Heavy-load pneumatic tire

Abstract

Provided is a heavy-load pneumatic tire capable of maintaining or improving rolling resistance, uneven wear resistance performance, and durability, and achieving a high level of balance of the properties. In this heavy-load pneumatic tire, a tread rubber layer 10 is composed of a tread rubber formed of a rubber composition in which at most 60 parts by mass of a filler containing carbon black and silica is blended with 100 parts by mass of a rubber component containing at least 90 mass% of an isoprene-based rubber and the blending amount ratio of the carbon black and the silica in the filler is 3:2 to 1:1, a belt cushion layer 40 is composed of a belt cushion rubber formed of a rubber composition in which at most 45 parts by mass of carbon black is blended with 100 parts by mass of a rubber component containing at least 90 mass% of an isoprene-based rubber, and the hardness Hc of the tread rubber and the hardness Hb of the belt cushion rubber satisfy the relationship of 1.2≤Hc / Hb≤1.5.

Description

Heavy-duty pneumatic tires

[0001] The present invention relates to a heavy-duty tire including a belt cushion layer disposed between the end portions of a belt layer in the tire width direction and a carcass layer.

[0002] Tires used on vehicles such as trucks and buses (heavy-duty pneumatic tires) are subject to higher loads than passenger car tires, making excellent wear resistance a key consideration. Furthermore, even heavy-duty pneumatic tires are prone to uneven wear, such as localized wear in the shoulder area, when frequently driven at high speeds. This uneven wear can cause the tire's primary lifespan to expire before the entire tread wears out, necessitating improved resistance to uneven wear. On the other hand, as mentioned above, when vehicles frequently drive at high speeds (compared to, for example, urban driving), the rate at which wear progresses is slower, resulting in a longer primary tire lifespan (because the wear rate is slower, the time until the tire reaches its end of life due to total or uneven wear tends to be longer). Therefore, excellent durability of the tire body (tire frame) is also required. In addition, with growing concern about global environmental issues, environmental regulations are becoming stricter for heavy-duty tires, necessitating further improvements in fuel economy.

[0003] For example, Patent Document 1 proposes blending silica into a rubber component primarily composed of natural rubber and a specific silane coupling agent to improve silica dispersion and thereby improve wear resistance and fuel economy. However, such measures alone are unable to sufficiently improve the aforementioned uneven wear resistance and other properties. Known methods for improving uneven wear resistance include increasing the hardness of the tread rubber and belt cushion rubber located in the shoulder region of the tire. Known methods for increasing rubber hardness include increasing crosslink density and adding filler. However, increasing crosslink density raises concerns about a deterioration in the durability of the belt cushion, and increasing the amount of filler raises concerns about an impact on fuel economy due to increased heat generation. For these reasons, it is difficult to achieve rolling resistance, uneven wear resistance, and durability in tires equipped with tread rubber and belt cushion rubber, and measures are needed to achieve these qualities in a balanced and highly effective manner.

[0004] Japanese Patent Application Publication No. 2019-056068

[0005] An object of the present invention is to provide a heavy-duty pneumatic tire equipped with tread rubber and belt cushion rubber, which maintains or improves rolling resistance, uneven wear resistance, and durability, and achieves a high degree of compatibility among these in a well-balanced manner.

[0006] The heavy-duty pneumatic tire of the present invention that achieves the above object comprises a tread portion extending in the tire circumferential direction 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 on the tire radially inward side of these sidewall portions, and has at least one carcass layer mounted between the pair of bead portions and a plurality of belt layers disposed on the outer peripheral side of the carcass layer in the tread portion, and a belt cushion layer disposed between the tire width direction end portions of the belt layer and the carcass layer, wherein the tread rubber constituting the tread portion contains 90 mass % or more of isoprene-based rubber. The tread rubber is characterized in that it is composed of a rubber composition in which 60 parts by mass or less of a filler containing carbon black and silica is blended with 100 parts by mass of a rubber component containing the tread rubber, and the ratio of the amount of the carbon black to the amount of the silica in the filler is 3:2 to 1:1, and the belt cushion rubber constituting the belt cushion layer is composed of a rubber composition in which 45 parts by mass or less of a filler containing carbon black and silica is blended with 100 parts by mass of a rubber component containing 90% by mass or more of an isoprene-based rubber, and the hardness Hc of the tread rubber at 20°C and the hardness Hb of the belt cushion rubber at 20°C satisfy the relationship of 1.2≦Hc / Hb≦1.5.

[0007] In the heavy-duty pneumatic tire of the present invention, the tread rubber is composed of a rubber composition containing 60 parts by mass or less of a filler containing carbon black and silica per 100 parts by mass of a rubber component containing 90% by mass or more of isoprene-based rubber, with the carbon black to silica ratio in the filler being between 3:2 and 1:1. This suppresses heat generation in the tread portion and improves rolling resistance. Furthermore, the belt cushion rubber is composed of a rubber composition containing 45 parts by mass or less of a filler containing carbon black and silica per 100 parts by mass of a rubber component containing 90% by mass or more of isoprene-based rubber, thereby improving shoulder resistance and uneven wear resistance. Furthermore, the hardness ratio Hc / Hb between the tread rubber and the belt cushion rubber is set within a moderate range of 1.2 to 1.5, thereby ensuring durability and exhibiting the aforementioned uneven wear resistance. These cooperations maintain or improve rolling resistance, uneven wear resistance, and durability, achieving a good balance between these.

