JP2018058421A - Pneumatic tire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pneumatic tire which has low rolling resistance, can prevent occurrence of groove cracking even in thin groove lower gage, and has good operation stability.SOLUTION: A tread rubber layer 10 is formed of a cap tread 11 and an undertread 12, a ratio of the undertread in a main groove 8 lower area is set at 95% or less, a height Ht from the outer peripheral surface of a reinforcement layer 7 to a surface of a land part 9 is set at 30 mm or less, a height Hg from the outer peripheral surface of the reinforcement layer to the bottom surface of the main groove is set at 5 mm or less, a height H from the outer peripheral surface of the reinforcement layer in a width direction center position of the land part 9 to a boundary between the cap tread and the undertread is 0.5 mm or more and less than Hg, the undertread contains a butadiene structure and contains a filler of 20-80 pts.wt. with respect to a rubber component, elastic modulus Eu of a rubber composition for undertread is set at 12 MPa or less, and elastic modulus Ec of a rubber composition for cap tread satisfies a relationship of Ec/Eu≤5.0 with respect to the elastic modulus Eu.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a pneumatic tire in which a sub-groove gauge is thinned to reduce rolling resistance. More specifically, even if the sub-groove gauge is thin, the occurrence of groove cracks can be prevented, and steering stability can be improved. The present invention relates to a pneumatic tire that can be well maintained.

  In recent years, in order to reduce the rolling resistance of pneumatic tires and reduce fuel consumption, the gauge of the rubber layer (under-groove gauge) constituting the lower part of the main groove extending in the tire circumferential direction in the tread portion has been reduced. It has been broken. However, when the sub-groove gauge becomes extremely thin, the rigidity of the rubber layer constituting the lower part of the main groove with respect to the reinforcing layer including the reinforcing cord such as the belt layer or the belt reinforcing layer is remarkably reduced, and the bottom of the main groove There is a problem that when the tire is used over a long period of time and deteriorates with time, groove cracks are likely to occur.

  On the other hand, the rubber layer (tread rubber layer) constituting the tread portion is generally composed of a cap tread exposed on the tread surface and an under tread disposed on the inner peripheral side thereof. In this case, the cap tread needs to have high hardness physical properties in order to improve the steering stability of the tire, and on the other hand, tends to have a characteristic that it is difficult to stretch. On the other hand, the under tread is intended to reduce the heat generation of the tire and is composed of a rubber composition that is more flexible than the cap tread. For example, as shown in Patent Document 1, the thickness of the under tread is increased so that the under tread has a groove. If it is exposed to the bottom, it is considered to be a countermeasure for the groove crack. However, there is a problem that when the amount of undertread used increases, the cap tread that inevitably contributes to steering stability decreases and steering stability deteriorates.

JP-A-4-118305

  An object of the present invention is an air that can prevent the occurrence of groove cracks and maintain good steering stability even when the under-groove gauge is thinned to reduce rolling resistance. The purpose is to provide tires.

  In order to achieve the above object, a pneumatic tire according to the present invention includes a tread portion that extends in the tire circumferential direction to form an annular shape, a pair of sidewall portions disposed on both sides of the tread portion, and the sidewall portions. A pair of bead portions disposed on the inner side in the tire radial direction, a carcass layer mounted between the pair of bead portions, a reinforcing layer disposed on an outer peripheral side of the carcass layer in the tread portion, and the tread. A tread rubber layer disposed on the outer peripheral side of the carcass layer and the reinforcing layer in a portion, and a main groove extending in a tire circumferential direction and a land portion partitioned by the main groove are formed on an outer surface of the tread portion A cap tread in which the tread rubber layer is exposed on the surface of the tread portion, and an under tread disposed on an inner peripheral side of the cap tread In the meridian cross section, the ratio of the under tread in the region below the main groove is 95% or more, and the height Ht from the outer peripheral surface of the reinforcing layer to the surface of the land portion is 30 mm or less. The height Hg from the outer peripheral surface of the reinforcing layer to the bottom surface of the main groove is 5 mm or less, and the boundary between the cap tread and the under tread from the outer peripheral surface of the reinforcing layer at the center position in the width direction of the land portion. The rubber composition constituting the undertread contains at least a butadiene structure in the rubber component, and has a height h of 0.5 mm or more and satisfies the relationship of h <Hg with respect to the height Hg. Containing 20 to 80 parts by weight of filler with respect to 100 parts by weight of the component, and the elastic modulus Eu of the rubber composition constituting the undertread is 12 MPa or less; and , Wherein the elastic modulus Ec of the rubber composition constituting the cap tread satisfies the relationship Ec / Eu ≦ 5.0 with respect to the elastic modulus Eu.

