JP2018052414A - Pneumatic tire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pneumatic tire that even when groove under gauge is made thinner for reducing rolling resistance can prevent occurrence of crack of a groove and maintain steering stability excellently.SOLUTION: Distances from bottom surfaces of main grooves 8 to an outer peripheral surface of a reinforcement layer 7 are set to 5 mm or less, and groove under rubber layers 40 constituted of rubber compositions different from tread rubber layers 10 are provided so as to cover the bottom surfaces of the main grooves 8. The rubber compositions constituting the groove under rubber layers 40 include diene rubber, fillers and antioxidants. The filler of 90 pts.wt. or less is mixed with the diene rubber of 100 pts.wt. The antioxidant is aromatic amine compounds. Elastic modulus of the rubber compositions constituting the groove under rubber layers 40 is set to 16 MPa or less, and elastic modulus of the rubber compositions constituting the tread rubber layers 10 is set to 5.0 times the elastic modulus of the rubber compositions constituting the groove under rubber layers 40 or less.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 rolling resistance and reduce fuel consumption, the gauge of the rubber layer (under-groove gauge) constituting the lower portion of the main groove extending in the tire circumferential direction in the tread portion has been reduced ( For example, see Patent Document 1). 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 tread rubber layer as a whole requires a tread rubber layer with low heat generation and high hardness in order to improve steering stability in addition to reducing fuel consumption of the tire. It is difficult to stretch, and there is a problem that the fatigue resistance against deformation stress during running is insufficient and the groove crack resistance is inferior. From the above, even when thinning the sub-groove gauge to reduce rolling resistance and reduce fuel consumption, measures to prevent the occurrence of groove cracks are required without deteriorating steering stability. It has been.

JP 2013-039864 A

  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 In the produced pneumatic tire, the distance from the bottom surface of the main groove to the outer peripheral surface of the reinforcing layer is 5 mm or less, and the rubber composition is different from the rubber composition constituting the tread rubber layer A configured under-groove rubber layer is provided so as to cover at least the bottom surface of the main groove, and the under-groove rubber layer passes through end points on the treads of the pair of land portions adjacent to the main groove. It exists in a region between a pair of imaginary lines that are inclined toward the outside of the main groove from a perpendicular line that is lowered from each end point toward the reinforcing layer and an angle formed with the reinforcing layer is 95 °, and The rubber composition constituting the under-groove rubber layer includes a diene rubber, a filler, and an anti-aging agent, and the filler is blended in an amount of 90 parts by weight or less with respect to 100 parts by weight of the diene rubber, and the anti-aging agent Is an aromatic amine compound, the elastic modulus of the rubber composition constituting the under-groove rubber layer is 16 MPa or less, and the elastic modulus of the rubber composition constituting the tread rubber layer constitutes the under-groove rubber layer Less than 5.0 times the elastic modulus of the rubber composition It is characterized by being.

  In the pneumatic tire of the present invention, the distance from the bottom surface of the main groove to the outer peripheral surface of the reinforcing layer is reduced to 5 mm or less to reduce rolling resistance, while having the above characteristics, being flexible and resistant to strain. In addition, since the under-groove rubber layer made of a rubber composition that can be flexibly handled is provided, the occurrence of groove cracks can be suppressed. At this time, since the under-groove rubber layer is disposed in a limited manner in the above-described region, the rigidity of the entire tread portion (particularly, the rigidity of the land portion) is not reduced, and the steering stability is maintained well. Can do. 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 diene rubber is preferably a diene rubber containing at least a butadiene structure. In particular, the diene rubber preferably contains at least polybutadiene. By limiting the rubber component of the rubber composition constituting the under-groove rubber layer to one having excellent fatigue and heat generation, the anti-groove rubber layer has good fatigue resistance and reduced rolling resistance. It is advantageous to achieve both the prevention of the occurrence of groove cracks and the maintenance of steering stability in a balanced manner.

In the present invention, it is preferable to include carbon black having a CTAB adsorption specific surface area of 130 m ² / g or less as a filler. By limiting the types of fillers in this way, the balance between exothermic properties and elastic modulus of the rubber layer under the groove is improved, and both the prevention of groove cracks and the maintenance of exothermic properties and steering stability are balanced. To be advantageous. In the present invention, the CTAB adsorption specific surface area of the filler is measured according to JIS K6217-3.

