Heavy Duty Tires

Abstract

To provide a tire for a heavy load which can be improved in durability without being deteriorated in low heat generation performance.SOLUTION: In a pneumatic tire which has five or more belt layers 7 and a tread part 1 constituted of two layers of a cap tread layer 11 and an under tread layer 12, a distance A between a widest belt 7A and a carcass layer 4 which is measured on a perpendicular line P of a carcass layer passing through a grounding end E and a distance B between a groove bottom line L and an inner surface of the tire satisfy a relation of 0.3≤A / B≤0.7, and the under tread layer 12 is constituted of a rubber composition in which 0.5 pts.mass-3.0 pts.mass of a hydrazide compound represented by the formula (1) and 30 pts.mass-60 pts.mass of carbon black having a nitrogen adsorption specific surface area N2SA of 60 m2 / g-120 m2 / g with respect to 100 pts.mass of a rubber constituent containing 70 pts.mass or more of natural rubber are combined.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a heavy duty tire having five or more belt layers embedded on the outer peripheral side of a carcass layer in a tread portion. [Background technology]

[0002] In order to ensure durability, heavy-duty tires used for trucks, buses, construction vehicles, etc., have a plurality of belt layers mainly made of steel wires arranged on the outer periphery of the carcass layer in the tread portion. In particular, heavy-duty tires mounted on large construction vehicles such as large dump trucks operating in quarries and large construction work sites are often used for long periods of time under heavy loads, and are therefore required to have excellent durability. For example, the heavy-duty tire described in Patent Document 1 has five or more steel belt layers to ensure durability against heavy loads.

[0003] On the other hand, heavy-duty tires for large construction vehicles tend to overheat due to the fact that they are used for long periods of time under heavy loads as described above, and so measures to suppress heat generation are required. In addition, suppressing heat generation in heavy-duty tires is expected to prevent bursts caused by heat generation, so reducing heat generation is also important from the perspective of durability. Furthermore, in recent years, tires have become larger in size to improve transportation efficiency, so reducing heat generation in heavy-duty tires has become a very important issue. However, in tires with five or more steel belt layers as described above, the large number of belt layers makes it difficult to suppress heat generation, and there is a problem in that it is difficult to achieve both durability and low heat generation. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Unexamined Patent Publication No. 7-117408 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a heavy-duty tire which is capable of improving durability without impairing low heat generation properties. [Means for solving the problem]

[0006] The heavy-duty tire of the present invention that achieves the above object comprises a tread portion extending in a circumferential direction of the tire to form an annular shape, a pair of sidewall portions disposed on both sides of the tread portion, and a pair of bead portions disposed radially inward of the pair of sidewall portions, the tire having a carcass layer mounted between the pair of bead portions, and five or more belt layers disposed on the outer peripheral side of the carcass layer in the tread portion, the tread portion being composed of two layers, a cap tread layer that constitutes the contact surface of the tread portion, and an under tread layer disposed radially inward of the cap tread layer, and a plurality of grooves are formed on the outer surface of the tread portion, the tire comprising: In a meridian cross section, when a straight line passing through a ground contact edge and perpendicular to the surface of the carcass layer radially outward from the tire is defined as a perpendicular line P, a distance A measured on the perpendicular line P between a widest belt having the largest width among the five or more belt layers and the carcass layer, and a distance B measured on the perpendicular line P between a groove bottom line L passing through a groove bottom of a deepest groove having the greatest depth among the plurality of grooves and parallel to a contour line of the outer surface of the tread portion and an inner surface of the tire satisfy a relationship of 0.3≦A / B≦0.7, and the undertread layer is a rubber component containing 70% by mass or more of natural rubber, 0.5 to 3.0 parts by mass of a hydrazide compound represented by the following formula (1), and a nitrogen adsorption specific surface area N2SA of 60 m 2 / g~120m 2 and 30 to 60 parts by mass of carbon black, the total weight of which is 1 / g (in the following formula (1), R1 and R2 each independently represent an alkyl group having 1 to 18 carbon atoms).

