Pneumatic tire for heavy load

Abstract

To provide a pneumatic tire for a heavy load improved in low fuel consumption performance and rib tear resistance performance.SOLUTION: A pneumatic tire for a heavy load comprises a tread 1. The tread has a cap rubber layer 5 constituting a tread face and a base rubber layer 4 arranged adjacent to inside in a tire radial direction of the cap rubber layer. The cap rubber layer and the base rubber layer are constituted of rubber compositions containing rubber constituents. Extraction amounts of acetone of the rubber compositions constituting the base rubber layer are less than 4.0 mass%. When modulus at the time when the rubber compositions constituting the base rubber layer is stretched by 200% at 23°C is defined as M200b (MPa) and modulus at the time when the rubber compositions constituting the cap rubber layer is stretched by 200% at 23°C is defined as M200c (MPa), M200b and M200c satisfy the following formula (1): 0≤M200c-M200b≤5 (1).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a heavy-duty pneumatic tire. [Background technology]

[0002] As a method for improving the breaking strength of truck and bus tires, a technique of micronizing or highly structuring carbon black is known (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 6-279624 Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned methods of microparticulating or highly structuring carbon black are not sufficient to improve the fuel economy or rib tear resistance of tires. Furthermore, the deterioration of processability associated with microparticulation can also worsen the dispersibility of carbon black, which in turn can worsen the abrasion resistance of tires. Therefore, there are limitations to the conventional methods of improving performance by improving carbon black.

[0005] Furthermore, due to the influence of recent environmental regulations, there is an increasing demand for truck and bus tires to not only have high wear resistance, but also high levels of fuel efficiency and fracture resistance.

[0006] An object of the present invention is to provide a heavy-duty pneumatic tire having improved fuel economy and rib tear resistance. [Means for solving the problem]

[0007] The inventors discovered that the above problem can be solved by setting the difference in modulus between the base rubber layer and the cap rubber layer at 200% stretch within a predetermined range and by setting the acetone extractable amount of the base rubber layer within a predetermined range, and thus completed the present invention.

[0008] That is, the present invention is [1] A heavy-duty pneumatic tire having a tread, the tread having a cap rubber layer constituting a tread surface and a base rubber layer adjacent to the cap rubber layer on the radially inner side thereof, the cap rubber layer and the base rubber layer being made of a rubber composition containing a rubber component, the acetone extractable amount of the rubber composition constituting the base rubber layer being less than 4.0% by mass, and the modulus of the rubber composition constituting the base rubber layer at 200% elongation at 23°C being M200b (MPa), and the modulus of the rubber composition constituting the cap rubber layer at 200% elongation at 23°C being M200c (MPa), M200b and M200c satisfy the following formula (1): 0≦M200c-M200b≦5 (1) [2] The heavy-duty pneumatic tire according to the above [1], wherein the tan δ (70°C tan δ) at 70°C of the rubber composition constituting the base rubber layer is less than 0.040. [3] The heavy-duty pneumatic tire according to [1] or [2] above, wherein, when the strength at break of the rubber composition constituting the base rubber layer at 23°C is TB (MPa) and the elongation at break at 23°C is EB (%), the tan δ at 70°C (70°C tan δ), TB, and EB of the rubber composition constituting the base rubber layer satisfy the following formula (2): TB×EB / 70℃ tanδ≧4.5×10 5 ···(2) [4] The heavy-duty pneumatic tire according to any one of [1] to [3] above, wherein the rubber component constituting the base rubber layer contains butadiene rubber. [5] A heavy-duty pneumatic tire according to any one of [1] to [4] above, wherein the thickness of the base rubber layer is 1 to 70% of the total thickness of the tread. [6] A heavy-duty pneumatic tire according to any one of the above [1] to [5], which includes four or more belt layers on the radially inner side of the base rubber layer, and at least one of the radially outer side surface of the radially outermost belt layer of the tire and the radially outer side surface of the widest belt layer is covered with a reinforcing rubber layer that ends without reaching the tire equatorial plane, and the value of tan δ (70°C tan δ) at 70°C of the rubber composition constituting the reinforcing rubber layer is larger than the value of tan δ (70°C tan δ) at 70°C of the rubber composition constituting the base rubber layer. [7] A heavy-duty pneumatic tire according to any one of [1] to [6] above, wherein the width w of the reinforcing rubber layer is 10 to 40% of the tread width W of the tread. [8] A heavy-duty pneumatic tire according to any one of the above [1] to [7], wherein the maximum thickness of the reinforcing rubber layer is 5 to 25% of the groove depth of a widthwise groove located at a position 1 / 4 of the tread width from the tire equator line. [9] The heavy-duty pneumatic tire according to any one of the above [1] to [8], wherein the rubber composition constituting the base rubber layer contains a glycerin fatty acid ester. [Effects of the Invention]