  In the pneumatic tire of the present invention, most of the region below the groove is composed of the undertread composed of the above-mentioned composition, so that the groove bottom can be made flexible and able to flexibly follow strain, and the occurrence of groove cracks. Can be suppressed. At this time, after setting the height Hg and the height Ht to the above ranges, the height h is limited to the above range, and the ratio of the under tread occupying the land portion is suppressed. In particular, it is possible to maintain good steering stability without lowering the rigidity of the land portion. Further, since the height Hg (that is, the rubber gauge under the groove) is suppressed within the above range, the rolling resistance can also be reduced. In the present invention, the “under-groove region” is a region below the bottom surface of the main groove in the meridian section. Specifically, the region is lowered from the connection point between the wall surface and the bottom surface of the main groove toward the reinforcing layer. It is the area between each perpendicular. When the connecting portion between the wall surface and the bottom surface of the main groove forms a smooth arc, the starting point on the wall surface side of this arc is regarded as the aforementioned connecting point. The ratio of the under tread to the under-groove area is the ratio of the cross-sectional area of the under tread included in the under-groove area to the cross-sectional area of the entire under-groove area in the meridian section. In the present invention, the elastic modulus is a condition of a frequency of 20 Hz, an initial strain of 10%, a dynamic strain of ± 2%, and a temperature of 20 ° C. using a viscoelastic spectrometer (manufactured by Toyo Seiki Seisakusho) in accordance with JIS K6394. The storage elastic modulus (E ′) measured at

  In the present invention, the rubber composition constituting the undertread contains 15% to 80% by weight of natural rubber or polyisoprene rubber and 10% to 60% by weight of polybutadiene rubber in the rubber component, and carbon as a filler. It is preferable that black is contained alone or together with silica and sulfur is contained in an amount of 2.0 to 6.0 parts by weight with respect to 100 parts by weight of the rubber component. By limiting the composition of the rubber composition constituting the undertread in this way, the heat generation and fatigue properties of the undertread are improved, rolling resistance is reduced, groove cracks are prevented, and steering stability is maintained. It is advantageous to achieve both in a balanced manner.

  In the present invention, it is preferable that the undertread is exposed on at least a part of the bottom surface of the inner surface of the main groove. Thus, it becomes possible to prevent a groove crack more effectively by exposing an under tread.

  In the present invention, the center position in the width direction of the land portion for measuring the “height h” is the tire width between the end portions on the tread surface of each land portion in the case of a land portion partitioned between a pair of main grooves. The distance from the end of the land portion to the center position in the width direction is ½ of the width W of each land portion. On the other hand, in the case of a land portion (a so-called shoulder land portion) partitioned on the outer side in the tire width direction of the outermost main groove in the tire width direction, between the end portion on the main groove side on the tread surface of the land portion and the ground contact end The distance from the end on the main groove side to the center position in the width direction or the distance from the ground contact end to the center position in the width direction is ½ of the width W of the land portion. The ground contact end is an end portion in the tire axial direction when a normal load is applied by placing the tire on a normal rim and filling the normal internal pressure in a vertical state on a plane. The “regular rim” is a rim determined for each tire in the standard system including the standard on which the tire is based, for example, a standard rim for JATMA, “Design Rim” for TRA, or ETRTO. Then, “Measuring Rim” is set. “Regular internal pressure” is the air pressure that each standard defines for each tire in the standard system including the standard on which the tire is based. The maximum air pressure is JATMA, and the table “TIRE ROAD LIMITS AT VARIOUS” is TRA. The maximum value described in “COLD INFRATION PRESURES”, “INFLATION PRESURE” for ETRTO, but 180 kPa when the tire is a passenger car. “Regular load” is a load determined by each standard for each tire in the standard system including the standard on which the tire is based. The maximum load capacity is JATMA, and the table “TIRE ROAD LIMITS AT VARIOUS” is TRA. In the case of ETRTO, the maximum value described in “COLD INFORATION PRESSURES” is “LOAD CAPACITY”.