[0007] [ka] Effect of the Invention

[0008] In the pneumatic tire of the present invention, the range of the ratio A / B is set as described above when five or more belt layers are provided to improve durability, so that the end of the belt layer (particularly the widest belt) can be positioned at an appropriate position between the groove bottom line and the tire inner surface, and the distortion of the belt edge can be appropriately distributed and durability can be effectively improved. On the other hand, there is a concern that the heat generation property will be deteriorated due to the large number of belt layers, but since the undertread layer is composed of the above-mentioned rubber composition, heat generation can be effectively suppressed. In particular, since the above-mentioned hydrazide compound and carbon black are used, the carbon black can be effectively dispersed to reduce heat generation. These cooperations can achieve a good balance between excellent durability and low heat generation.

[0009] In the present invention, the number of belt layers is preferably 5 to 6. By optimizing the number of belt layers in this manner, it is possible to improve durability while preventing deterioration of low heat build-up properties due to an excessive number of belt layers.

[0010] In the present invention, it is preferable that each belt layer includes a plurality of steel wires inclined with respect to the tire circumferential direction, and the inclination angle of the steel wires in each belt layer with respect to the tire circumferential direction is 3° or more, which is advantageous for improving durability.

[0011] In the present invention, it is preferable that the rubber constituting the rim cushion rubber layer arranged on the outer circumferential side of the carcass layer in the bead portion has a JIS-A hardness of at least 64. In particular, it is preferable that the rubber constituting the rim cushion rubber layer has a JIS-A hardness of at least 67. Ensuring sufficient hardness of the rim cushion rubber layer in this way is advantageous in improving the durability of the tire.

[0012] In the present invention, the nitrogen adsorption specific surface area N2SA of carbon black is measured in accordance with JIS K6217-2. The hardness is the JIS-A hardness of the rubber constituting each rubber layer, and is the hardness of the rubber measured at a temperature of 20°C using a type A durometer in accordance with JIS K6253.

[0013] In the present invention, the term "ground contact edge" refers to both ends in the tire axial direction of the ground contact area formed when a tire is mounted on a standard rim, inflated to the standard internal pressure, placed vertically on a flat surface, and subjected to a standard load. The term "standard rim" refers to a rim determined for each tire by a standard system including the standard on which the tire is based, for example, a standard rim for JATMA, a "Design Rim" for TRA, or a "Measuring Rim" for ETRTO. The term "standard internal pressure" refers to an air pressure determined for each tire by a standard system including the standard on which the tire is based, for example, the maximum air pressure for JATMA, the maximum value listed in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" for TRA, and "INFLATION PRESSURE" for ETRTO. "Normal load" is the load that each standard specifies for each tire in the system of standards on which the tire is based. For JATMA, it is the maximum load capacity. For TRA, it is the maximum value listed in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES." For ETRTO, it is "LOAD CAPACITY." [Brief description of the drawings]

[0014] [Figure 1] 1 is a meridian half cross-sectional view showing an example of a pneumatic tire of the present invention. [Diagram 2] FIG. 2 is an explanatory diagram showing an enlarged view of a part of the tread portion of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The configuration of the present invention will be described in detail below with reference to the accompanying drawings.

[0016] As shown in Fig. 1, the pneumatic tire of the present invention includes a tread portion 1, a pair of sidewall portions 2 arranged on both sides of the tread portion 1, and a pair of bead portions 3 arranged on the tire radially inward side of the sidewall portions 2. In Fig. 1, the symbol CL indicates the tire equator. Although not depicted in Fig. 1 because it is a meridian cross section, the tread portion 1, the sidewall portions 2, and the bead portions 3 each extend in the tire circumferential direction to form an annular shape, thereby forming a basic toroidal structure of a pneumatic tire. The following explanation using Fig. 1 is basically based on the meridian cross section shown in the figure, but each tire component extends in the tire circumferential direction to form an annular shape.

[0017] A carcass layer 4 is installed between a pair of left and right bead portions 3. This carcass layer 4 includes a plurality of reinforcing cords extending in the tire radial direction, and is folded back from the inside to the outside of the vehicle around a bead core 5 arranged in each bead portion 3. In addition, a bead filler 6 is arranged on the outer periphery of the bead core 5, and this bead filler 6 is sandwiched between the main body portion and the folded back portion of the carcass layer 4.

[0018] Five or more belt layers 7 (six layers in the illustrated example) are embedded on the outer circumferential side of the carcass layer 4 in the tread portion 1. By increasing the number of belt layers 7 in this way, durability can be ensured. If the number of belt layers 7 is less than five, sufficient durability cannot be ensured. From the viewpoint of preventing deterioration of low heat build-up properties due to an excessive number of belt layers 7, the number of belt layers 7 is preferably set to five to six.