[0009] According to the present invention, a heavy-duty pneumatic tire is provided which has improved fuel economy and rib tear resistance. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view in the tread width direction showing half of a tread portion of a heavy-duty pneumatic tire of the present disclosure. [Figure 2] 1 is a front view schematically showing an example of a test device used to evaluate the rib tear resistance performance of a heavy-duty pneumatic tire in the present disclosure. FIG. [Figure 3] FIG. 2 is a plan view of a protrusion jig and a reference surface of the testing device. [Figure 4] FIG. 2 is an enlarged view of a main part of the testing device. DETAILED DESCRIPTION OF THE INVENTION

[0011] The heavy-duty pneumatic tire according to the present disclosure is a heavy-duty pneumatic tire having a tread, the tread having a cap rubber layer constituting the tread surface and a base rubber layer adjacent to the cap rubber layer radially inward of the tire, the cap rubber layer and the base rubber layer being composed of a rubber composition containing a rubber component, the acetone extractable amount of the rubber composition constituting the base rubber layer being less than 4.0 mass%, and when the modulus of the rubber composition constituting the base rubber layer at 200% elongation at 23°C is M200b (MPa) and the modulus of the rubber composition constituting the cap rubber layer at 200% elongation at 23°C is M200c (MPa), M200b and M200c satisfy the following formula (1): 0≦M200c-M200b≦5 (1)

[0012] Although not intending to be bound by theory, the mechanism by which the effects of the present disclosure are exhibited is thought to be, for example, as follows.

[0013] If the modulus balance between the base rubber layer and the cap rubber layer becomes poor, this may cause damage to the tread, such as peeling at the interface between the cap rubber layer and the base rubber layer or cracks within the base tread rubber.

[0014] Therefore, by setting the difference in modulus between the base rubber layer and the cap rubber layer at 200% stretch within a predetermined range and setting the amount of acetone extractable from the base rubber layer within a predetermined range, it is believed that the modulus balance between the base rubber layer and the cap rubber layer can be optimized, suppressing heat generation in the tread rubber while improving rib tear resistance.

[0015] Furthermore, it is believed that the structure of the reinforcing rubber layer, which will be described later, and the above-mentioned physical properties of the rubber composition constituting each layer of the tread cooperate to improve fuel economy and rib tear resistance.

[0016] The rubber composition constituting the base rubber layer preferably has a tan δ at 70° C. (70° C. tan δ) of less than 0.040.

[0017] When the strength at break of the rubber composition constituting the base rubber layer at 23°C is TB (MPa) and the elongation at break at 23°C is EB (%), it is preferable that the tan δ at 70°C (70°C tan δ), TB, and EB of the rubber composition constituting the base rubber layer satisfy the following formula (2): TB×EB / 70℃ tanδ≧4.5×10 5 ···(2)

[0018] The rubber component constituting the base rubber layer preferably contains butadiene rubber.

[0019] The thickness of the base rubber layer is preferably 1 to 70% of the total thickness of the tread.

[0020] The heavy-duty pneumatic tire according to the present disclosure includes four or more belt layers radially inward of the base rubber layer, and at least one of the radially outer side surface of the radially outermost belt layer and the radially outer side surface of the widest belt layer is covered with a reinforcing rubber layer that ends without reaching the tire equatorial plane, and it is preferable that the value of tan δ (70°C tan δ) at 70°C of the rubber composition constituting the reinforcing rubber layer is larger than the value of tan δ (70°C tan δ) at 70°C of the rubber composition constituting the base rubber layer.