[0008] In the present invention, the rubber composition constituting the tread rubber contains a guanidine-based vulcanization accelerator and a sulfenamide-based vulcanization accelerator, and the compounding amount Ac of the guanidine-based vulcanization accelerator is G and the amount of sulfenamide vulcanization accelerator Ac S and 0.3≦Ac G / Ac S It is preferable to satisfy the relationship of ≦5.0. This improves the rubber physical properties of the tread rubber, which is advantageous for achieving both low rolling resistance and chipping resistance.

[0009] In the present invention, when a plurality of circumferential grooves extending along the tire circumferential direction are formed on the surface of the tread portion and a pair of the plurality of circumferential grooves located on the outermost sides in the tire width direction are shoulder main grooves, shoulder land portions are defined on the tire width outer sides of each shoulder main groove, and it is preferable that the width Wsh of each shoulder land portion satisfies the relationship 0.15≦Wsh / W≦0.27 with respect to the contact width W of the tread portion. This optimizes the land portions (amount of rubber) located in the shoulder regions that contribute to uneven wear resistance, which is advantageous for improving uneven wear resistance.

[0010] In the present invention, it is preferable that the plurality of circumferential grooves include shoulder main grooves and a center circumferential groove disposed more inward in the tire width direction than the shoulder main grooves, and that the groove width Wo of the shoulder main groove and the groove width Wi of the center circumferential groove satisfy the relationship 0.15≦Wi / Wo≦0.24. Providing grooves in this manner is advantageous for improving uneven wear resistance.

[0011] In the present invention, the tread gauge Gcc measured at the tire equator is preferably 12.0 mm or more and 30.0 mm or less. This ensures a sufficient amount of tread rubber, which is advantageous for improving uneven wear resistance. The tread gauge Gcc is the thickness of the tread rubber measured at the tire equator, and is the distance measured at the tire equator from the outer surface of the tread layer to the radially outermost surface of the layer located radially outward of the various reinforcing layers (carcass layer, belt layer, belt reinforcing layer, etc.) that make up the tire.

[0012] In the present invention, when the contact length of the tread center land row is Lc and the contact length of the axially outermost land row is Ls, it is preferable that the maximum contact length Lc and the outer contact length Ls satisfy the relationship 0.80≦Ls / Lc≦1.05, which is advantageous for improving uneven wear resistance because a sufficient contact length can be secured in the shoulder area when the tire is in contact with the ground.

[0013] In the present invention, the "contact width" and "contact length" of the tread portion refer to the axial length (contact width) and circumferential length (contact length) of the contact area when the tire is mounted on a standard rim, inflated to a standard internal pressure, placed vertically on a flat surface, and subjected to a standard load. Specifically, the contact width W is the distance in the tire width direction between the contact edges measured under the above conditions, and the shoulder land portion width Wsh is the distance in the tire width direction between the inner edge of each shoulder land portion in the tire width direction (the outer groove wall in the tire width direction of the shoulder main groove that defines the shoulder land portion) and the contact edge. The contact length Lc is measured as the circumferential length of the tread center land portion included in the contact area under the above conditions, at the center in the tire width direction. The contact length Ls is measured as the circumferential length of the axially outermost land portion row included in the contact area under the above conditions, at the center in the tire width direction. "Regular rim" refers to the rim specified for each tire by the standard system including the standard on which the tire is based, for example, the standard rim for JATMA, the "Design Rim" for TRA, or the "Measuring Rim" for ETRTO. "Regular internal pressure" refers to the air pressure specified for each tire by the 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 the "INFLATION PRESSURE" for ETRTO, but is 180 kPa if the tire is for a passenger car. The "normal load" is the load determined for each tire by the respective standards, including the standards on which the tire is based, and is the maximum load capacity in the case of JATMA, the maximum value listed in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in the case of TRA, and the "LOAD CAPACITY" in the case of ETRTO. However, when the tire is for a passenger car, the normal load is a load equivalent to 88% of the above load.

[0014] Fig. 1 is a meridian cross-sectional view showing an example of a heavy-duty pneumatic tire of the present invention, and Fig. 2 is a front view showing an example of a tread portion of a heavy-duty pneumatic tire of the present invention.

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

[0016] As shown in Figure 1, the heavy-duty pneumatic tire of the present invention includes a tread portion 1 that contacts the road surface, a pair of sidewall portions 2 arranged on either side of the tread portion 1, and a pair of bead portions 3 arranged radially inward of the sidewall portions 2. In Figure 1, the symbol CL indicates the tire equator, and the symbol E indicates the contact edge. Although not depicted in Figure 1 because it is a meridian cross-section, the tread portion 1, sidewall portions 2, and bead portions 3 each extend circumferentially in the tire direction and form an annular shape, thereby constituting the basic toroidal structure of the pneumatic tire. The following explanation using Figure 1 will be based basically on the meridian cross-section shape shown, but each tire component also extends circumferentially in the tire direction and forms an annular shape.

[0017] A carcass layer 4 is installed between the pair of left and right bead portions 3. This carcass layer 4 includes a plurality of reinforcing cords extending in the tire radial direction, and is folded back from the inner side to the outer side in the tire width direction around 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 portion and the folded back portion of the carcass layer 4.