1 is a meridian cross-sectional view of a pneumatic tire according to an embodiment of the present invention. It is meridian sectional drawing which expands and shows the tread part of FIG. It is meridian sectional drawing which expands and shows the tread part of the pneumatic tire which consists of another embodiment of this invention. It is meridian sectional drawing which expands and shows the tread part of the pneumatic tire which consists of another embodiment of this invention.

  Hereinafter, the configuration of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 1, the pneumatic tire of the present invention includes a tread portion 1 that extends in the tire circumferential direction and has an annular shape, a pair of sidewall portions 2 that are disposed on both sides of the tread portion 1, And a pair of bead portions 3 disposed inside the wall portion 2 in the tire radial direction. In FIG. 1, CL indicates the tire equator.

  A carcass layer 4 is mounted between the pair of left and right bead portions 3. The carcass layer 4 includes a plurality of reinforcing cords extending in the tire radial direction, and is folded back around the bead core 5 disposed in each bead portion 3 from the vehicle inner side to the outer side. A bead filler 6 is disposed on the outer periphery of the bead core 5, and the bead filler 6 is wrapped by the main body portion and the folded portion of the carcass layer 4.

  A belt layer 7 a (two belt layers 7 a in the example of FIG. 1) is embedded on the outer peripheral side of the carcass layer 4 in the tread portion 1. Each belt layer 7a 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 7a, the inclination angle of the reinforcing cord with respect to the tire circumferential direction is set, for example, in a range of 10 ° to 40 °. Further, a belt reinforcing layer 7b (in the example of FIG. 1, two belt reinforcing layers 7b) is provided on the outer peripheral side of the belt layer 7a. The belt reinforcing layer 7b includes an organic fiber cord oriented in the tire circumferential direction. In the belt reinforcing layer 7b, the organic fiber cord has an angle of, for example, 0 ° to 5 ° with respect to the tire circumferential direction. In the present invention, the belt layer 7a and the belt reinforcing layer 7b are collectively referred to as “reinforcing layer 7”. In the present invention, the belt layer 7a is an indispensable element, but the belt reinforcement layer 7b is an optional element. Therefore, in addition to the case where the belt layer 7a and the belt reinforcement layer 7b are provided as shown, the reinforcement layer 7a is provided. As an alternative, only the belt layer 7a may be provided.

  A main groove 8 (four main grooves 8 in FIG. 1) extending in the tire circumferential direction is provided on the outer surface of the tread portion 1, and the main grooves 8 form land portions 9 (five rows of land portions 9 in FIG. 1). ) Is marked. The land portion 9 may be either a rib continuously extending over the entire circumference of the tire or a block defined by lug grooves (not shown) extending in the tire width direction. At this time, the height Hg from the outer peripheral surface of the reinforcing layer 7 to the bottom surface of the main groove 8 (hereinafter also referred to as a sub-groove gauge Hg) is set to 5 mm or less. Further, the height Ht from the outer peripheral surface of the reinforcing layer 7 to the surface (tread surface) of the land portion 9 is set to 30 mm or less. The heights Hg and Ht are heights based on the outer peripheral surface of the belt layer 7a when only the belt layer 7a is provided as the reinforcing layer 7, and the belt layer 7a and the belt reinforcing layer 7b are provided. In this case, the outer peripheral surface of the belt reinforcing layer 7b is the reference and lower height. The outer peripheral surface of the reinforcing layer 7 (the belt layer 7a and the belt reinforcing layer 7b) is covered with a reinforcing cord (a belt cord in the case of the belt layer 7a and an organic fiber cord in the case of the belt reinforcing layer 7b) constituting the reinforcing layer 7. This is the outer surface of the coated rubber in the tire radial direction.