[0019] Each belt layer 7 includes a plurality of reinforcing cords (steel wires) extending at an incline with respect to the tire circumferential direction. From the viewpoint of durability, the inclination angle of the steel wires in each belt layer with respect to the tire circumferential direction is preferably 3° or more, and more preferably 3° to 35°. Note that, in order to prevent edge separation of the outermost belt layer, a belt cover sheet (not shown) may be provided on the outer peripheral side of the belt layer 7 (the outer peripheral side of the outermost belt layer in the tire radial direction) that is made of rubber only and does not include reinforcing materials such as wires, and is arranged to cover the edge of the outermost belt layer.

[0020] An inner liner layer 8 is provided on the inner surface of the tire along the carcass layer 4. This inner liner layer 8 is a layer for preventing air filled in the tire from permeating to the outside of the tire. The inner liner layer 9 is composed of, for example, a rubber composition mainly composed of butyl rubber having air permeation prevention properties. Alternatively, it can be composed of a resin layer having a thermoplastic resin as a matrix. In the case of a resin layer, an elastomer component may be dispersed in a thermoplastic resin matrix.

[0021] In the tread portion 1, a tread rubber layer 10 is disposed on the outer peripheral side of the carcass layer 4 and the belt layer 7. In the present invention, the tread rubber layer 10 has a structure in which two types of rubber layers (cap tread layer 11 and under tread layer 12) with different physical properties are laminated in the tire radial direction. The cap tread layer 11 is disposed on the outer peripheral side of the under tread layer 12 to form the tread surface of the tread portion 1. The under tread layer 12 is disposed at a position sandwiched between the cap tread layer 11 and the belt layer 7 (the outermost belt layer in the tire radial direction). However, when the above-mentioned belt cover sheet is provided, the under tread layer 12 is disposed at a position sandwiched between the cap tread layer 11 and the belt cover sheet. In addition, a side rubber layer 20 is disposed on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the sidewall portion 2, and a rim cushion rubber layer 30 is disposed on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the bead portion 3.

[0022] A plurality of grooves 40 are formed on the outer surface of the tread portion 1. The grooves 40 may be grooves that are generally formed in tires, i.e., main grooves extending along the tire circumferential direction, lug grooves extending along the tire width direction, etc. In the following description, the groove having the greatest depth may be referred to as the deepest groove 41, regardless of the extension direction of these grooves.

[0023] In the present invention, as shown in FIG. 2, in the tire meridian cross section, a straight line passing through the ground contact edge E and perpendicular to the tire radially outer surface of the carcass layer 4 is defined as a perpendicular line P. The layer with the largest width among the above-mentioned five or more belt layers 7 is defined as the widest belt 7A (in the illustrated example, the layer is the second outermost layer among the six belt layers 7), and the distance measured on the perpendicular line P between the tire radially inner surface of the widest belt 7A and the tire radially outer surface of the carcass layer 4 is defined as A. Furthermore, a curve passing through the groove bottom of the above-mentioned deepest groove 41 and parallel to the contour line of the outer surface of the tread portion 1 is defined as a groove bottom line L, and the distance measured on the perpendicular line P between the groove bottom line L and the tire inner surface (the surface of the inner liner layer 8 facing the tire cavity) is defined as B. With regard to the groove bottom line L, "parallel to the contour line of the outer surface" means that all points on the groove bottom line L share a normal line with the contour line of the outer surface. When the distances A and B are defined as above, the ratio A / B of these distances A and B satisfies the relationship of 0.3≦A / B≦0.7. Since the range of the ratio A / B is set as above, the end of the belt layer 7 (particularly the widest belt 7A) can be positioned at an appropriate position between the groove bottom line L and the tire inner surface, and the distortion of the belt edge can be appropriately distributed and durability can be effectively improved. If the ratio A / B is less than 0.3, the end of the widest belt 7A is too close to the tire inner cavity side, and the distortion of the belt edge cannot be sufficiently distributed, so that the effect of improving durability cannot be expected. If the ratio A / B exceeds 0.7, the end of the widest belt 7A is too close to the tire outer periphery side, and the distortion of the belt edge cannot be sufficiently distributed, so that the effect of improving durability cannot be expected.