[0018] Multiple belt layers 7 (four layers in FIG. 1 ) are embedded on the outer periphery of the carcass layer 4 in the tread portion 1. Each belt layer 7 includes multiple reinforcing cords (steel cords) oriented in a predetermined direction. The multiple belt layers 7 may include a cross belt pair. A cross belt pair is a combination of at least two belt layers in which the inclination angle of the reinforcing cords with respect to the tire circumferential direction is set to a range of 10° to 40° and the inclination direction of the reinforcing cords is reversed between layers, so that the reinforcing cords cross each other. In addition to the cross belt pair, other optional layers may include a high-angle belt layer in which the inclination angle of the reinforcing cords with respect to the tire circumferential direction is set to a range of 40° to 70°, a protective belt layer disposed in the outermost layer and having a width 85% or less of the other belt layers, and a circumferential reinforcing layer in which the reinforcing cords are set at an angle with respect to the tire circumferential direction in a range of 0° to 5°. For example, in FIG. 1 , one protective belt layer is disposed in the outermost layer, one high-angle belt layer is disposed in the innermost layer, and the other two layers are cross belt pairs. Furthermore, a belt reinforcing layer (not shown) may be provided on the outer peripheral side of the belt layer 7. The belt reinforcing layer may be made of, for example, organic fiber cords oriented in the tire circumferential direction. The organic fiber cords constituting the belt reinforcing layer 8 may be angled at, for example, 0° to 5° with respect to the tire circumferential direction.

[0019] A tread rubber layer 10 is disposed on the outer peripheral side of the carcass layer 4 in the tread portion 1, 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, and 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 in the bead portion 3. In addition, a belt cushion layer 40 is disposed in a region with a generally triangular cross section between the end portion in the tire width direction of the belt layer 7 and the carcass layer 4. The tread rubber layer 10 may have a structure in which two types of rubber layers with different physical properties (a cap tread layer 11 that forms the tread surface of the tread portion 1 and an under tread layer 12 disposed on its inner peripheral side) are laminated in the tire radial direction.

[0020] Since the present invention mainly relates to the tread rubber layer 10 (particularly the cap tread layer 11) and the belt cushion layer 40, as long as the tread portion 1 (tread rubber layer 10) is composed of the cap tread layer 11 and the under tread layer 12 and is provided with the belt cushion layer 40, the basic structure of the other parts is not limited to the above-mentioned structure.

[0021] In the present invention, the tread rubber refers to the rubber that constitutes the above-mentioned tread rubber layer 10. When the tread portion 1 (tread rubber layer 10) is composed of a cap tread layer 11 and an under tread layer 12, the tread rubber refers to the rubber that constitutes the cap tread layer 11. The rubber composition that constitutes this tread rubber contains a diene rubber as a rubber component, and in particular, necessarily contains an isoprene rubber. It may also optionally contain butadiene rubber or styrene butadiene rubber.

[0022] The isoprene-based rubber used in the tread rubber can be any rubber commonly used in rubber compositions for tires. Examples include various natural rubbers, epoxidized natural rubbers, and various synthetic polyisoprene rubbers. The content of the isoprene-based rubber is 90% by mass or more, preferably 93% by mass to 100% by mass, when the total rubber components in the rubber composition constituting the tread rubber are taken as 100% by mass. By including a specific amount of isoprene-based rubber in this way, a good balance between abrasion resistance and fuel economy can be achieved. If the content of isoprene-based rubber is less than 90% by mass, chipping resistance will be impaired.

[0023] Tread rubber always contains silica and carbon black as fillers. The inclusion of silica as a filler in tread rubber is advantageous for improving fuel economy. Furthermore, the inclusion of carbon black as a filler in tread rubber is advantageous for improving uneven wear resistance. As described above, the filler always contains silica and carbon black, but other fillers can also be contained. Examples of other fillers include calcium carbonate, magnesium carbonate, talc, clay, alumina, aluminum hydroxide, titanium oxide, and calcium sulfate. These other fillers may be used alone or in combination of two or more.

[0024] The compounding amount of the filler used in the tread rubber (total compounding amount of silica and carbon black) is 60 parts by mass or less, preferably 40 to 60 parts by mass, and more preferably 45 to 60 parts by mass, per 100 parts by mass of the rubber component. If the compounding amount of the filler exceeds 60 parts by mass, heat buildup and processability decrease.

[0025] When silica and carbon black are blended as fillers, the ratio of the amount of carbon black to the amount of silica in the filler is set to 3:2 to 1:1, preferably 4:3 to 1:1. In other words, the amount of carbon black in the filler is set to 1 to 1.5 times, preferably 1 to 1.3 times, the amount of silica in the filler. Blending carbon black and silica in an appropriate ratio like this is advantageous for achieving both low rolling resistance and uneven wear resistance. If the ratio of the amount of carbon black to silica in the filler is outside the above range, low rolling resistance or uneven wear resistance will deteriorate.