  A tread rubber layer 10 is disposed on the outer peripheral side of the carcass layer 4 in the tread portion 1, and a side rubber layer 20 is disposed on the outer peripheral side (outer side in the tire width direction) of the carcass layer 4 in the sidewall portion 2. A rim cushion rubber layer 30 is disposed on the outer peripheral side (outer side in the tire width direction) of the carcass layer 4. The tread rubber layer 10 has a structure in which two types of rubber layers (cap tread 11 and under tread 12) having different physical properties are laminated in the tire radial direction.

  The present invention mainly defines the structure and physical properties of the undertread rubber layer 12 as described later in such a general pneumatic tire. Therefore, the basic cross-sectional structure of the pneumatic tire excluding the under tread 12 (and the cap tread 11 whose structure can be changed according to the under tread 12) is not limited to the above-described structure.

  In the present invention, as shown in FIG. 2, the ratio of the under tread 12 in the lower groove region of the main groove 8 in the meridian cross section is 95% or more, and most of the lower groove region is composed of the under tread 12. In contrast, the height h from the outer peripheral surface of the reinforcing layer 7 to the boundary between the cap tread 11 and the under tread 12 at the center position in the width direction of the land portion 9 is 0.5 mm or more, and the height h Satisfies the relationship of h <Hg, preferably h ≦ 0.95Hg, with respect to the height Hg (under-groove gauge Hg). The under tread 12 is composed of a rubber composition different from the cap tread 11 as described above. However, the rubber composition constituting the under tread 12 always includes a butadiene structure in the rubber component, and the rubber component 100 It always contains 20 to 80 parts by weight of filler with respect to parts by weight.

  The undertread 12 made of the rubber composition having the above-mentioned composition is more flexible than the cap tread 11 and has a characteristic that can flexibly follow strain. Therefore, the undertread 12 occupies most of the region below the groove as described above. By configuring the tread portion 1 as described above, the occurrence of groove cracks can be suppressed. At this time, for the lower portion of the land portion 9, the height h is limited to the above range to suppress the proportion of the undertread 11, so that the rigidity of the entire tread portion 1 (particularly the rigidity of the land portion 9) is reduced. The steering stability can be maintained satisfactorily. Moreover, since the height Hg and the height Ht are set in the above range as the structure of the entire tread portion 1, rolling resistance can also be reduced.

  If the ratio of the under tread 12 occupying the region below the main groove 8 in the meridian cross section is less than 95%, the effect of making the bottom surface of the main groove 8 flexible by the under tread 12 cannot be obtained sufficiently, and groove cracks are generated. The effect which suppresses is not acquired. Ideally, it is preferable that the ratio of the undertread 12 in the region below the main groove 8 is close to 100%, and the undertread 12 is exposed on the bottom surface of the main groove 8. As described above, the cap tread 11 may exist on the bottom surface of the main groove 8. Thus, even if the cap tread 11 is exposed at the groove bottom of the main groove 8, if the under tread 12 satisfies the above-described ratio (that is, the ratio of the cap tread 11 exposed at the groove bottom in the groove lower region). If it is less than 5%), the effect of the present invention that suppresses the occurrence of groove cracks can be sufficiently exhibited. In FIG. 3, the cap tread 11 is exposed on the entire surface of the bottom of the main groove 8. In this manner, the cap tread 11 is present on the groove bottom side of the undertread 12 in the region below the groove. However, if the under tread 12 is exposed at a part of the groove bottom, an effective groove crack suppressing effect can be obtained by the under tread 12 exposed at a part of the groove bottom. That is, the under tread 12 occupying 95% or more of the region below the groove can suppress the groove crack even if it is not exposed to the groove bottom, but in order to enhance the effect of suppressing the groove crack, It is preferable that at least a part of the bottom surface of the inner surface is exposed, more preferably, there is more undertread 12 exposed on the inner surface of the main groove 8, and most preferably the undertread is exposed on the entire bottom surface of the main groove 8. 12 may be exposed.

