the syndiotactic 1,2-polybutadiene component is preferably 10 to 40% by weight in 100% by weight of the diene rubber. When the content of the butadiene rubber containing the syndiotactic 1,2-polybutadiene component is less than 10% by weight, the reinforcing property of the rubber composition cannot be sufficiently obtained. On the other hand, when the content of the butadiene rubber exceeds 40% by weight, rolling resistance is deteriorated.

  The content of the syndiotactic-1,2-polybutadiene component in the butadiene rubber containing the syndiotactic-1,2-polybutadiene component suitably used in the present invention is preferably 1 to 50% by weight, more preferably 3 to 3%. 25% by weight. When the content of the syndiotactic-1,2-polybutadiene component is less than 1% by weight, sufficient reinforcement performance may not be obtained. On the other hand, when the content of the syndiotactic-1,2-polybutadiene component exceeds 50% by weight, the rubber composition becomes too hard and the tensile elongation at break is lowered, which is not preferable.

  As such a butadiene rubber containing syndiotactic-1,2-polybutadiene, a commercially available rubber can be used, and examples thereof include UBEPOL VCR412, VCR617, VCR450, and VCR800 manufactured by Ube Industries. .

  In the present invention, the diene rubber can be composed of butadiene rubber and natural rubber. Further, the diene rubber may include other diene rubbers other than butadiene rubber and natural rubber. Examples of other diene rubbers include isoprene rubber, styrene-butadiene rubber, and acrylonitrile-butadiene rubber. Of these, isoprene rubber and styrene-butadiene rubber are preferable. These diene rubbers can be used alone or as any blend.

  In the rubber composition for run-flat tires of the present invention, the reinforcing filler is blended in an amount of 20 to 100 parts by weight, preferably 30 to 90 parts by weight, based on 100 parts by weight of the diene rubber. When the compounding amount of the reinforcing filler is less than 20 parts by weight, the reinforcing property of the rubber composition cannot be sufficiently obtained. Moreover, when the compounding quantity of a reinforcing filler exceeds 100 weight part, rolling resistance when it is set as a tire will become large.

  Examples of the reinforcing filler include carbon black, silica, clay, mica, talc, calcium carbonate, aluminum oxide, titanium oxide, activated zinc white and the like. Of these, carbon black, silica, and clay are preferable.

In the present invention, the following specific carbon black is necessarily included as the reinforcing filler. That is, a novel carbon black having a specific nitrogen adsorption specific surface area N ₂ SA and DBP absorption and limiting the relationship between the N ₂ SA and the mode diameter Dst in the mass distribution curve of the Stokes diameter of the aggregate is blended. Accordingly, mechanical properties such as tensile strength, tensile elongation at break, rubber hardness, elastic modulus, and wear resistance are not deteriorated while reducing tan δ (60 ° C.) of the rubber composition. The amount of the specific carbon black is 50% by weight or more out of 100% by weight of the reinforcing filler described above. If the blending amount of carbon black is less than 50% by weight, the rubber hardness and elastic modulus of the rubber composition cannot be sufficiently improved.

The carbon black used in the present invention has a nitrogen adsorption specific surface area N ₂ SA of 55 to 81 m ² / g. When N ₂ SA is less than 55 m ² / g, mechanical properties such as rubber hardness and dynamic elastic modulus of the rubber composition are lowered. When N ₂ SA exceeds 81 m ² / g, tan δ (60 ° C.) increases. N ₂ SA shall be measured according to JIS K6217-2.

Carbon DB has a DBP absorption of 120 to 150 ml / 100 g. If the DBP absorption is less than 120 ml / 100 g, the rubber composition cannot be sufficiently reinforced, resulting in low hardness and reduced rigidity when used as a tire. Further, since the molding processability of the rubber composition is lowered and the dispersibility of the carbon black is deteriorated, the reinforcing performance of the carbon black cannot be sufficiently obtained. If the DBP absorption exceeds 150 ml / 100 g, the rubber becomes too hard and the riding comfort of the tire is deteriorated. Moreover, workability deteriorates due to an increase in the viscosity of the rubber composition. The DBP absorption amount shall be measured according to JIS K6217-4 oil absorption amount A method.

The carbon black used in the present invention has the above-mentioned colloidal characteristics, and the mode diameter Dst and the nitrogen adsorption specific surface area N ₂ SA in the mass distribution curve of the Stokes diameter of the aggregate are expressed by the following equation (1). When expressed, the coefficient α is 1979 or more and 2215 or less.
Dst = α (N ₂ SA) ^−0.61 (1)
(However, Dst is the mode diameter (nm) in the mass distribution curve of the Stokes diameter of the aggregate, which is 152 nm or more , N ₂ SA is the nitrogen adsorption specific surface area (m ² / g), and α is a coefficient.)