[0026] The amount of carbon black used as a filler in the tread rubber is preferably 50% by mass to 60% by mass, and more preferably 50% by mass to 56% by mass, when the total amount of fillers used in the tread rubber is taken as 100% by mass. If the carbon black content is less than 50% by mass, the effect of improving uneven wear resistance is limited. If the carbon black content exceeds 60% by mass, the effect of improving low rolling resistance is limited. Furthermore, the amount of carbon black per 100 parts by mass of the rubber component constituting the tread rubber is preferably 23 parts by mass to 35 parts by mass, and more preferably 28 parts by mass to 35 parts by mass. If the amount of carbon black is less than 23 parts by mass per 100 parts by mass of the rubber component, the effect of improving uneven wear resistance is limited. If the amount of carbon black is more than 36 parts by mass per 100 parts by mass of the rubber component, the effect of improving low rolling resistance is limited.

[0027] The nitrogen adsorption specific surface area NSA of the carbon black used in the tread rubber is not particularly limited, but is preferably 85 m 2 / g~150m 2 / g, more preferably 85m 2 / g~120m 2 / g. In this way, the use of carbon black with an appropriate particle size is advantageous in achieving both uneven wear resistance and low rolling resistance. 2 If the nitrogen adsorption specific surface area N2SA of the carbon black is less than 150 m / g, the effect of improving uneven wear resistance cannot be expected to be sufficient. 2 If the nitrogen adsorption specific surface area N2SA of carbon black exceeds 1 / g, the effect of improving low rolling resistance cannot be expected to be sufficient. The nitrogen adsorption specific surface area N2SA of carbon black is measured in accordance with JIS K6217-2.

[0028] The silica used as a filler in the tread rubber may be any silica typically used in rubber compositions for tires, such as wet silica (hydrated silicic acid), dry silica (anhydrous silicic acid), calcium silicate, aluminum silicate, etc., which may be used alone or in combination of two or more.

[0029] The content of silica used in the tread rubber is preferably 40% to 50% by mass, and more preferably 40% to 45% by mass, when the entire filler is taken as 100% by mass. If the silica content is less than 40% by mass, the effect of improving fuel economy performance is limited. If the silica content exceeds 50% by mass, the effect of improving uneven wear resistance is limited. Furthermore, the amount of silica blended per 100 parts by mass of the rubber component constituting the tread rubber is 18 parts by mass to 30 parts by mass, preferably 20 parts by mass to 28 parts by mass. By thus including a sufficient amount of silica in the entire tread rubber, further improvement in fuel economy performance can be achieved. If the silica blending amount is less than 18 parts by mass, the effect of improving fuel economy performance is limited. If the silica blending amount exceeds 30 parts by mass, the effect of improving uneven wear resistance is limited.

[0030] The CTAB adsorption specific surface area of ​​the silica used in the tread rubber is not particularly limited, but is preferably 150 m 2 / g~220m 2 / g, more preferably 150m 2 / g~180m2 / g. The CTAB adsorption specific surface area of ​​the silica is preferably 150 m 2 If the CTAB adsorption specific surface area of ​​the silica is less than 220 m / g, there is a risk that the effect of improving uneven wear resistance will not be sufficiently obtained. 2 When the CTAB adsorption specific surface area of ​​silica exceeds 1 / g, the effect of improving low rolling resistance may not be sufficiently obtained. In this specification, the CTAB adsorption specific surface area of ​​silica is a value measured in accordance with ISO 5794.

[0031] In the tread rubber, it is preferable to compound a silane coupling agent together with silica. By compounding a silane coupling agent, the dispersibility of silica in the rubber component can be improved, and abrasion resistance and fuel economy performance can be improved in a balanced manner. The type of silane coupling agent is not particularly limited as long as it is usable in a rubber composition compounded with silica. Examples include sulfur-containing silane coupling agents such as bis-(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)disulfide, 3-trimethoxysilylpropylbenzothiazoletetrasulfide, γ-mercaptopropyltriethoxysilane, and 3-octanoylthiopropyltriethoxysilane.

[0032] When a silane coupling agent is compounded into the tread rubber, the compounding amount is not particularly limited, but is preferably 1% by mass to 20% by mass, more preferably 5% by mass to 15% by mass, relative to the compounding amount of silica described above. If the compounding amount of the silane coupling agent is less than 1% by mass of the compounded amount of silica, the dispersion of the silica cannot be sufficiently improved. If the compounding amount of the silane coupling agent exceeds 20% by mass of the compounded amount of silica, the silane coupling agents condense with each other, and the desired tensile strength of the rubber composition cannot be obtained.

[0033] In tread rubber, it is preferable to compound a guanidine-based vulcanization accelerator and a sulfenamide-based vulcanization accelerator as vulcanization accelerators. Examples of guanidine-based vulcanization accelerators include diphenyl guanidine (DPG), di(o-tolyl)guanidine, and o-tolylbiguanide. Examples of sulfenamide-based vulcanization accelerators include N,N-dicyclohexyl-1,3-benzothiazole-2-sulfenamide (DZ), N-cyclohexyl-2-benzothiazole sulfenamide (CZ), N-oxydiethylene-2-benzothiazole sulfenamide (OBS), and N-(tert-butyl)benzothiazole-2-sulfenamide (NS). In this way, the combined use of two types of vulcanization accelerators, a guanidine-based vulcanization accelerator and a sulfenamide-based vulcanization accelerator, is advantageous for achieving both low rolling resistance and chipping resistance.