[0024] In the present invention, in order to suppress the influence on heat generation caused by the large number of belt layers 7, not only the above-mentioned structure of the belt layers 7 but also the rubber composition described below is used for the undertread layer 12.

[0025] In the rubber composition constituting the undertread layer 12 (hereinafter referred to as the rubber composition for tires of the present invention), the rubber component is a diene rubber, and natural rubber is always used. By including natural rubber, durability can be improved. As the natural rubber, any natural rubber generally used in rubber compositions for tires can be used. The content of natural rubber is 70% by mass or more, preferably 80% by mass to 100% by mass, and more preferably 100% by mass, based on 100% by mass of the rubber component. If the content of natural rubber is less than 70% by mass, durability cannot be sufficiently improved.

[0026] The rubber composition for tires of the present invention may contain other diene rubbers in addition to the above-mentioned natural rubber. As the other diene rubbers, rubbers that can be generally used in rubber compositions for tires may be used. For example, butadiene rubber, styrene butadiene rubber, acrylonitrile butadiene rubber, etc. may be mentioned. The other diene rubbers may be used alone or as any blend. The molecular weight and microstructure of the diene rubber are not particularly limited, and it may be terminally modified with amine, amide, silyl, alkoxysilyl, carboxyl, hydroxyl groups, etc., or may be epoxidized.

[0027] The rubber composition for tires of the present invention necessarily contains a hydrazide compound represented by the following formula (1) (in the following formula (1), R1 and R2 each independently represent an alkyl group having 1 to 18 carbon atoms).

[0028] [ka]

[0029] Specific examples thereof include 1-hydroxy-N'-(1-methylethylidene)-2-naphthoic acid hydrazide, 1-hydroxy-N'-(1-methylpropylidene)-2-naphthoic acid hydrazide, 1-hydroxy-N'-(1-methylbutylidene)-2-naphthoic acid hydrazide, 1-hydroxy-N'-(1,3-dimethylbutylidene)-2-naphthoic acid hydrazide, 3-hydroxy-N'-(1-methylethylidene)-2-naphthoic acid hydrazide, 3-hydroxy-N'-(1-methylpropylidene)-2-naphthoic acid hydrazide, 3-hydroxy-N'-(1-methylbutylidene)-2-naphthoic acid hydrazide, 3-hydroxy-N'-(1-methylbutylidene)-2-naphthoic acid hydrazide, and 3-hydroxy-N'-(1,3-dimethylbutylidene)-2-naphthoic acid hydrazide. Among these, from the viewpoint of the effects of the present invention, a hydrazide compound represented by the following formula (2) can be preferably used.

[0030] [ka]

[0031] The rubber composition for tires of the present invention necessarily contains carbon black. The carbon black used in the present invention has a nitrogen adsorption specific surface area (N2SA) of 60 m 2 / g~120m 2 / g, preferably 70m 2 / g~120m 2 / g. By using carbon black with such a particle size, it is possible to improve mechanical properties such as rubber hardness, tensile strength at break, and tensile elongation at break while suppressing heat generation of the rubber composition, thereby improving tire durability. 2 If the nitrogen adsorption specific surface area (N2SA) of carbon black is less than 120 m 2 If it exceeds 1 / g, heat buildup will be impaired.

[0032] The amount of carbon black is 30 to 60 parts by mass, preferably 35 to 50 parts by mass, based on 100 parts by mass of the rubber component. If the amount of carbon black is less than 30 parts by mass, the reinforcing performance of the rubber composition cannot be sufficiently obtained, and durability is reduced. If the amount of carbon black is more than 60 parts by mass, heat buildup of the rubber composition increases, and tensile elongation at break decreases, resulting in reduced durability.

[0033] In addition to the compounding agents described above, the rubber composition for tires of the present invention may contain various additives that are generally used in rubber compositions for tires, such as various fillers such as silica, clay, talc, calcium carbonate, vulcanization or crosslinking agents, vulcanization accelerators, various oils, antioxidants, plasticizers, zinc oxide, etc., within the range that does not impair the object of the present invention.

[0034] The various additives described above can be kneaded in a general manner to prepare a rubber composition, which can then be used for vulcanization or crosslinking. The amounts of these additives can be conventional amounts, provided that they do not go against the object of the present invention. The rubber composition for tires can be produced by mixing the above-mentioned components using a general rubber kneading machine, such as a Banbury mixer, kneader, roll, etc.