  In the present invention, the elastic modulus of the rubber composition constituting the sub-groove rubber layer is preferably 1.0 MPa to 15.0 MPa. Use of the under-groove rubber layer having such physical properties is advantageous in preventing the occurrence of groove cracks without impairing the steering stability.

  In this invention, it is preferable that the compounding quantity of an anti-aging agent is 1 to 5 weight part with respect to 100 weight part of diene rubbers. In this way, by limiting the blending amount of the anti-aging agent, it is advantageous to improve fatigueability without impairing the elastic modulus of the under-groove rubber layer, and to balance the prevention of the occurrence of groove cracks and the steering stability in a balanced manner. Become.

  In the present invention, the tread rubber layer is composed of a cap tread rubber layer exposed on the surface of the tread portion and an under tread rubber layer laminated on the inner peripheral side, and the rubber composition constituting the cap tread rubber layer and the under tread rubber layer are formed. The rubber composition constituting the tread rubber layer is preferably different from the rubber composition constituting the under-groove rubber layer. As a result, the rubber layer under the groove is a rubber composition that emphasizes the anti-groove crack resistance without deteriorating the steering stability by optimizing the elastic modulus ratio with the cap tread rubber layer, and the cap tread layer emphasizes the steering stability. The rubber composition, the under and red rubber layers can be made into a rubber composition in which heat generation is emphasized, and the balance of the entire tire can be optimized.

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. 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. 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 distance G from the bottom surface of the main groove 8 to the outer peripheral surface of the reinforcing layer 7 (hereinafter referred to as a sub-groove gauge G) is set to 5 mm or less. The sub-groove gauge G is a distance from the bottom surface of the main groove 8 to 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. Is provided, the distance from the bottom surface of the main groove 8 to the outer peripheral surface of the belt reinforcing layer 7b. Further, the outer peripheral surface of the reinforcing layer 7 (the belt layer 7a and the belt reinforcing layer 7b) is a reinforcing cord constituting the reinforcing layer 7 (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). This is a surface on the outer side in the tire radial direction of the coated rubber covering the tire.

  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 may have a structure in which two types of rubber layers (cap tread rubber layer 11 and under tread rubber layer 12) having different physical properties are laminated in the tire radial direction.

  In this general pneumatic tire, the present invention is such that an under-groove rubber layer 40 is provided under the groove of the main groove 8 as described later. Therefore, the basic cross-sectional structure of the pneumatic tire excluding the sub-groove rubber layer 40 is not limited to the above-described structure. Therefore, for example, the under-groove rubber layer 40 of the present invention can also be applied to a so-called run flat tire in which a side reinforcing layer having a crescent cross section is provided on the inner side in the tire width direction of the carcass layer 4 in the sidewall portion 2. it can.

  In the present invention, as shown in FIG. 2, a sub-groove rubber layer 40 is provided below the main groove 8. That is, the tread portion 1 is composed of the tread rubber layer 10 that occupies most of the tread portion 1 and the tread rubber layer 10 locally provided below the main groove 8. The under-groove rubber layer 40 is provided so as to cover at least the bottom surface of the main groove 8. 3, that is, perpendicular lines passing through the end points P on the treads of the pair of land portions 9 adjacent to one main groove 8 and descending from each end point P toward the reinforcing layer 7. It exists in a region between a pair of imaginary lines L that is inclined toward the outer side of the main groove 8 than (dotted line in the figure) and the angle θ formed with the reinforcing layer 7 is 95 °. In the illustrated example, the under-groove rubber layer 40 extends from the bottom surface of the main groove 8 to the reinforcing layer 7 and divides the tread rubber layer 10 in the tire width direction.

  The under-groove rubber layer 40 is composed of a rubber composition different from the rubber composition that constitutes the tread rubber layer 10. Specifically, it contains a diene rubber, a filler, and an antioxidant, and the filler is blended in an amount of 90 parts by weight or less based on 100 parts by weight of the diene rubber, and the antioxidant is an aromatic secondary amine. It is comprised with the rubber composition which is aromatic amine compounds, such as. Further, the rubber composition constituting the sub-groove rubber layer 40 has an elastic modulus E1 of 16 MPa or less. When the elastic modulus of the rubber composition constituting the sub-groove rubber layer 40 is E1, and the elastic modulus of the rubber composition constituting the tread rubber layer 10 is E2, the ratio E2 / E1 of these elastic moduli is 5. It is set to 0 or less.