[0034] When a guanidine vulcanization accelerator and a sulfenamide vulcanization accelerator are used in combination, the amount Ac of the guanidine vulcanization accelerator is G and the amount of sulfenamide vulcanization accelerator Ac S and preferably 0.3≦Ac G / Ac S ≦5.0, more preferably 0.4≦Ac G / Ac S It is preferable that the relationship of Ac≦2.0 is satisfied. This improves the rubber physical properties of the tread rubber, which is advantageous for achieving both low rolling resistance and chipping resistance. G / Ac S If the blending ratio Ac is less than 0.3, the reaction between the sulfenamide vulcanization accelerator and the silane coupling agent will result in excessive tread hardness, and the effect of improving uneven wear resistance will be limited. G / Ac S If the value exceeds 5.0, the vulcanization rate will be increased, causing the tread portion to be over-vulcanized, and the effect of improving low rolling resistance will be limited.

[0035] The individual amounts of the guanidine vulcanization accelerator and sulfenamide vulcanization accelerator are not particularly limited, but the amount of the guanidine vulcanization accelerator is preferably 0.2 to 1.2 parts by mass, more preferably 0.4 to 1.0 parts by mass, per 100 parts by mass of the rubber composition constituting the tread rubber. The amount of the sulfenamide vulcanization accelerator is preferably 0.6 to 1.6 parts by mass, more preferably 0.8 to 1.2 parts by mass, per 100 parts by mass of the rubber composition constituting the tread rubber. Furthermore, the total amount of the guanidine vulcanization accelerator and sulfenamide vulcanization accelerator is preferably 1.4 to 2.0 parts by mass, more preferably 1.6 to 2.0 parts by mass, per 100 parts by mass of the rubber composition constituting the tread rubber. Adding an appropriate amount of vulcanization accelerator in this way is advantageous for achieving both low rolling resistance and uneven wear resistance.

[0036] In the present invention, the belt cushion rubber is the rubber that constitutes the belt cushion layer 40. The rubber composition that constitutes this belt cushion rubber contains a diene rubber, and in particular, necessarily contains an isoprene rubber, as a rubber component. It may also contain a butadiene rubber as an option.

[0037] The isoprene-based rubber used in the belt cushion rubber can be any rubber commonly used in rubber compositions for tires. Examples include various natural rubbers, epoxidized natural rubbers, and various synthetic polyisoprene rubbers. The content of the isoprene-based rubber is 90% by mass or more, preferably 93% by mass to 100% by mass, when the total rubber components in the rubber composition constituting the belt cushion rubber are taken as 100% by mass. By including a specific amount of isoprene-based rubber in this way, a good balance between abrasion resistance and fuel economy can be achieved. If the content of isoprene-based rubber is less than 90% by mass, durability deteriorates.

[0038] Carbon black is always blended as a filler in belt cushion rubber. The inclusion of carbon black as a filler in belt cushion rubber is advantageous for improving durability. As mentioned above, the filler always contains carbon black, but other fillers can also be blended. Examples of other fillers include silica, calcium carbonate, magnesium carbonate, talc, clay, alumina, aluminum hydroxide, titanium oxide, and calcium sulfate. These other fillers may be used alone or in combination of two or more.

[0039] The amount of carbon black used in the belt cushion rubber is 45 parts by mass or less, preferably 30 to 45 parts by mass, and more preferably 35 to 45 parts by mass, per 100 parts by mass of the rubber component constituting the belt cushion rubber. If the amount of carbon black exceeds 45 parts by mass, heat buildup and processability decrease. Multiple types of carbon black may be compounded in the belt cushion rubber, and in that case, it is preferable that the total amount of the multiple types of carbon black compounded satisfies the above range. Note that when carbon black is used in combination with other fillers, particularly when carbon black is used in combination with silica, it is preferable that the amount of filler compounded (total amount of silica and carbon black compounded) is set to 45 parts by mass or less per 100 parts by mass of the rubber component constituting the belt cushion rubber.

[0040] The nitrogen adsorption specific surface area N2SA of the carbon black used in the belt cushion rubber is not particularly limited, but is preferably 40 m 2 / g~95m 2 / g, more preferably 65m 2 / g~80m 2 / g. In this way, the use of carbon black with an appropriate particle size is advantageous in achieving both low rolling resistance and durability. 2 If the nitrogen adsorption specific surface area N2SA of the carbon black is less than 95 m / g, the effect of improving durability cannot be expected to be sufficient. 2If the nitrogen adsorption specific surface area N2SA of carbon black exceeds 1 / g, the effect of improving low rolling resistance cannot be expected to be sufficient. The nitrogen adsorption specific surface area N2SA of carbon black is measured in accordance with JIS K6217-2.

[0041] The carbon black used in the belt cushion rubber preferably has a particle size different from that of the carbon black used in the tread rubber. Specifically, the nitrogen adsorption specific surface area N2SA of the carbon black used in the belt cushion rubber is preferably smaller than the nitrogen adsorption specific surface area N2SA of the carbon black used in the tread rubber. By using different types of carbon black in the tread rubber and the belt cushion rubber in this way, it is possible to impart rubber physical properties suitable for each component.