[0021] The width w of the reinforcing rubber layer is preferably 10 to 40% of the tread width W of the tread.

[0022] The maximum thickness of the reinforcing rubber layer is preferably 5 to 25% of the groove depth of a widthwise groove located at a position 1 / 4 of the tread width from the tire equator line.

[0023] The rubber composition constituting the base rubber layer preferably contains a glycerin fatty acid ester.

[0024] A heavy-duty pneumatic tire including a rubber composition for treads according to an embodiment of the present disclosure will be described in detail below. However, the following description is an example for explaining the present disclosure, and is not intended to limit the technical scope of the present invention to the described range. In this specification, when a numerical range is indicated using "to", it is intended to include both ends of the range.

[0025] <Heavy-duty pneumatic tires> FIG. 1 shows a cross-sectional view in the tread width direction illustrating half of the tread portion of a heavy-duty pneumatic tire of the present disclosure, but the present disclosure is not limited thereto.

[0026] In Fig. 1, 1 denotes a tread portion, 2 denotes a carcass consisting of one or more carcass plies extending toroidally between one bead core and the other bead core, and 3 denotes a belt consisting of five laminated belt layers 3a to 3e disposed radially outward of carcass 2. A base rubber layer 4 and a cap rubber layer 5 are laminated radially outward of belt 3.

[0027] Here, at least one of the radially outer side surface of the radially outermost belt layer 3e of the tire and the radially outer side surface of the widest belt layer 3c (in FIG. 1, the radially outer side surface of the radially outermost belt layer 3e of the tire) is covered with a reinforcing rubber layer 7 that ends radially inside the tread rubber 6 without reaching the tire equatorial plane E.

[0028] By making the value of tan δ at 70°C (70°C tan δ) of the rubber composition constituting the reinforcing rubber layer 7 larger than the value of tan δ at 70°C (70°C tan δ) of the rubber composition constituting the base rubber layer, it is possible to achieve a high level of both heat buildup durability of the tire and durability of the belt.

[0029] The width w of the reinforcing rubber layer 7 is preferably 10 to 40% of the tread width W, and more preferably 15 to 30%. The maximum thickness t of the reinforcing rubber layer 7 is preferably 5 to 25% of the groove depth of the widthwise groove located at 1 / 4 of the tread width from the tire equator, and more preferably 7 to 20%. These characteristics effectively prevent damage from progressing into the base rubber 4, while allowing the low-heat-generation base rubber layer 4 to fully demonstrate its inherent functions, thereby imparting high durability to the tread rubber 6. In this disclosure, the term "tread width" refers to the tread contact width, which is the linear distance parallel to the tire axis between the axially outermost contact positions (tread edges) of the tire when the tire is mounted on an applicable rim, inflated to a specified air pressure, and placed perpendicularly with a camber angle of zero degrees relative to a flat plate and subjected to a load equivalent to the maximum load capacity.

[0030] In the present disclosure, "70°C tan δ" conforms to JIS K 6394:2007 and refers to the loss tangent tan δ under the conditions of a temperature of 70°C, an initial strain of 5%, a dynamic strain of ±1%, and a frequency of 10 Hz.

[0031] From the viewpoint of fuel economy, the 70° C. tan δ of the rubber composition constituting the base rubber layer 4 is preferably less than 0.040, more preferably less than 0.039, and even more preferably less than 0.037.

[0032] The 70° C. tan δ of the base rubber layer 4 can be adjusted appropriately by the types and amounts of rubber components, fillers, silane coupling agents, oils, glycerin fatty acid esters, etc., which will be described later.

[0033] In this disclosure, the strength at break (TB) (MPa) and elongation at break (EB) (%) refer to the strength at break and elongation at break (elongation at break) measured in accordance with JIS K 6251:2017 in an atmosphere of 23°C at a tensile speed of 3.3 mm / sec.

[0034] In the present disclosure, the modulus at 200% elongation refers to the tensile stress at 200% elongation in the grain direction, measured in accordance with JIS K 6251:2017 under conditions of an atmosphere of 23°C and a tensile speed of 3.3 mm / sec.