  The under-groove rubber layer 40 made of the rubber composition having such characteristics is softer than the tread rubber layer 10 and can flexibly follow the strain. Generation of cracks can be suppressed. In particular, even if the sub-groove gauge G is set to be 5 mm or less in order to reduce rolling resistance and the groove cracks tend to occur, the sub-groove rubber layer 40 is provided as described above. Groove cracks can be prevented. At this time, since the under-groove rubber layer 40 is provided only in the above-described region, the rigidity of the tread portion 1 as a whole (particularly the rigidity of the land portion 9) is not reduced, and the steering stability is good. Can be maintained.

  If the under-groove rubber layer 40 is provided outside the above range, the proportion of the under-groove rubber layer 40 in the tread portion 1 increases, the rigidity of the tread portion 1 decreases, and steering stability is improved. It becomes difficult to maintain. The under-groove rubber layer 40 does not have to be provided in the entire region (the shaded portion in FIG. 3) and exists at least below the flat portion of the bottom surface of the main groove 8 as shown in FIG. Just do it. In the illustrated example, the under-groove rubber layer 40 divides the tread rubber layer 10, but the under-groove rubber layer 40 does not reach the reinforcing layer 7 as shown in FIG. It may be continuous in the tire width direction on the 7 side. When the tread rubber layer 10 is composed of the cap tread rubber layer 11 and the under tread rubber layer 12 as described above, the under-groove rubber layer 40 reaches the reinforcing layer 7 as shown in FIG. Both the cap tread rubber layer 11 and the under tread rubber layer 12 may be divided. As shown in FIG. 6, the cap tread rubber layer 11 only reaches the under tread rubber layer 12 without reaching the reinforcing layer 7 as shown in FIG. May be divided. Alternatively, both the cap tread rubber layer 11 and the under tread rubber layer 12 remain between the under-groove rubber layer 40 and the reinforcing layer 7 in a state where the under-groove rubber layer 40 does not reach the reinforcing layer 7 as shown in FIG. You may do it.

  When the under-groove rubber layer 40 is provided on the entire hatched portion in FIG. 3, the virtual line L coincides with the boundary between the under-groove rubber layer 40 and the tread rubber layer 10. Therefore, the above-described condition is that the boundary between the sub-groove rubber layer 40 and the tread rubber layer 10 passes through the end point P on the tread surface of the land portion 9, and the angle θ on the tread rubber layer 10 side formed by this boundary and the reinforcing layer 7. In other words is 95 ° or less. In this way, when the angle θ at the boundary between the groove-under-rubber layer 40 and the tread rubber layer 10 exceeds 95 °, the groove-under-rubber layer 40 expands outside the above-described range (shaded portion in FIG. 3). Therefore, it becomes difficult to maintain good steering stability as described above.

  If the elastic modulus E1 of the under-groove rubber layer 40 is greater than 16 MPa, the under-groove rubber layer 40 is not sufficiently flexible, and the occurrence of groove cracks cannot be suppressed appropriately. The elastic modulus E1 of the under-groove rubber layer 40 is preferably 1.0 MPa to 15.0 MPa. If the elastic modulus ratio E1 / E2 between the under-groove rubber layer 40 and the tread rubber layer 10 is greater than 5.0, the balance of physical properties between the under-groove rubber layer 40 and the tread rubber layer 10 is poor, and groove cracking is prevented. It becomes difficult to maintain a good balance between maintaining steering stability. The elastic modulus ratio E1 / E2 of the sub-groove rubber layer 40 and the tread rubber layer 10 is preferably 4.5 to 1.5.

  As a rubber composition which comprises the tread rubber layer 10, the rubber composition (elastic modulus E2 is 4 MPa-20 MPa) normally used as the tread rubber layer 10 in a general pneumatic tire can be used. As described above, the tread rubber layer 10 may be composed of a cap tread rubber layer and an under tread rubber layer. In this case, the under-groove rubber layer 40 is composed of the cap tread rubber layer and the under tread rubber layer. It is comprised with the rubber composition different from both. When the elastic modulus of the rubber composition constituting the cap tread rubber layer is E2c and the elastic modulus of the rubber composition constituting the undertread rubber layer is E2u, each of E2c / E1 and E2u / E1 is 5.0 or less. .