[0042] The heavy-duty pneumatic tire of the present invention comprises a tread rubber layer 10 made of the above-described tread rubber and a belt cushion layer 40 made of the above-described belt cushion rubber. In providing these two layers, the hardness Hc of the tread rubber at 20°C and the hardness Hb of the belt cushion rubber at 20°C satisfy the relationship 1.20≦Hc / Hb≦1.50, preferably 1.20≦Hc / Hb≦1.40. By setting the hardness ratio Hc / Hb of the tread rubber and the belt cushion rubber within an appropriate range, the tire can exhibit the aforementioned uneven wear resistance while ensuring durability. If the hardness ratio Hc / Hb is less than 1.20 or exceeds 1.50, durability decreases.

[0043] Although the hardness of each rubber layer is not particularly limited, the hardness Hc of the tread rubber at 20°C is preferably 62 to 72, and more preferably 64 to 70. Having a moderate hardness in the tread rubber is advantageous for improving uneven wear resistance. If the hardness Hc is less than 62 or exceeds 72, uneven wear resistance decreases. Similarly, the hardness Hb of the belt cushion rubber at 20°C is preferably 50 to 60, and more preferably 50 to 57. Having a moderate hardness in the belt cushion rubber is advantageous for improving durability. If the hardness Hb is less than 50 or exceeds 60, durability decreases.

[0044] The heavy-duty pneumatic tire of the present invention may have any structure, as long as it includes a tread rubber layer 10 made of the above-described tread rubber and a belt cushion layer 40 made of the above-described belt cushion rubber. Preferably, however, as shown in Figures 1 and 2, the tire surface has a plurality of circumferential grooves 50 (four in the figure) extending circumferentially along the tire circumferential direction. When a pair of the circumferential grooves 50 located at the outermost positions in the tire width direction is defined as shoulder main grooves 51, shoulder land portions 60 are defined on the tire widthwise outer side of each shoulder main groove 51. In this case, the width Wsh of each shoulder land portion 60 preferably satisfies the relationship 0.15≦Wsh / W≦0.27, more preferably 0.18≦Wsh / W≦0.24, relative to the contact width W of the tread 1. This structure optimizes the land portions (amount of rubber) located in the shoulder regions that contribute to uneven wear resistance, which is advantageous for improving uneven wear resistance. If the ratio Wsh / W is less than 0.15, the amount of rubber in the shoulder land portion 60, which contributes to uneven wear resistance, is insufficient, resulting in poor uneven wear resistance.If the ratio Wsh / W exceeds 0.27, wet performance (traction performance on wet roads) is poor.

[0045] In particular, as shown in FIG. 1 , the multiple circumferential grooves 50 preferably include a pair of shoulder main grooves 51 and multiple (two in the figure) center circumferential grooves 52 arranged on the tire widthwise inward side of the shoulder main grooves 51. When such center circumferential grooves 52 are included, the center circumferential grooves 52 preferably have a smaller groove width than the shoulder main grooves 51. Specifically, the groove width Wo of the shoulder main grooves 51 and the groove width Wi of the center circumferential groove 52 preferably satisfy the relationship 0.20≦Wi / Wo≦1.20, more preferably 0.20≦Wi / Wo≦0.90. Setting the groove widths of the multiple circumferential grooves 50 within an appropriate range like this is advantageous for achieving both wet performance and low rolling resistance. If the groove width ratio Wi / Wo is less than 0.20, wet performance deteriorates. If the groove width ratio Wi / Wo exceeds 1.20, low rolling resistance deteriorates. Although there are no particular limitations on the individual groove widths of the respective grooves, the groove width Wo of the shoulder main groove 51 can be set to, for example, 10.5 mm to 14.0 mm, and the groove width Wi of the center circumferential groove 52 can be set to, for example, 3.5 mm to 14.5 mm.

[0046] Furthermore, as shown in Figure 1, when the thickness of the tread rubber measured at the tire equator CL is defined as the tread rubber gauge Gcc, this tread gauge Gcc is preferably 12.0 mm or more and 30.0 mm or less, and more preferably 16.0 mm or more and 30.0 mm or less. Ensuring a sufficient amount of tread rubber in this way is advantageous for improving wear resistance. If the tread gauge Gcc is less than 12.0 mm, wear resistance decreases. If the tread gauge Gcc exceeds 30.0 mm, low rolling resistance decreases.

[0047] 2, the area indicated by the dashed line schematically shows the contact shape (contact area) when a heavy-duty pneumatic tire of the present invention is mounted on a standard rim, inflated to the standard internal pressure, placed vertically on a flat surface, and subjected to a standard load. In this contact shape, if the contact length of the tread center land row is Lc and the contact length of the axially outermost land row is Ls, the heavy-duty pneumatic tire of the present invention preferably has a maximum contact length Lc and an outer contact length Ls that satisfy the relationship 0.80≦Ls / Lc≦1.05, and more preferably 0.85≦Ls / Lc≦1.00. This ensures sufficient contact length in the shoulder region when the tire is in contact with the ground, which is advantageous for improving uneven wear resistance.

[0048] The heavy-duty pneumatic tire of the present invention maintains or improves rolling resistance, uneven wear resistance, and durability, and achieves a good balance between these, thanks to the cooperation of the compositions and physical properties of the tread rubber and belt cushion rubber, as well as the effects of the tire structure.

[0049] The present invention will be further explained below with reference to examples, but the scope of the present invention is not limited to these examples.