Carbon black has the N ₂ SA and DBP absorption amounts described above, and the coefficient α is set to 1979 or more, so that the tan δ (60 ° C.) of the rubber composition is reduced and the rubber hardness, the dynamic elastic modulus, etc. It can be maintained and improved. The upper limit of the coefficient α is, you view of productivity such as yield and cost of the carbon black in 2215 below. In the present invention, the mode diameter Dst in the mass distribution curve of the Stokes diameter of the aggregate refers to the mode diameter of the maximum frequency in the mass distribution curve of the Stokes diameter of the aggregate obtained by centrifugal sedimentation of carbon black. In the present invention, Dst is measured according to the method for obtaining the aggregate distribution by JIS K6217-6 disc centrifugal light sedimentation method.

FIG. 2 is a graph showing the relationship between Dst and N ₂ SA of the carbon black used in the present invention. In FIG. 2, the horizontal axis represents N ₂ SA (m ² / g), and the vertical axis represents Dst (nm). Representative carbon black having the ASTM standard number was plotted with square marks, and carbon black obtained by trial production was plotted with circle marks and triangle marks. Here, numbers 1 to 13 that refer to carbon blacks CB1 to CB13 used in Examples and Comparative Examples described later are attached to the plots, respectively. As shown in FIG. 2, Dst and N ₂ SA of the conventional standardized carbon black black generally satisfy the relationship of the following formula (2).
Dst = 1650 × (N ₂ SA) ^−0.61 (2)
(However, Dst represents the mode diameter (nm) in the mass distribution curve of the Stokes diameter of the aggregate, and N ₂ SA represents the nitrogen adsorption specific surface area (m ² / g).)
In FIG. 2, the relationship of the above formula (2) is represented by a dotted curve.

On the other hand, in the carbon black used in the present invention, Dst and N ₂ SA are plotted on the upper right from the curve (solid line) of the following formula (3).
Dst = 1979 × (N ₂ SA) ^−0.61 (3)
(However, Dst is a mode diameter (nm) in the mass distribution curve of the Stokes diameter of the aggregate and is 152 nm or more, and N ₂ SA represents a nitrogen adsorption specific surface area (m ² / g).)

That is, in the present invention, a new carbon black in which Dst is larger than Dst of conventional carbon black is used in the range of N ₂ SA of 55 to 81 m ² / g. Such carbon black means that the Stokes diameter of the aggregate is large and the form of the aggregate is close to a sphere even when N ₂ SA is at the same level as conventional carbon black. Thereby, in order to raise the reinforcement performance with respect to rubber | gum, mechanical characteristics, such as rubber hardness of a rubber composition and a dynamic elastic modulus, can be improved more than the conventional level.

  Carbon black having the colloidal characteristics described above is, for example, feedstock introduction conditions, fuel oil and feedstock supply rate, fuel oil combustion rate, reaction time (combustion from the last feedstock introduction position to reaction stoppage) in the carbon black production furnace. The production conditions such as gas residence time) can be adjusted.

  The rubber composition for run-flat tires includes various additives generally used in rubber compositions for run-flat tires such as vulcanization or crosslinking agents, vulcanization accelerators, various oils, anti-aging agents, and plasticizers. These additives can be compounded and kneaded by a general method to form a rubber composition, which can be used for vulcanization or crosslinking. As long as the amount of these additives is not contrary to the object of the present invention, a conventional general amount can be used. The rubber composition for run-flat tires of the present invention can be produced by mixing the above components using a normal rubber kneading machine such as a Banbury mixer, a kneader, or a roll.

  The rubber composition for run-flat tires of the present invention can be suitably used for a side rubber reinforcing layer having a crescent-shaped cross section in a run-flat tire. Since the pneumatic tire using the rubber composition of the present invention for the side rubber reinforcing layer has low heat generation during running, it can reduce rolling resistance and improve fuel efficiency. At the same time, by improving the rubber hardness and elastic modulus of the rubber composition, the handling stability when the tire is made can be improved to a level higher than the conventional level. Further, during run flat running, the run flat running performance is excellent, and the heat generation of the side rubber reinforcing layer having a crescent-shaped cross section of the sidewall portion is suppressed, and the durability performance is also excellent.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further, the scope of the present invention is not limited to these Examples.