  In the rubber composition constituting the sub-groove rubber layer 40, the rubber component includes a diene rubber as described above. As the diene rubber, rubbers usually used in rubber compositions for tires such as natural rubber, butadiene rubber, styrene butadiene rubber, isoprene rubber, modified butadiene rubber having a modified molecular end, modified styrene butadiene rubber, and the like are used. be able to. Among the above-described diene rubbers, those having a butadiene structure (for example, butadiene rubber, styrene butadiene rubber, modified butadiene rubber having a modified molecular end, modified styrene butadiene rubber), and those having a polybutadiene structure (butadiene) Rubber) can be preferably used. The butadiene rubber may be modified with a modifier, and the catalyst used at the time of polymerization is not limited, and butadiene rubber produced by a catalyst such as nickel, titanium, chromium, neodymium, lithium or the like 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 under-groove rubber layer 40 necessarily includes a filler as described above, but when the blending amount of the filler exceeds 90 parts by weight with respect to 100 parts by weight of the diene rubber, the under-groove rubber layer When 40 is not obtained, sufficient flexibility cannot be obtained, and not only the effect of suppressing the occurrence of groove cracks cannot be obtained sufficiently, but also the heat generation is greatly deteriorated. The blending amount of the filler is preferably 85 to 40 parts by weight with respect to 100 parts by weight of the diene rubber. As the filler, carbon black usually used in a rubber composition for tires, and as silica, those manufactured by a wet method or a dry method such as precipitated silica can be used. As a commercial product, Ultrasil 7000GR ( Evonik), Ultrasil 9100GR (Evonik), Zeosil 1165MP (Solvay), Zeosil Premium 200MP (Solvay), Zeosil 115GR (Solvay), UltraSil 5G Evonik) can be used. In particular, the present invention, CTAB adsorption specific surface area is preferably not more than 130m ² / g as a filler, may more preferably comprise carbon black as a 129m ² / g~30m ² / g. By including such carbon black, the physical properties of the under-groove rubber layer 40 are improved, and it is advantageous to achieve a good balance between reduction in rolling resistance, prevention of groove cracks and maintenance of steering stability. .

  The rubber composition constituting the sub-groove rubber layer 40 always contains an aromatic amine compound such as an aromatic secondary amine as an anti-aging agent as described above, and more preferably N- (1,3-dimethylbutyl). -N'-phenyl-p-phenylenediamine is preferred. The blending amount of the antioxidant is preferably 1 to 5 parts by weight, for example, with respect to 100 parts by weight of the diene rubber. By blending the anti-aging agent in this way, the anti-groove rubber layer 40 has good anti-aging properties, and it is advantageous for achieving both a balance between prevention of groove cracking and maintenance of steering stability.

  A compounding agent other than the above can be added to the rubber composition constituting the under-groove rubber layer 40. As other compounding agents, fillers other than the above-mentioned carbon black, vulcanization or crosslinking agents, vulcanization accelerators, liquid polymers, thermosetting resins, thermoplastic resins, etc. are generally used for pneumatic tires. Various compounding agents can be exemplified. 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.

  First, ten types of tire rubber compositions (rubbers 1 to 10) 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
CB1: Carbon black, VULCAN 10H (CTAB adsorption specific surface area: 129 m ² / g) manufactured by CABOT
CB2: carbon black, VULCAN M (CTAB adsorption specific surface area 95 m ² / g) manufactured by CABOT
-Silica: Ultrasil VN3GR manufactured by Evonik
Coupling agent: Si69 manufactured by Evonik
・ Oil: Showa Shell Sekiyu Extract 4 S
・ Stearic acid: manufactured by NOF Co., Ltd. Beads stearic acid ・ Zinc flower: Zinc oxide 3 types manufactured by Shodo Chemical 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, using the tire rubber composition (rubbers 1 to 10) described above, the tire size is 245 / 50R18, the basic structure shown in FIG. 1, and the type of rubber composition constituting the under-groove rubber layer (Type of rubber under the groove), type of rubber composition constituting the tread rubber layer (type of tread rubber), elastic modulus E1 of the rubber composition constituting the rubber layer under the groove and rubber composition constituting the tread rubber layer Comparative Example 1-7, Example 1-5 12 in which the ratio E2 / E1 of the elastic modulus E2 of the rubber, the groove depth gauge, and the angle θ of the boundary between the grooved rubber layer and the tread rubber layer are set as shown in Table 2. Various types of pneumatic tires were made.

  In the column of “type of rubber under groove” in Table 2, the elastic modulus of the rubber composition constituting the rubber groove under groove is also shown. Comparative Example 1 and Comparative Example 2 are examples in which the entire tread portion was composed of a single rubber composition, but in Table 1, the under-groove rubber layer and the tread rubber layer were composed of the same rubber composition. Numerical values and the like are shown, but the angle θ is left blank because the boundary between the sub-groove rubber layer and the tread rubber layer cannot be determined.