[0035] As described above, the present invention is basically related to the tread portion 1, and other portions and components are not particularly limited. However, when the above-mentioned structure and rubber composition are adopted, the JIS-A hardness of the rubber constituting the rim cushion rubber layer 30 is set to preferably 64 or more, more preferably 67 or more, and even more preferably 67 to 73 inclusive, taking into consideration the balance of the entire tire. This allows the tread portion 1 and the bead portion 3 to work together to effectively improve the durability of the tire. If the hardness of the rim cushion rubber layer 30 falls below the above-mentioned range, the effect of improving durability by the rim cushion rubber layer 30 cannot be expected to be sufficient. The JIS-A hardness of the rubber constituting the rim cushion rubber layer 30 can be set, for example, by adjusting the amount of filler such as carbon black or silica, resin, and sulfur blended into the rubber constituting the rim cushion rubber layer 30.

[0036] In addition, when compared with the rubber constituting the above-mentioned rim cushion rubber layer 30, the rubber constituting the undertread layer 12 (the above-mentioned rubber composition for a tire of the present invention) preferably has a lower hardness than the rubber constituting the rim cushion rubber layer 30. The specific hardness of the rubber constituting the undertread layer 12 (the above-mentioned rubber composition for a tire of the present invention) is not particularly limited.

[0037] The present invention will be further described below with reference to examples, but the scope of the present invention is not limited to these examples. EXAMPLES

[0038] 29 kinds of rubber compositions for tires (Standard Example 1, Comparative Examples 1-14, Examples 1-14) having the formulations shown in Tables 1-3 were used in the undertread layer (undertread rubber), and a pneumatic tire (test tire) having the basic structure shown in FIG. 1 and the structure (number of belt layers, minimum value of the inclination angle of the belt cord with respect to the tire circumferential direction, ratio A / B) and hardness (undertread layer, rim cushion rubber layer) were set as shown in Tables 1-3, and the tire size was 46 / 90R57. In addition, when preparing these 29 kinds of rubber compositions for tires, each compounding component except for the vulcanization accelerator and sulfur was weighed, kneaded for 4 minutes in a 1.7L closed Banbury mixer, and the master batch was released and cooled at room temperature. Then, this master batch was fed to a 1.7L closed Banbury mixer, and the vulcanization accelerator and sulfur were added and mixed for 3 minutes to obtain 29 kinds of rubber compositions for tires.

[0039] The column "angle of belt cord (minimum value)" in Tables 1 to 3 indicates the minimum value of the inclination angle of the belt cord with respect to the tire circumferential direction in each of the multiple belt layers. "Distance A" is the distance measured on the perpendicular line P between the widest belt, which is the widest among the multiple belt layers, and the carcass layer. "Distance B" is the distance measured on the perpendicular line P between the groove bottom line L and the tire inner surface. The hardness of the undertread layer and the rim cushion rubber layer in each example was measured by vulcanizing the rubber composition constituting each layer at 145°C for 35 minutes using a mold of a predetermined shape, and creating a vulcanized rubber test piece made of each tire rubber composition. Specifically, it is a durometer hardness measured in accordance with JIS K6253 using a durometer type A at a temperature of 20°C.

[0040] Each test tire was evaluated for low heat generation and durability by the methods described below.

[0041] Low heat generation in the undertread layer The rubber compositions for tires used in the undertread layer of each example (Standard Example 1, Comparative Examples 1-14, Examples 1-14) were vulcanized at 145°C for 35 minutes using a mold of a predetermined shape to prepare vulcanized rubber test pieces made of each rubber composition for tires. Using these vulcanized rubber test pieces, tan δ(60°C) was measured using a viscoelasticity spectrometer (manufactured by Toyo Seiki Seisakusho) in accordance with JIS K6394:2007 under the conditions of an elongation deformation rate of 10±2%, a vibration frequency of 20Hz, and a temperature of 60°C. The results were expressed as an index using the reciprocal of the measured value, with the value of Standard Example 1 being set to 100. The larger this index value, the lower the heat buildup of the undertread layer.