[0050] Ten types of tread rubber (CAP1-10) with the formulations shown in Table 1 and five types of belt cushion rubber (BC1-5) with the formulations shown in Table 2 were prepared, and test tires (tire size: 11R22.5) were produced using a tread layer made of any of the tread rubbers (CAP1-10) and a belt cushion layer made of any of the belt cushion rubbers (BC1-5) in combination as shown in Tables 3 to 5. Each test tire has the basic structure shown in Figure 1 and the tread pattern shown in Figure 2, with the hardness Hc of the tread rubber, the hardness Hb of the belt cushion rubber, the hardness ratio Hc / Hb, the tread width W, the width Wsh of the shoulder land portion, the ratio Wsh / W of these widths, the tread gauge Gcc, the groove width Wo of the shoulder main groove, the groove width Wi of the center circumferential groove, the ratio Wi / Wo of these groove widths, and the contact length ratio Ls / Lc set as shown in Tables 3 to 5, respectively.

[0051] Using each test tire, evaluation of low rolling resistance, uneven wear resistance, and durability was carried out by the methods described below.

[0052] Low Rolling Resistance Each test tire was mounted on a wheel with a rim size of 22.5 x 9.0, and the rolling resistance coefficient of the test tire was measured using a drum testing machine with a drum diameter of 1707 mm in accordance with ISO 28580 under conditions of a load of 28.76 kN, an air pressure of 900 kPa, and a speed of 60 km / h. The evaluation results were expressed as an index, with Standard Example 1 being 100. The smaller the index value, the lower the rolling resistance, meaning that the tire has excellent low rolling resistance.

[0053] Each test tire was mounted on a wheel with a rim size of 22.5 x 9.0, and the tire was mounted on a test vehicle (2-D tractor head) with an air pressure of 830 kPa. After driving 50,000 km on a test course consisting of a paved road surface, the wear amount of the center circumferential groove and the shoulder main groove (the difference in groove depth before and after driving) was measured at five points around the tire, and the difference between the wear amount of the center circumferential groove and the wear amount of the shoulder main groove was calculated. The evaluation results were expressed as an index using the reciprocal of the measured value, with Standard Example 1 being 100. The larger the index value, the smaller the difference in wear amount between the center region and the shoulder region, and the better the resistance to uneven wear.

[0054] Durability Each test tire was mounted on a wheel with a rim size of 22.5 x 9.0, and the running distance until the tire failed was measured using a drum testing machine with a drum diameter of 1707 mm under the conditions of a load of 41.19 kN, an air pressure of 830 kPa, a speed of 45 km / h, and a slip angle of ±2° (frequency of 0.1 Hz). The evaluation results were expressed as an index, with Standard Example 1 being 100. The higher the index value, the longer the running distance until the tire failed, indicating superior durability.

[0055]

[0056] The types of raw materials used in Table 1 are as follows: NR: Natural rubber, RSS No. 3 BR: Butadiene rubber, Nipol 1220 manufactured by Nippon Zeon Co., Ltd. CB1: Carbon black (grade: ISAF), Nitelon #300IH manufactured by Nippon Steel Carbon Co., Ltd. (Dst: 83 nm, nitrogen adsorption specific surface area NSA: 115 m) 2 / g) CB2: Carbon black (grade: HAF), manufactured by Nippon Steel Carbon Co., Ltd., Nitelon #200IN (Dst: 114 nm, nitrogen adsorption specific surface area NSA: 72 m 2 / g) Silica: ZEOSIL 1165MP manufactured by Solvay Japan (CTAB adsorption specific surface area: 165 m 2 / g) Antioxidant: Santoflex 6PPD manufactured by Solutia Europe Wax: OZOACE-0015A manufactured by Nippon Seiro Co., Ltd. Zinc oxide: 3 types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. Stearic acid: Stearic acid: Stearic acid: Stearic acid YR manufactured by NOF Corporation Vulcanization accelerator 1: Sulfenamide-based vulcanization accelerator Noccela NS-P (NS) manufactured by Ouchi Shinko Chemical Co., Ltd. Vulcanization accelerator 2: Guanidine-based vulcanization accelerator Soxinol D-G (DPG) manufactured by Sumitomo Chemical Co., Ltd. Sulfur: Oil-treated sulfur manufactured by Karuizawa Refinery Co., Ltd.

[0057]

[0058] The types of raw materials used in Table 2 are as follows: NR: Natural rubber, RSS No. 3 BR: Butadiene rubber, Nipol 1220 manufactured by Nippon Zeon Co., Ltd. CB1: Carbon black (grade: ISAF), Nitelon #300IH manufactured by Nippon Steel Carbon Co., Ltd. (Dst: 83 nm, nitrogen adsorption specific surface area NSA: 115 m) 2 / g) CB2: Carbon black (grade: HAF), manufactured by Nippon Steel Carbon Co., Ltd., Nitelon #200IN (Dst: 114 nm, nitrogen adsorption specific surface area NSA: 72 m 2 / g) CB3: Carbon black (grade: FAF), Nitelon #10N (Dst: 166 nm, nitrogen adsorption specific surface area NSA: 41 m) manufactured by Nippon Steel Carbon Co., Ltd. 2 / g) Antioxidant: Santoflex 6PPD manufactured by Solutia Europe Oil: Diana Process NH-70S manufactured by Idemitsu Kosan Zinc oxide: Three types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. Stearic acid: Stearic acid YR manufactured by NOF Corporation Vulcanization accelerator 1: Noccela NS-P (NS) manufactured by Ouchi Shinko Chemical Co., Ltd. Sulfur: Oil-treated sulfur manufactured by Karuizawa Refinery

[0059]

[0060]

[0061]

[0062] As is clear from Tables 3 to 5, Examples 1 to 18 maintained or improved low rolling resistance, uneven wear resistance, and durability compared to Reference Example 1, and achieved a good balance of these performance properties.