[0035] The modulus M200b of the rubber composition constituting the base rubber layer 4 at 200% stretching is preferably 4.0 MPa or more, more preferably 4.5 MPa or more, even more preferably 5.0 MPa or more, and particularly preferably 5.5 MPa or more, from the viewpoints of preventing deterioration of fuel economy due to distortion of the rubber, preventing a decrease in steering stability, and preventing uneven wear. On the other hand, M200b is preferably 15.0 MPa or less, more preferably 14.0 MPa or more, even more preferably 13.0 MPa or less, and particularly preferably 12.0 MPa or less. If M200b exceeds 15.0 MPa, external forces are difficult to release, and input is concentrated at the interface between the cap rubber layer 5 and the base rubber layer 4, which may cause cracks to grow at the interface.

[0036] When the modulus of the rubber composition constituting the base rubber layer 4 of the present disclosure at 200% elongation at 23°C is M200b (MPa), and the modulus of the rubber composition constituting the cap rubber layer 5 at 23°C at 200% elongation is M200c (MPa), M200b and M200c satisfy the following formula (1). 0≦M200c-M200b≦5 (1)

[0037] It is believed that by setting M200c-M200b to 5 MPa or less and improving the modulus balance between the base rubber layer 4 and the cap rubber layer 5, damage to the tread such as peeling at the interface between the base rubber layer 4 and the cap rubber layer 5 and cracks within the base rubber layer 4 can be suppressed.

[0038] M200c-M200b is preferably 4 MPa or less, more preferably 3 MPa or less.

[0039] The modulus, TB and EB at 200% stretching of each rubber layer can be adjusted appropriately by the types and amounts of rubber components, fillers, silane coupling agents, softeners, etc., which will be described later.

[0040] The acetone extractable amount in this disclosure is an index of the concentration of organic low-molecular-weight compounds in the plasticizer contained in the vulcanized rubber composition. In accordance with JIS K 6229-3:2015, each vulcanized rubber test piece is immersed in acetone for 24 hours to extract the soluble components, and the mass of each test piece is measured before and after extraction, and the acetone extractable amount can be calculated using the following formula: Acetone extractable amount (%) = {(mass of rubber test piece before extraction - mass of rubber test piece after extraction) / (mass of rubber test piece before extraction)} × 100

[0041] The acetone extractable amount of the rubber composition constituting the base rubber layer 4 is less than 4.0% by mass, preferably less than 3.9% by mass, more preferably less than 3.8% by mass, even more preferably less than 3.7% by mass, and particularly preferably less than 3.6% by mass. By setting the difference in acetone extractable amount within the above range, a high level of balance between fuel economy and fracture resistance can be achieved.

[0042] From the viewpoint of a balance between tire durability and fuel economy, it is preferable that the 70°C tan δ, the strength at break TB (MPa) at 23°C, and the elongation at break EB (%) at 23°C of the rubber composition constituting the base rubber layer 4 satisfy the following formula (2): TB×EB / 70℃ tanδ≧4.5×10 5 ···(2)

[0043] TB×EB / 70℃ tanδ is 4.6×10 5 More than 4.7 × 10 is preferable. 5 More than 4.8 × 10 is more preferable. 5 More preferably, 4.9 × 10 5 The above is particularly preferred.

[0044] [Rubber composition] The heavy-duty pneumatic tire of the present disclosure can improve fuel economy and rib tear resistance due to the cooperation of the above-described structure of the reinforcing rubber layer and the above-described physical properties of the rubber composition constituting each layer of the tread.

[0045] <Rubber component> The rubber composition constituting each rubber layer of the tread according to the present disclosure (rubber composition for tread) preferably contains at least one rubber component selected from the group consisting of isoprene-based rubber, styrene butadiene rubber (SBR), and butadiene rubber (BR). In particular, natural rubber is preferably blended to ensure durability and abrasion resistance. The rubber component constituting each rubber layer of the tread may be a rubber component consisting only of isoprene-based rubber, or a rubber component consisting only of isoprene-based rubber and BR.

[0046] (Isoprene rubber) Examples of isoprene-based rubbers that can be used include those commonly used in the tire industry, such as isoprene rubber (IR) and natural rubber. Natural rubber includes unmodified natural rubber (NR) as well as modified natural rubbers such as epoxidized natural rubber (ENR), hydrogenated natural rubber (HNR), deproteinized natural rubber (DPNR), highly purified natural rubber, and grafted natural rubber. These isoprene-based rubbers may be used alone or in combination of two or more.