[0042] Low heat generation throughout the tire Each test tire was mounted on a rim specified by the TRA standard, inflated to the standard air pressure of the TRA standard, and attached to all wheels of a test vehicle (construction vehicle). The load specified by the TRA standard was applied, and the tire was driven for 60 minutes at a speed of 10 km / h. The temperature of the inner surface of the tread part of the tire was measured before and after the tire was driven, and the amount of temperature rise of the inner surface of the tire was calculated. The evaluation results were expressed as an index using the reciprocal of the measured value, with the value of Standard Example 1 being set at 100. The higher the index value, the smaller the amount of temperature rise, meaning that the tire as a whole has low heat generation.

[0043] durability Each test tire was mounted on a rim specified by the TRA standard, inflated to the standard air pressure of the TRA standard, and attached to a drum testing machine with a drum diameter of 1707.6 mm, and run on the drum for 200 hours at a speed of 10 km / h under a load of 120% of the TRA standard load, and the tire cross section after disassembly was visually evaluated. Evaluation was based on the following two-level criteria. ◯: No cracks were generated at the belt end, the belt-undertread interface, the inside of the undertread, or the interface with the peripheral members of the undertread, and the appearance was good. Δ: Cracks occurred at the belt edge, the belt-undertread interface, the inside of the undertread, or the interface with a surrounding member.

[0044] [Table 1]

[0045] [Table 2]

[0046] [Table 3]

[0047] The types of raw materials used in Tables 1 to 3 are shown below. NR: Natural rubber, RSS#3 CB1: Carbon black, Cabot Japan Co., Ltd. Show Black N339 (Type: HAF, Nitrogen adsorption specific surface area N2SA: 88 m 2 / g) CB2: Carbon black, Cabot Japan Co., Ltd. Show Black S118 (Type: SAF, Nitrogen adsorption specific surface area N2SA: 130 m 2 / g) CB3: Carbon black, Nitelon #GN (type: GPF, nitrogen adsorption specific surface area: 33 m 2 / g) Silica: EVONIK ULTRASIL VN3GR Hydrazide compound 1: A hydrazide compound represented by the above formula (2), DC-01 manufactured by Otsuka Chemical Co., Ltd. Hydrazide compound 2: A hydrazide compound prepared by the following method: Hydrazide compound 3: A hydrazide compound prepared by the following method: Hydrazide compound 4: Adipic acid dihydrazide, manufactured by Otsuka Chemical Co., Ltd. Adipic acid dihydrazide (ADH) Hydrazide compound 5: Sebacic acid dihydrazide (SDH), manufactured by Otsuka Chemical Co., Ltd. Stearic acid: NOF Corporation Beads Stearic Acid YR Zinc oxide: 3 types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. Vulcanization accelerator: Noccela NS manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Sulfur: Tsurumi Chemical Industry Co., Ltd. Kinkaji Oil-filled Powdered Sulfur

[0048] Method for preparing hydrazide compound 2: 3-Hydroxy-2-naphthoic acid hydrazide and 3-methyl-2-pentanone were stirred while heating. The reaction solution was concentrated and cooled, and the precipitated crystals were filtered and dried under reduced pressure to obtain hydrazide compound 2 having the structure represented by the following formula (3).

[0049] [ka]

[0050] Method for preparing hydrazide compound 3: 3-Hydroxy-2-naphthoic acid hydrazide and 3-pentanone were stirred while being heated. After the reaction solution was concentrated and cooled, the precipitated crystals were filtered and dried under reduced pressure to obtain hydrazide compound 3 having a structure represented by the following formula (4).

[0051] [ka]