  When the cap tread 11 covers the entire bottom surface of the main groove 8 as shown in FIG. 3, the thickness of the undertread 12 below the bottom surface of the main groove 8 (the reinforcing layer at the position where the height Hg is measured). 7) (height from the outer peripheral surface of the cap 7 to the boundary between the cap tread 11 and the under tread 12), h ′, and the height h and the height h ′ preferably satisfy the relationship h <h ′. Preferably, the relationship of h ≦ 0.95h ′ is satisfied. When the ratio of the under tread 12 in the region below the main groove 8 is 100%, the height h ′ coincides with the height Hg, so the relationship between the height h and the height Hg (h <Hg, preferably h ≦ 0.95 Hg).

  When the height Ht from the outer peripheral surface of the reinforcing layer 7 to the surface of the land portion 9 exceeds 30 mm, or the height Hg from the outer peripheral surface of the reinforcing layer 7 to the bottom surface of the main groove 8 exceeds 5 mm, the entire tread portion 1 Therefore, in addition to the deterioration of steering stability, the rolling resistance cannot be reduced sufficiently. When the height h from the outer peripheral surface of the reinforcing layer 7 at the center of the land portion 9 in the width direction to the boundary between the cap tread 11 and the under tread 12 is smaller than 0.5 mm, the under tread 12 is formed below the land portion 9. Since it does not exist substantially, the original performance of the tire is impaired, and the heat build-up is remarkably deteriorated. If the height h is equal to or higher than Hg, the amount of the undertread 12 increases below the land portion 9, so that it becomes difficult to maintain the rigidity of the entire tread portion 1 (rigidity of the land portion 9), and steering stability. Decreases. In the present invention, the rigidity of the land portion 9 can be sufficiently maintained if the height h at the center position in the width direction of the land portion 9 is smaller than the height Hg. That is, for example, as shown in FIG. 4, the under tread 12 is arranged so as to cover the bottom surface and the wall surface of the main groove 8, while the thickness of the under tread decreases toward the center of the land portion 9 in the width direction. In the meridian section, the boundary between the cap tread 11 and the under tread 12 is substantially V-shaped, and the height h satisfies the relationship of h <Hg only in the vicinity of the center position in the width direction of the land portion 9. There may be.

  As described above, the cap tread 11 and the under tread 12 are composed of rubber compositions having different physical properties. Specifically, the elastic modulus Eu of the rubber composition constituting the under tread 12 is 12 MPa or less, Moreover, it is preferable that the elastic modulus Ec of the rubber composition constituting the cap tread 11 satisfies the relationship of Ec / Eu ≦ 5.0 with respect to the elastic modulus Eu of the rubber composition constituting the undertread 12. By setting the elastic modulus Eu in this way, the undertread 12 can be made sufficiently flexible, which is advantageous for suppressing the occurrence of groove cracks. In addition, since the elastic modulus Eu and the elastic modulus Ec satisfy the above relationship, the balance between the physical properties of the two becomes good, and both reduction in rolling resistance, prevention of groove cracks, and maintenance of steering stability are balanced. To be advantageous. The elastic modulus Ec of the rubber composition constituting the cap tread 11 is not particularly limited as long as the above relationship is satisfied, but can be set to 4 MPa to 20 MPa, for example.

  In the rubber composition constituting the undertread 12, the rubber component is a diene rubber and necessarily includes a butadiene structure as described above. As such a diene rubber, rubbers usually used in tire rubber compositions such as natural rubber, butadiene rubber, styrene butadiene rubber, isoprene rubber, modified butadiene rubber having a modified molecular end, modified styrene butadiene rubber, etc. are used. can do. In particular, the rubber component may contain 15% to 80% by weight of natural rubber or polyisoprene rubber and 10% to 60% by weight of polybutadiene rubber. The butadiene rubber may be modified with a modifying agent, and the catalyst used during the polymerization is not limited, and a butadiene rubber produced by a catalyst such as nickel, titanium, chromium, neodymium, or lithium may be used. . These diene rubbers can be used alone or as any blend. By using such a rubber component, the fatigue and heat generation properties of the under-groove rubber layer 40 are improved, and a reduction in rolling resistance, prevention of groove cracks, and maintenance of steering stability are balanced. Will be advantageous.