17 types of rubber compositions (Examples 1 and 3 to 7 , Reference Example 1 and Comparative Examples 1 to 10) were prepared using 13 types of carbon black (CB1 to CB13). Of these, eight types of carbon black (CB1, CB2, CB8 to CB13) are commercially available grades, and five types of carbon black (CB3 to CB7) are prototypes. Tables 1 and 2 show their colloidal characteristics. In FIG. 2, the relationship between Dst and N ₂ SA of each carbon black CB1 to CB13 is plotted, and numbers for referring to the respective carbon blacks are attached.

In Tables 1 and 2, each abbreviation represents the following colloidal characteristics.
N ₂ SA: Nitrogen adsorption specific surface area measured based on JIS K6217-2 IA: Iodine adsorption amount measured based on JIS K6217-1 CTAB: CTAB measured based on JIS K6217-3 Adsorption specific surface area DBP: DBP absorption measured based on JIS K6217-4 (uncompressed sample) 24M4: 24M4-DBP absorption measured based on JIS K6217-4 (compressed sample) TINT: JIS Specific tinting strength measured based on K6217-5, Dst: Mode diameter, ΔD50, which is the maximum value of the mass distribution curve of the Stokes diameter of the aggregate by the disc centrifugal light sedimentation method measured based on JIS K6217-6 : Mass fraction of the Stokes diameter of the aggregate measured by the disc centrifugal light sedimentation method measured based on JIS K6217-6 In curves, the width of the distribution when its mass frequency is half height of the maximum point (half-width)
· Alpha: coefficient when the Dst and N ₂ SA fit to the relationship of the formula (1) described above alpha

In Tables 1 and 2, carbon blacks CB1, CB2, and CB8 to CB13 represent the following commercial grades, respectively.
・ CB1: Seest KHP made by Tokai Carbon
・ CB2: Toast Carbon Co., Ltd. Seast KH
・ CB8: Tokai Carbon Co., Ltd. Seest 300
・ CB9: Show Black 330T manufactured by Cabot Japan
-CB10: Niteron # 10N manufactured by Nippon Kayaku Carbon
・ CB11: Tokai Carbon Co., Ltd. Seest 3
・ CB12: Tokai Carbon Co., Ltd. Seest NH
CB13: Tokai Carbon Co. Seast 6

Production of carbon blacks CB3 to CB7 Using a cylindrical reactor, carbon black CB3 was changed by changing the total air supply amount, fuel oil introduction amount, fuel oil combustion rate, feedstock oil introduction amount, reaction time as shown in Table 3. -CB7 was produced.

Preparation and Evaluation of Run-Flat Tire Rubber Composition 17 kinds of rubber compositions (Examples 1 and 3 to 7) having the composition shown in Tables 4 and 5 using the 13 kinds of carbon blacks (CB1 to CB13) described above. In preparing Reference Example 1 and Comparative Examples 1 to 10), the components excluding sulfur and the vulcanization accelerator were weighed and kneaded in a 55 L kneader for 15 minutes, and then the master batch was discharged and cooled at room temperature. This master batch was subjected to a 55 L kneader, and sulfur and a vulcanization accelerator were added and mixed to obtain a rubber composition for a run flat tire.

  A run-flat tire having a tire size of 195 / 65R15 was vulcanized and molded so as to constitute a side rubber reinforcing layer having a crescent-shaped cross section shown in FIG. 1 using the obtained 17 kinds of rubber compositions. The rolling resistance and steering stability of the 17 types of pneumatic tires obtained were evaluated by the methods shown below. Moreover, about the comparative example 1 and Example 7, riding comfort performance was evaluated by the method shown below.

Rolling resistance The obtained tire is assembled to a standard rim (size 15 × 6J wheel) and attached to an indoor drum tester (drum diameter 1707 mm) in accordance with JIS D4230. A high-speed durability test was carried out in accordance with the described high-speed durability test, and the resistance force at a test load of 2.94 kN and a speed of 50 km / hour was measured to obtain a rolling resistance. The obtained results are shown in the “Rolling resistance” column of Tables 4 and 5 as an index with the reciprocal of the value of Comparative Example 1 as 100. The larger this index, the smaller the rolling resistance and the better the fuel efficiency.

Steering stability The tires obtained were assembled on a standard rim (size 15 x 6J wheel) and mounted on a domestic 2.0 liter class test vehicle. The course was run on a real vehicle, and the handling stability at that time was scored by a sensitive evaluation by three expert panelists. The obtained results are shown in the “Steering stability” column of Tables 4 and 5 as an index with the value of Comparative Example 1 as 100. The larger this index, the better the steering stability performance.