[0052] As is clear from Tables 1 to 3, the heavy-duty tires of Examples 1 to 14 improved the low heat generation property of the undertread layer itself, further improved the low heat generation property of the entire tire, and exhibited good durability, compared with Standard Example 1. On the other hand, Comparative Example 1 had a small number of belt layers, and the rubber composition constituting the undertread layer did not contain a hydrazide compound, so the low heat generation property of the undertread layer itself deteriorated, and the durability also deteriorated. Comparative Example 2 had an appropriate belt layer structure, but the rubber composition constituting the undertread layer did not contain a hydrazide compound, so the effect of improving the low heat generation property was not obtained. Comparative Example 3 had a small ratio A / B, and the rubber composition constituting the undertread layer did not contain a hydrazide compound, so the low heat generation property of the undertread layer itself deteriorated, and the durability also deteriorated. Comparative Example 4 had a small number of belt layers, so the durability was deteriorated, although the rubber composition constituting the undertread layer contained a hydrazide compound. In Comparative Example 5, the rubber composition constituting the undertread layer contained a hydrazide compound, but the ratio A / B was small, so durability was deteriorated. In Comparative Example 6, the rubber composition constituting the undertread layer contained a hydrazide compound, but the ratio A / B was large, so durability was deteriorated. In Comparative Example 7, the N2SA of the carbon black was large, so low heat buildup was deteriorated. In Comparative Example 8, the N2SA of the carbon black was small, so the reinforcement of the undertread rubber decreased and durability was deteriorated. In Comparative Example 9, the durability was deteriorated because the amount of carbon black was small. In Comparative Example 10, the amount of carbon black was large, so heat buildup was deteriorated. In Comparative Example 11, the amount of hydrazide compound was small, so the effect of improving tire heat buildup was not obtained. In Comparative Example 12, the amount of hydrazide compound was large, so processability was deteriorated, and heat buildup and durability were deteriorated. In Comparative Example 13, adipic acid dihydrazide was included instead of the hydrazide compound that satisfies the conditions of the present invention, so heat buildup was deteriorated. Comparative Example 14 contained sebacic dihydrazide instead of the hydrazide compound satisfying the conditions of the present invention, and therefore the heat generation property was deteriorated. [Explanation of symbols]

[0053] 1 Tread section 2 Sidewall 3 Bead section 4 Carcass layer 5 Bead Core 6 Bead Filler 7 Belt layer 7A Widest Belt 8 Inner liner layer 10 Tread rubber layer 11 Cap tread layer 12 Undertread layer 20 Side rubber layer 30 Rim cushion rubber layer 40 grooves 41 Deepest groove CL Tire Equator E Ground end L Groove bottom line

Claims

1. A pneumatic tire comprising a tread portion extending in a circumferential direction of the tire and forming an annular shape, a pair of sidewall portions disposed on both sides of the tread portion, and a pair of bead portions disposed radially inward of the pair of sidewall portions, a carcass layer mounted between the pair of bead portions, and five or more belt layers disposed on the outer peripheral side of the carcass layer in the tread portion, the tread portion being composed of two layers, a cap tread layer forming a contact surface of the tread portion, and an under tread layer disposed radially inward of the cap tread layer, and a plurality of grooves formed on the outer surface of the tread portion, In a tire meridian cross section, when a straight line passing through a ground contact edge and perpendicular to a surface of the carcass layer radially outward is defined as a perpendicular line P, a distance A measured on the perpendicular line P between a widest belt having the largest width among the five or more belt layers and the carcass layer, and a distance B measured on the perpendicular line P between a groove bottom line L passing through a groove bottom of a deepest groove having the greatest depth among the plurality of grooves and parallel to a contour line of the outer surface of the tread portion and an inner surface of the tire satisfy a relationship of 0.3≦A / B≦0.7, The undertread layer comprises 100 parts by mass of a rubber component containing 70% by mass or more of natural rubber, 0.5 to 3.0 parts by mass of a hydrazide compound represented by the following formula (1), and a nitrogen adsorption specific surface area N2SA of 60 m 2 / g ~ 120m 2 and 30 to 60 parts by mass of carbon black having a viscosity of 100 / g. 【Chemistry 1】 (In the above formula (1), R 1 and R 2 each independently represents an alkyl group having 1 to 18 carbon atoms.

2. 2. The heavy duty tire according to claim 1, wherein the number of the belt layers is 5 to 6.

3. 3. The heavy-duty tire according to claim 1, wherein each of the belt layers includes a plurality of steel wires inclined with respect to the tire circumferential direction, and an inclination angle of the steel wires in each belt layer with respect to the tire circumferential direction is 3° or more.

4. 4. The heavy-duty tire according to claim 1, wherein the rim cushion rubber layer disposed on the outer circumferential side of the carcass layer in the bead portion has a JIS-A hardness of 64 or more.

5. 5. The heavy duty tire according to claim 4, wherein the rubber constituting the rim cushion rubber layer has a JIS-A hardness of 67 or more.

6. A heavy-duty tire as described in any one of claims 1 to 5, characterized in that a rim cushion rubber layer is arranged on the outer circumferential side of the carcass layer in the bead portion, and the JIS-A hardness of the rubber composition constituting the undertread layer is lower than the JIS-A hardness of the rubber constituting the rim cushion rubber layer.

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