  The rubber composition constituting the undertread necessarily includes 20 to 80 parts by weight of the filler with respect to 100 parts by weight of the rubber component described above, but the amount of filler is 20 parts by weight with respect to 100 parts by weight of the rubber component. When the amount is less than the weight part, the hardness of the rubber cannot be maintained and the steering stability is deteriorated. When the blending amount of the filler is more than 80 parts by weight with respect to 100 parts by weight of the rubber component, the rolling resistance cannot be sufficiently reduced. As the filler, carbon black may be contained alone or together with silica.

  The rubber composition constituting the undertread 12 preferably contains 2.0 parts by weight to 6.0 parts by weight of sulfur with respect to 100 parts by weight of the rubber component. By blending a predetermined amount of sulfur in this manner, the toughness of the rubber composition constituting the undertread 12 is improved, which is advantageous for preventing groove cracks, and having hardness and exothermicity that affect steering stability. Good balance can be achieved. When the amount of sulfur is less than 2.0 parts by weight with respect to 100 parts by weight of the rubber component, the crosslinking density is lowered, the elastic modulus is remarkably lowered, and the steering stability is deteriorated. If the amount of sulfur is more than 6.0 parts by weight with respect to 100 parts by weight of the rubber component, the vulcanization density increases, the elastic modulus becomes excessive, and the effect of preventing groove cracks cannot be sufficiently obtained.

  To the rubber composition constituting the undertread 12, other compounding agents other than those described above can be added. Examples of other compounding agents include various compounding agents generally used for pneumatic tires such as a vulcanization accelerator, a liquid polymer, a thermosetting resin, and a thermoplastic resin. The compounding amounts of these compounding agents can be conventional conventional compounding amounts as long as they do not contradict the purpose of the present invention.

  In the present invention, the physical properties of the undertread 12 that directly contribute to the suppression of groove cracks are important, and the composition of the undertread 12 is limited as described above. The blending of the composition is not particularly limited. For example, as a rubber component of the rubber composition constituting the cap tread 11, rubber for tires such as natural rubber, butadiene rubber, styrene butadiene rubber, isoprene rubber, modified butadiene rubber having a modified molecular terminal, modified styrene butadiene rubber or the like is used. A rubber usually used in the composition can be used. As the filler, carbon black usually used in a rubber composition for tires, and as silica, those produced by a wet method or a dry method such as precipitated silica are used. Commercially available products include Ultrasil 7000GR (manufactured by Evonik), Ultrasil 9100GR (manufactured by Evonik), Zeosil 1165MP (manufactured by Solvay), Zeosil Premium 200MP (manufactured by Solvay), and Zeosil. 15GR (manufactured by Solvay Co., Ltd.), Ultrasil 5000GR (manufactured by Evonik Co., Ltd.), or Ultrasil VN3GR (manufactured by Evonik Co., Ltd.) and the like can be used.

  First, eight types of tire rubber compositions (rubbers 1 to 8) having the composition shown in Table 1 were prepared. In preparing these rubber compositions, the components excluding sulfur and the vulcanization accelerator were weighed and kneaded in a 1.8 L closed mixer at 160 ° C. for 5 minutes, and then the master batch was discharged. Each rubber composition was obtained by kneading this master batch with an open roll, adding sulfur and a vulcanization accelerator, and mixing them. In Table 1, in accordance with JIS K6394, a viscoelastic spectrometer (manufactured by Toyo Seiki Seisakusho) was used, and the measurement was performed under the conditions of frequency 20 Hz, initial strain 10%, dynamic strain ± 2%, and temperature 20 ° C. The elastic modulus of each rubber composition is also shown.