[0047] The NR is not particularly limited, and those commonly used in the tire industry can be used, such as SIR20, RSS#3, and TSR20.

[0048] The content of the isoprene-based rubber in the rubber component is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more, and the rubber component may consist solely of isoprene-based rubber.

[0049] (BR) The BR is not particularly limited, and can be one commonly used in the tire industry, such as BR with a cis content (cis-1,4 bond content) of 90% by mass or more (high-cis BR), rare earth butadiene rubber synthesized using a rare earth catalyst (rare earth BR), BR containing syndiotactic polybutadiene crystals (SPB-containing BR), or modified BR (high-cis modified BR, low-cis modified BR). Modified BR is preferred for its reinforcing properties with filler. SPB-containing BR can also be used to ensure strength when the filler is reduced.

[0050] Examples of high-cis BR include those manufactured by Zeon Corporation, Ube Industries, Ltd., and JSR Corporation. The inclusion of high-cis BR can improve low-temperature properties and wear resistance. The cis content of the high-cis BR is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 96% by mass or more, and particularly preferably 97% by mass or more. In this specification, the cis content is a value calculated by infrared absorption spectroscopy.

[0051] The rare earth BR is synthesized using a rare earth catalyst, has a vinyl bond content (amount of 1,2-bonded butadiene units) of preferably 1.8 mol % or less, more preferably 1.0 mol % or less, and even more preferably 0.8 mol % or less, and has a cis content (cis-1,4 bond content) of preferably 95 mass % or more, more preferably 96 mass % or more, and even more preferably 97 mass % or more. Examples of rare earth BR that can be used include those manufactured by Lanxess K.K.

[0052] SPB-containing BR is one in which 1,2-syndiotactic polybutadiene crystals are dispersed chemically with the BR, rather than simply dispersed in the BR. Examples of such SPB-containing BR include those manufactured by Ube Industries, Ltd.

[0053] The modified BR preferably used is a terminal-modified BR coupled with tin or a terminal-modified BR having an alkoxysilyl group and / or an amino group. The modified BR may be either non-hydrogenated or hydrogenated. Examples of such modified BR include those manufactured by Zeon Corporation and Asahi Kasei Chemicals Corporation.

[0054] The terminally modified BR coupled with tin is preferably obtained by polymerizing 1,3-butadiene using a lithium initiator and then adding a tin compound, and the terminals of the modified BR molecule are preferably bonded with a tin-carbon bond. Examples of lithium initiators include lithium-based compounds such as alkyllithium, aryllithium, vinyllithium, organotinlithium, and organonitrogenlithium compounds, as well as lithium metal. Using the lithium initiator as the initiator for the modified BR allows for the production of a modified BR with a high vinyl and low cis content. Examples of tin compounds include tin tetrachloride, butyltin trichloride, dibutyltin dichloride, dioctyltin dichloride, tributyltin chloride, triphenyltin chloride, diphenyldibutyltin, triphenyltin ethoxide, diphenyldimethyltin, ditolyltin chloride, diphenyltin dioctanoate, divinyldiethyltin, tetrabenzyltin, dibutyltin distearate, tetraallyltin, and p-tributyltin styrene. These tin compounds may be used alone or in combination of two or more.

[0055] The BRs listed above may be used alone or in combination of two or more.

[0056] The weight-average molecular weight (Mw) of the BR is preferably 300,000 or more, more preferably 350,000 or more, and even more preferably 400,000 or more, from the viewpoints of abrasion resistance and grip performance. Furthermore, from the viewpoint of crosslink uniformity, it is preferably 2,000,000 or less, more preferably 1,000,000 or less. The Mw can be calculated in terms of standard polystyrene based on measurements obtained by gel permeation chromatography (GPC) (GPC-8000 series manufactured by Tosoh Corporation, detector: differential refractometer, column: TSKGEL SUPERMALTPORE HZ-M manufactured by Tosoh Corporation).