  About these 12 types of pneumatic tires, the groove crack resistance, steering stability, and heat generation were evaluated by the following evaluation methods, and the results are also shown in Table 2.

Groove crack resistance Each test tire was assembled on a wheel with a rim size of 18 × 8 JJ, filled with air pressure of 50 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 Comparative Example 1 was evaluated in five stages with 2 points (reference).

Steering stability Each test tire (245 / 50R18) is assembled to a rim 18 × 8JJ wheel, mounted on a 2.5L class test vehicle with an air pressure of 230kPa, and a test driver on a circuit course consisting of a dry road surface The sensory evaluation was performed. 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.

  As is clear from Table 2, all of Examples 1 to 5 improved the groove crack resistance while maintaining or reducing the steering stability and heat generation compared to Comparative Example 1. On the other hand, Comparative Example 2 was excellent in groove crack resistance due to its large sub-groove gauge, but the steering stability and heat generation were deteriorated. In Comparative Example 3, since the angle θ is large and the ratio of the rubber layer under the groove in the tread portion is large, the steering stability is deteriorated. In Comparative Example 4, since rubber (rubber 3) having a remarkably large elastic modulus was used as the under-groove rubber layer, the groove crack resistance could not be improved, and the heat generation was also deteriorated. In Comparative Example 5, since the elastic modulus ratio E2 / E1 is excessive, the elastic modulus of the under-groove rubber layer is low, and the elastic modulus of the tread rubber layer is high, the steering stability balance deteriorates and the steering stability deteriorates. did. In Comparative Example 6, since the rubber composition (rubber 6) in which the compounding amount of the filler (carbon black) was excessive was used as the under-groove rubber layer, the heat generation was deteriorated. Since the rubber composition (rubber 7) which does not contain an antiaging agent was used for the comparative example 7 as a rubber layer under a groove | channel, the groove crack resistance 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 rubber layer 12 Under tread rubber layer 20 Side rubber Layer 30 Rim cushion rubber layer 40 Under-groove rubber layer CL Tire equator

Claims (7)

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 distance from the bottom surface of the main groove to the outer peripheral surface of the reinforcing layer is 5 mm or less, and the under-groove rubber layer made of a rubber composition different from the rubber composition constituting the tread rubber layer is at least of the main groove. It is provided so as to cover the bottom surface, and the under-groove rubber layer is formed from a perpendicular line that passes through the end points on the treads of the pair of land portions adjacent to the one main groove and descends from each end point toward the reinforcing layer. Is present in a region between a pair of imaginary lines that are inclined toward the outside of the main groove and have an angle of 95 ° with the reinforcing layer, and the rubber composition constituting the sub-groove rubber layer is a diene-based rubber composition. A rubber, a filler, and an anti-aging agent, wherein the filler is blended in an amount of 90 parts by weight or less based on 100 parts by weight of the diene rubber, the anti-aging agent is an aromatic amine compound, and the under-groove rubber layer The elastic modulus of the rubber composition constituting And the MPa or less, a pneumatic tire, wherein the elastic modulus of the rubber composition constituting the tread rubber layer is less than 5.0 times the elastic modulus of the rubber composition constituting the groove bottom rubber layer.   The pneumatic tire according to claim 1, wherein the diene rubber is a diene rubber containing at least a butadiene structure.   The pneumatic tire according to claim 2, wherein the diene rubber contains at least polybutadiene. The pneumatic tire according to claim 1, wherein the filler contains carbon black having a CTAB adsorption specific surface area of 130 m ² / g or less.   The pneumatic tire according to any one of claims 1 to 4, wherein an elastic modulus of the rubber composition constituting the under-groove rubber layer is 1.0 MPa to 15.0 MPa or less.   The pneumatic tire according to any one of claims 1 to 5, wherein the blending amount of the antioxidant is 1 part by weight to 5 parts by weight with respect to 100 parts by weight of the diene rubber.   The tread rubber layer is composed of a cap tread rubber layer exposed on the surface of the tread portion and an under tread rubber layer laminated on the inner peripheral side thereof, and a rubber composition constituting the cap tread rubber layer and the under tread. The pneumatic tire according to any one of claims 1 to 6, wherein a rubber composition constituting the rubber layer is different from a rubber composition constituting the under-groove rubber layer.

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