The types of raw materials used in Table 1 are shown below.
SBR: styrene butadiene rubber, Nipol 1502 manufactured by Nippon Zeon
-BR: Butadiene rubber, Nippon Zeon Corporation Nipol BR 1220
・ NR: Natural rubber, STR20
-CB: THAIBLACK N550 made by THAI CARBON BLACK PUBLICK
-Silica: Ultrasil VN3GR manufactured by Evonik
・ Oil: Showa Shell Sekiyu Extract 4 S
Coupling agent: Si69 manufactured by Evonik
-Stearic acid: manufactured by NOF Co., Ltd. Beads stearic acid- Zinc oxide: Zinc oxide, 3 types manufactured by Shodo Chemical Industry Co., Ltd.-Anti-aging agent: PILFLEX 13 manufactured by NICOL LIMITED
・ Sulfur: Tsurumi Chemical Industry Co., Ltd. Jinhua Oil-filled fine powder sulfur ・ Vulcanization accelerator: FLEXSYS SANTOCURE CBS

  Next, the tire rubber composition (rubbers 1 to 8) described above is used for an under tread, the tire size is 245 / 50R18, the basic structure shown in FIG. The ratio Ec / Eu between the elastic modulus Ec of the rubber composition constituting the cap tread and the elastic modulus Eu of the rubber composition constituting the under tread, the height Hg from the outer peripheral surface of the reinforcing layer at the bottom surface of the main groove, The height Ht from the outer peripheral surface of the reinforcing layer on the surface of the land portion, the height h from the outer peripheral surface of the reinforcing layer at the boundary between the cap tread and the under tread at the center position in the width direction of the land portion, the under occupying in the region below the groove Seventeen types of pneumatic tires of Conventional Example 1, Comparative Examples 1 to 10, and Examples 1 to 6 in which the tread ratio was set as shown in Table 2 were produced.

  In addition, in the column of “type of under tread” in Tables 2 and 3, the elastic modulus (described in Table 1) of each rubber composition is also shown. As the cap tread, a rubber composition generally used for cap treads in pneumatic tires was used, and its elastic modulus Ec was 8.0 MPa.

  About these 17 types of pneumatic tires, the groove crack resistance, steering stability, and heat generation were evaluated by the following evaluation methods, and the results are shown in Tables 2 and 3.

Groove crack resistance Each test tire was assembled on a wheel with a rim size of 18 × 8 JJ, filled with air pressure of 230 kPa, left in a container irradiated with 100 ppmh of ozone at a temperature of 50 ° C., and then left on the bottom surface of the main groove. The crack generation state was visually confirmed, and the result of Conventional Example 1 was evaluated in five stages with 2 points (reference).

Steering stability Each test tire is mounted on a wheel with a rim size of 18 x 8 JJ, mounted on a 2.5 L class test vehicle with an air pressure of 230 kPa, and a sensory evaluation is performed by a test driver on a circuit course consisting of a dry road surface. It was. The evaluation results are shown as an index with Comparative Example 1 as 100. It means that it is excellent in steering stability, so that this index value is large. When the index value is “97” or more, it means that the conventional level is maintained and sufficiently good steering stability is maintained.

Exothermic property The rubber composition used for the under-groove rubber layer in each test tire was press vulcanized at 160 ° C. for 30 minutes in a predetermined mold to produce a test piece. , Using a viscoelastic spectrometer (manufactured by Toyo Seiki Seisakusho Co., Ltd.) under the conditions of initial strain 10%, amplitude ± 2%, frequency 20 Hz, and ambient temperature 60 ° C. The evaluation results are shown as an index with the reciprocal of the measured value of Comparative Example 1 as 100. The larger the index value, the smaller the tan δ at 60 ° C., and the lower the exothermic property. If the index value is “95” or more, it means that the conventional level is maintained and a sufficiently good exothermic property is maintained.

  As is clear from Tables 2 and 3, all of Examples 1 to 6 improved the groove crack resistance while maintaining or reducing steering stability and heat generation compared to Conventional Example 1.