[0057] When BR is contained, the content in the rubber component is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 12% by mass or more, and particularly preferably 15% by mass or more from the viewpoint of fuel economy performance. Also, from the viewpoint of breaking strength, the content is preferably 28% by mass or less, more preferably 25% by mass or less, even more preferably 22% by mass or less, and particularly preferably 20% by mass or less.

[0058] (SBR) The SBR is not particularly limited, and examples thereof include solution-polymerized SBR (S-SBR), emulsion-polymerized SBR (E-SBR), and modified SBRs thereof (modified S-SBR, modified E-SBR). Modified SBRs include SBRs whose terminals and / or main chains are modified, and modified SBRs (condensates, those having a branched structure, etc.) coupled with tin, silicon compounds, etc. Among these, E-SBR is preferred because it can satisfactorily improve fuel economy and wear resistance. These SBRs may be used alone or in combination of two or more.

[0059] When SBR is contained, the content in the rubber component is not particularly limited and can be, for example, 1% by mass or more, 5% by mass or more, 10% by mass or more, 30% by mass or less, 25% by mass or less, or 20% by mass or less.

[0060] (Other rubber components) The rubber component according to the present disclosure may contain rubber components other than the isoprene-based rubber, SBR, and BR. Examples of other rubber components that can be used include crosslinkable rubber components commonly used in the tire industry, such as styrene-isoprene-butadiene copolymer rubber (SIBR), styrene-isobutylene-styrene block copolymer (SIBS), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), hydrogenated nitrile rubber (HNBR), butyl rubber (IIR), ethylene propylene rubber, polynorbornene rubber, silicone rubber, chlorinated polyethylene rubber, fluororubber (FKM), acrylic rubber (ACM), and hydrin rubber. These other rubber components may be used alone or in combination of two or more.

[0061] <Filler> The rubber composition for a tread according to the present disclosure preferably contains a filler containing carbon black and / or silica. The filler may be a filler consisting of only carbon black, or a filler consisting of only carbon black and silica.

[0062] (silica) By blending silica into the rubber composition for treads according to the present disclosure, fuel economy, fracture resistance, and abrasion resistance can be improved. The silica is not particularly limited, and silica commonly used in the tire industry, such as silica prepared by a dry method (anhydrous silica) or silica prepared by a wet method (hydrated silica), can be used. Among these, hydrated silica prepared by a wet method is preferred because it contains a large number of silanol groups. These silicas may be used alone or in combination of two or more.

[0063] The average particle size of silica is preferably 50 nm or less, more preferably 40 nm or less, and even more preferably 30 nm or less. The average particle size of silica is preferably 13 nm or more, more preferably 15 nm or more, and even more preferably 17 nm or more. When the average particle size of silica is within the above range, the effects of the present disclosure tend to be more effectively exhibited. The average particle size of silica is a number average particle size, and is measured using a transmission electron microscope as the average value of 100 random particles.

[0064] The specific surface area of ​​silica cetyltrimethylammonium bromide (CTAB) is 80m 2 / g or more is preferable, and 90m 2 / g or more is more preferable, and 100m 2 / g or more is more preferable. The CTAB specific surface area of ​​silica is 300 m 2 / g or less is more preferable, and 170m 2 / g or less is more preferable, and 150m 2 / g or less is more preferable. When the CTAB specific surface area of ​​silica is within the above range, the effects of the present disclosure tend to be more favorably exhibited. Note that the CTAB specific surface area of ​​silica in this specification is a value measured in accordance with ASTM D3765-92.

[0065] The nitrogen adsorption specific surface area (N2SA) of silica is 80m 2 / g or more is preferable, and 110m 2 / g or more is more preferable, and 140m 2 / g or more is more preferable, and 170m 2 / g or more is particularly preferable. 2 / g or less is preferable, and 300m 2 / g or less is more preferable, and 250m 2 / g or less is more preferable. When the N2SA of silica is within the above range, the effects of the present disclosure tend to be more favorably exhibited. Note that the N2SA of silica in this specification is a value measured by the BET method in accordance with ASTM D3037-93.

[0066] The content of silica per 100 parts by mass of the rubber component is preferably 20 parts by mass or less, more preferably 18 parts by mass or less, even more preferably 15 parts by mass or less, and particularly preferably 10 parts by mass or less, from the viewpoint of fuel economy performance, while the content is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, from the viewpoint of breaking elongation.