[0063] On the other hand, Comparative Example 1 had a small hardness ratio Hc / Hb, resulting in reduced durability. Comparative Example 2 used CAP1, which does not satisfy the conditions of the present invention as the tread rubber (the ratio of the carbon black to silica compounding amounts was outside the range of the present invention: the carbon black amount was excessive), resulting in reduced uneven wear resistance and durability. Comparative Example 3 used CAP2, which does not satisfy the conditions of the present invention as the tread rubber (the ratio of the carbon black to silica compounding amounts was outside the range of the present invention: the silica amount was excessive), resulting in reduced uneven wear resistance. Comparative Example 4 used CAP3, which does not satisfy the conditions of the present invention as the tread rubber (the filler compounding amount was excessive), resulting in reduced low rolling resistance. Comparative Example 5 used CAP4, which does not satisfy the conditions of the present invention as the tread rubber (the isoprene-based rubber content was too low), resulting in reduced uneven wear resistance. Comparative Example 6 used BC1, which does not satisfy the conditions of the present invention as the belt cushion rubber (the filler compounding amount (total carbon black) was excessive), resulting in reduced durability. In Comparative Example 7, durability was reduced because BC2 (content of isoprene-based rubber is too low) which does not satisfy the conditions of the present invention was used as the belt cushion rubber. In Comparative Example 8, durability was reduced because the hardness ratio Hc / Hb was large.

[0064] REFERENCE SIGNS LIST 1 tread portion 2 sidewall portion 3 bead portion 4 carcass layer 5 bead core 6 bead filler 7 belt layer 8 belt reinforcing layer 10 tread rubber layer 11 cap tread layer 12 under tread layer 20 side rubber layer 30 rim cushion rubber layer 40 belt cushion layer 50 circumferential groove 51 shoulder main groove 52 center circumferential groove CL tire equator

Claims

1. A heavy-duty pneumatic tire comprising a tread portion extending in the 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 on the radially inward side of the sidewall portions, at least one carcass layer mounted between the pair of bead portions, and multiple belt layers disposed on the outer circumferential side of the carcass layer in the tread portion, with a belt cushion layer disposed between the tire width direction end of the belt layer and the carcass layer, wherein the tread rubber constituting the tread portion is made of a rubber composition in which 60 parts by mass or less of a filler containing carbon black and silica is blended with respect to 100 parts by mass of a rubber component containing 90% by mass or more of isoprene-based rubber, and the ratio of the amount of the carbon black to the amount of the silica in the filler is 3:2 to 1:1, and the belt cushion rubber constituting the belt cushion layer is made of a rubber composition in which 45 parts by mass or less of carbon black is blended with respect to 100 parts by mass of a rubber component containing 90% by mass or more of isoprene-based rubber, A heavy-duty pneumatic tire, characterized in that the hardness Hc of said tread rubber at 20° C. and the hardness Hb of said belt cushion rubber at 20° C. satisfy the relationship: 1.2≦Hc / Hb≦1.

5.

2. The rubber composition constituting the tread rubber contains a guanidine-based vulcanization accelerator and a sulfenamide-based vulcanization accelerator, and the blending amount Ac of the guanidine-based vulcanization accelerator is G and the blending amount Ac of the sulfenamide vulcanization accelerator S and 0.3≦Ac G / Ac S 2. The heavy duty pneumatic tire according to claim 1, wherein the relationship of ≦5.0 is satisfied.

3. A heavy-duty pneumatic tire as described in claim 1 or 2, characterized in that when a plurality of circumferential grooves extending circumferentially along the tire are formed on the surface of the tread portion, and a pair of the plurality of circumferential grooves located on the outermost sides in the tire width direction are shoulder main grooves, shoulder land portions are defined on the outer sides of each shoulder main groove in the tire width direction, and the width Wsh of each shoulder land portion satisfies the relationship 0.15≦Wsh / W≦0.27 with respect to the contact width W of the tread portion.

4. The heavy-duty pneumatic tire according to claim 3, characterized in that the plurality of circumferential grooves include the shoulder main groove and a central circumferential groove disposed more inward in the tire width direction than the shoulder main groove, and a groove width Wo of the shoulder main groove and a groove width Wi of the central circumferential groove satisfy the relationship: 0.20≦Wi / Wo≦1.

20.

5. A heavy-duty pneumatic tire according to any one of claims 1 to 4, characterized in that a tread gauge Gcc measured at the tire equator is 12.0 mm or more and 30.0 mm or less.

6. A heavy-duty pneumatic tire as described in any one of claims 1 to 5, characterized in that, when the contact length of the tread center land row is Lc and the contact length of the land row at the axially outermost side of the tire is Ls, the maximum contact length Lc and the outer contact length Ls satisfy the relationship: 0.80≦Ls / Lc≦1.05.

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