On the other hand, Comparative Example 1 has a high height Ht, and Comparative Example 2 has a high proportion of undertread but a high height Hg, which is excellent in groove crack resistance but deteriorated in handling stability and heat generation. . In Comparative Example 3, the effect of improving the groove crack resistance was not obtained because the ratio of the undertread that can be placed in the region below the groove was small. In Comparative Example 4, since the height h was too small, the under-tread substantially disappeared at the center of the land where the rubber greatly deformed and generated heat during running, and the heat generation deteriorated. In Comparative Example 5, since the height h was excessive, the rigidity of the land portion was lowered and the steering stability was deteriorated. In Comparative Example 6, since a rubber composition (rubber 4) with an excessive amount of filler was used as an under tread, the exothermic property deteriorated. In Comparative Example 7, since a rubber composition (rubber 5) with an excessive amount of filler as an undertread was used, the balance between elastic modulus and heat generation was deteriorated, and steering stability and heat generation were deteriorated. In Comparative Example 8, since the elastic modulus Eu of the undertread was excessive, the groove crack resistance was deteriorated. In Comparative Example 9, since the ratio of the elastic modulus Ec of the cap tread and the elastic modulus Eu of the undertread was excessively increased, the steering stability and the heat generation were deteriorated. Since the rubber composition (rubber 8) which does not use the butadiene rubber which contains a butadiene structure as an under tread was used for the comparative example 10, exothermic property deteriorated.

DESCRIPTION OF SYMBOLS 1 Tread part 2 Side wall part 3 Bead part 4 Carcass layer 5 Bead core 6 Bead filler 7 Reinforcement layer 7a Belt layer 7b Belt reinforcement layer 8 Main groove 9 Land part 10 Tread rubber layer 11 Cap tread 12 Under tread 20 Side rubber layer 30 Rim cushion Rubber layer 40 Under-groove rubber layer CL Tire equator

Claims (3)

An annular tread portion extending in the tire circumferential direction, a pair of sidewall portions disposed on both sides of the tread portion, a pair of bead portions disposed on the tire radial direction inside of the sidewall portions, A carcass layer mounted between the pair of bead portions; a reinforcing layer disposed on an outer peripheral side of the carcass layer in the tread portion; and an outer peripheral side of the carcass layer and the reinforcing layer in the tread portion. A pneumatic tire in which a main groove extending in the tire circumferential direction and a land portion defined by the main groove are formed on an outer surface of the tread portion.
The tread rubber layer is composed of a cap tread exposed on the surface of the tread portion, and an under tread disposed on the inner peripheral side of the cap tread, and the under tread occupies a sub-groove region of the main groove in a meridian section. The ratio is 95% or more, the height Ht from the outer peripheral surface of the reinforcing layer to the surface of the land portion is 30 mm or less, and the height Hg from the outer peripheral surface of the reinforcing layer to the bottom surface of the main groove is 5 mm. The height h from the outer peripheral surface of the reinforcing layer at the center position in the width direction of the land portion to the boundary between the cap tread and the under tread is 0.5 mm or more and h <with respect to the height Hg The rubber composition satisfying the relationship of Hg and constituting the undertread includes at least a butadiene structure in the rubber component, and 100 parts by weight of the rubber component The rubber composition constituting the undertread has an elastic modulus Eu of 12 MPa or less, and the elastic modulus Ec of the rubber composition constituting the cap tread is 20 parts by weight to 80 parts by weight. A pneumatic tire characterized by satisfying a relationship of Ec / Eu ≦ 5.0 with respect to the elastic modulus Eu.   The rubber composition constituting the undertread includes 15 wt% to 80 wt% of natural rubber or polyisoprene rubber and 10 wt% to 60 wt% of polybutadiene rubber with respect to 100 parts by weight of diene rubber, and the filler 2. The pneumatic tire according to claim 1, comprising carbon black alone or together with silica, and containing 2.0 parts by weight to 6.0 parts by weight of sulfur with respect to 100 parts by weight of the rubber component.   3. The pneumatic tire according to claim 1, wherein the under tread is exposed on at least a part of a bottom surface of an inner surface of the main groove.

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