[0067] (carbon black) The carbon black is not particularly limited, and can be, for example, one commonly used in the tire industry, such as GPF, FEF, HAF, ISAF, SAF, etc. These carbon blacks may be used alone or in combination of two or more.

[0068] The average particle size of the carbon black is preferably 90 nm or less, more preferably 70 nm or less, and even more preferably 50 nm or less. The average particle size of the carbon black is preferably 13 nm or more, more preferably 15 nm or more, and even more preferably 17 nm or more. When the average particle size of the carbon black is within the above range, the effects of the present disclosure tend to be more effectively exhibited. The average particle size of the carbon black is a number-average particle size, measured using a transmission electron microscope as the average value of 100 random particles.

[0069] The specific surface area of ​​carbon black cetyltrimethylammonium bromide (CTAB) is 80m 2 / g or more is preferable, and 90m 2 / g or more is more preferable, and 100m 2 / g or more is more preferable. The CTAB specific surface area of ​​carbon black is 170 m 2 / g or less is more preferable, and 165m 2 / g or less is more preferable, and 160m 2 / g or less is more preferable. When the CTAB specific surface area of ​​the carbon black is within the above range, the effects of the present disclosure tend to be more favorably exhibited. Note that the CTAB specific surface area of ​​the carbon black in this specification is a value measured in accordance with JIS K 6217-3:2001.

[0070] The nitrogen adsorption specific surface area (N2SA) of carbon black is 40m from the viewpoint of breaking strength. 2 / g or more is preferable, and 60m 2 / g or more is more preferable, and 80m 2 / g or more is more preferable. 2 / g or less is preferable, and 150m 2 / g or less is more preferable, and 130m 2 The N2SA of carbon black in this specification is a value measured in accordance with Method A of JIS K 6217-2 "Fundamental properties of carbon black for rubber use - Part 2: Determination of specific surface area - Nitrogen adsorption method - Single point method."

[0071] The amount of carbon black per 100 parts by mass of the rubber component is preferably 20 parts by mass or more, more preferably 25 parts by mass or more, and even more preferably 30 parts by mass or more, from the viewpoint of fuel economy performance. Also, from the viewpoint of fuel economy performance, the amount is preferably 60 parts by mass or less, more preferably 55 parts by mass or less, even more preferably 50 parts by mass or less, and particularly preferably 45 parts by mass or less.

[0072] Fillers other than silica and carbon black that can be used include those commonly used in the tire industry, such as aluminum hydroxide, calcium carbonate, alumina, clay, and talc.

[0073] The silica content in the total of 100% by mass of silica and carbon black is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 5% by mass or more, and particularly preferably 7% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less.

[0074] The total content of silica and carbon black per 100 parts by mass of the rubber component is preferably 25 parts by mass or more, more preferably 30 parts by mass or more, and even more preferably 35 parts by mass or more, from the viewpoint of abrasion resistance. Also, from the viewpoint of suppressing deterioration in fuel economy and abrasion resistance, the total content is preferably 80 parts by mass or less, more preferably 70 parts by mass or less, even more preferably 60 parts by mass or less, and particularly preferably 55 parts by mass or less.

[0075] (Silane coupling agent) Silica is preferably used in combination with a silane coupling agent.The silane coupling agent is not particularly limited, but examples thereof include silane coupling agents having a sulfide group such as bis(3-triethoxysilylpropyl)disulfide and bis(3-triethoxysilylpropyl)tetrasulfide; silane coupling agents having a mercapto group such as 3-mercaptopropyltrimethoxysilane, NXT-Z100, NXT-Z45, and NXT manufactured by Momentive; silane coupling agents having a vinyl group such as vinyltriethoxysilane and vinyltrimethoxysilane; 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane; Examples of suitable silane coupling agents include silane coupling agents having an amino group such as silane or 3-(2-aminoethyl)aminopropyltriethoxysilane; glycidoxy-based silane coupling agents such as γ-glycidoxypropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane; nitro-based silane coupling agents such as 3-nitropropyltrimethoxysilane or 3-nitropropyltriethoxysilane; and chloro-based silane coupling agents such as 3-chloropropyltrimethoxysilane or 3-chloropropyltriethoxysilane, among which silane coupling agents having a sulfide group are preferred. These silane coupling agents may be used alone or in combination of two or more.