Ride comfort performance The tires obtained according to Comparative Example 1 and Example 7 were assembled on a standard rim (size 15 × 6J wheel), mounted on a domestic 2.0-liter class test vehicle, and dried on a road with an air pressure of 230 kPa. The test course of 1 lap 2.6km consisting of was run on a real vehicle, and the ride performance at that time was scored by a sensitive evaluation by three special panelists. The obtained results were indexed with the value of Comparative Example 1 set to 100, and the index of ride comfort performance of Example 7 was 110. This means that the ride comfort performance of the run flat tire of Example 7 is significantly improved as compared with the run flat tire of Comparative Example 1.

The types of raw materials used in Tables 4 and 5 are shown below.
NR: natural rubber, RSS # 3
BR: butadiene rubber, Nipol BR1220 manufactured by Nippon Zeon
VCR: butadiene rubber containing 12% by weight of syndiotactic 1,2-polybutadiene component, UBEPOL VCR412 manufactured by Ube Industries, Ltd.
CB1 to CB13: Carbon black silica shown in Tables 1 and 2 above: VN-3 manufactured by Evonik Degussa
Aroma oil: Process X-140 manufactured by Japan Energy
Zinc Hana: Zinc Oxide Type 3 manufactured by Shodo Chemical Industry Co., Ltd. Stearic Acid: Beads Stearate Vulcanization Accelerator Promoted by NOF Co., Ltd.
Sulfur: Kristex HSOT20 manufactured by Akzo Nobel

  As is apparent from Table 4, it was confirmed that the pneumatic tires of Examples 1 to 7 were improved in the low rolling resistance and the steering stability over the conventional level.

As is clear from Table 4, it was confirmed that the pneumatic tires of Examples 1 and 3 to 7 improved the low rolling resistance and the steering stability to the conventional level or more.

As is clear from Table 4, the pneumatic tire of Comparative Example 1 has a low rolling property and steering stability in Examples 1 , 3, and 4 because the coefficient α of the formula (1) of the carbon black CB1 is less than 1979. Inferior to

The pneumatic tires of Comparative Examples 4, 5 and 7 cannot improve the low rolling property because the DBP absorption amount of carbon black CB8, CB9 and CB11 is less than 110 ml / 100 g and the coefficient α is less than 1979. In the pneumatic tire of Comparative Example 6, since N ₂ SA of the carbon black CB10 is less than 55 m ² / g and the coefficient α is less than 1979, the low rolling property is improved but the steering stability is inferior.

In the pneumatic tire of Comparative Example 9, since N ₂ SA of the carbon black CB13 exceeds 95 m ² / g and the coefficient α is less than 1979, the steering stability is improved but the low rolling property is inferior. In the pneumatic tire of Comparative Example 10, although the amount of carbon black CB1 of Comparative Example 1 was reduced and silica was blended instead, steering stability was poor.

DESCRIPTION OF SYMBOLS 1 Tread part 2 Side wall part 3 Bead part 4 Bead core 5 Bead filler 6 Carcass 7 Belt layer 8 Rubber reinforcement layer 9 Inner liner

Claims (3)

Nitrogen adsorption specific surface area N ₂ SA is 55 to 81 m ² / g and DBP absorption is 120 to 150 ml based on 100 parts by weight of diene rubber containing 10 to 80% by weight of butadiene rubber and 10 to 40% by weight of natural rubber. 20 to 100 parts by weight of a reinforcing filler containing 50% by weight or more of 100 g of carbon black is blended, and the mode diameter Dst (nm) in the mass distribution curve of the Stokes diameter of the carbon black aggregate is expressed as N _2. The relationship between SA and the following formula (1)
Dst = α (N ₂ SA) ^−0.61 (1)
(However, Dst is the mode diameter (nm) in the mass distribution curve of the Stokes diameter of the aggregate, which is 152 nm or more , N ₂ SA is the nitrogen adsorption specific surface area (m ² / g), and α is a coefficient.)
A rubber composition for a run-flat tire, wherein the coefficient α is 1979 or more and 2215 or less.   The rubber composition for a run-flat tire according to claim 1, wherein the butadiene rubber contains 10 to 40% by weight of a butadiene rubber containing a syndiotactic 1,2-polybutadiene component in the diene rubber. .   A pneumatic tire comprising the rubber composition for a run-flat tire according to claim 1 or 2, wherein a rubber reinforcing layer having a crescent-shaped cross section in left and right sidewall portions of the run-flat tire is formed.

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