[0076] When a silane coupling agent is contained, the content relative to 100 parts by mass of silica is preferably 8.0 parts by mass or more, more preferably 8.5 parts by mass or more, even more preferably 9.0 parts by mass or more, particularly preferably 9.5 parts by mass or more, from the viewpoint of improving the dispersibility of silica. Also, from the viewpoint of preventing the deterioration of abrasion resistance, it is preferably 18 parts by mass or less, more preferably 16 parts by mass or less, even more preferably 14 parts by mass or less, particularly preferably 12 parts by mass or less.

[0077] <Other compounding agents> In addition to the above components, the rubber composition for tread according to the present disclosure may contain, as appropriate, compounding agents that are generally used in the tire industry, such as oil, wax, glycerin fatty acid ester, processing aid, antioxidant, vulcanizing agent such as stearic acid, zinc oxide, sulfur, vulcanization accelerator, etc.

[0078] Examples of oils include process oil, vegetable oils, and animal fats. Examples of the process oil include paraffinic process oil, naphthenic process oil, and aromatic process oil. Furthermore, as an environmental measure, process oil with a low content of polycyclic aromatic compounds (PCA) can also be used. Examples of the low-PCA process oil include mild extract solvates (MES), treated distillate aromatic extracts (TDAE), and heavy naphthenic oil.

[0079] When oil is contained, the content per 100 parts by mass of the rubber component is preferably 10 parts by mass or less, more preferably 7 parts by mass or less, and even more preferably 4 parts by mass or less, from the viewpoint of the balance between fuel economy performance and breaking strength. In this specification, the oil content includes the amount of oil contained in the oil-extended rubber.

[0080] When wax is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, more preferably 1.0 part by mass or more, and even more preferably 1.5 parts by mass or more from the viewpoint of weather resistance of the rubber, and is preferably 8.0 parts by mass or less, more preferably 6.0 parts by mass or less from the viewpoint of preventing whitening of the tire due to bloom.

[0081] Glycerin fatty acid esters are ester-bonded to at least one of the three OH groups of glycerin, and are classified into glycerin fatty acid monoesters, glycerin fatty acid diesters, and glycerin fatty acid triesters depending on the number of fatty acids. The fatty acids constituting the glycerin fatty acid esters preferably have 8 to 28 carbon atoms, more preferably 8 to 22 carbon atoms, even more preferably 10 to 18 carbon atoms, and particularly preferably 12 to 18 carbon atoms. The fatty acids may be saturated, unsaturated, straight-chain, or branched, but straight-chain saturated fatty acids are preferred. Specific examples of fatty acids include capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, isostearic acid, oleic acid, and linoleic acid.

[0082] When a glycerin fatty acid ester is contained, the content per 100 parts by mass of the rubber component is not particularly limited, but is preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more, and even more preferably 1.0 part by mass or more. The content is preferably 10.0 parts by mass or less, more preferably 8.0 parts by mass or less, and even more preferably 6.0 parts by mass or less. By setting the content of the glycerin fatty acid ester within the above range, the dispersibility of the filler in the rubber composition can be improved, and fuel economy and abrasion resistance can be further improved.

[0083] Examples of processing aids include fatty acid metal salts, fatty acid amides, amide esters, silica surfactants, fatty acid esters, mixtures of fatty acid metal salts and amide esters, and mixtures of fatty acid metal salts and fatty acid amides. These processing aids may be used alone or in combination of two or more. Examples of processing aids that can be used include those commercially available from Schill + Seilacher, Performance Additives, etc.

[0084] When a processing aid is contained, the content thereof per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, from the viewpoint of improving processability, and is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, from the viewpoint of abrasion resistance and breaking strength.

[0085] The antioxidant is not particularly limited, but examples thereof include amine-based, quinoline-based, quinone-based, phenol-based, and imidazole-based compounds, as well as metal carbamates. Preferred are phenylenediamine-based antioxidants such as N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, N-isopropyl-N'-phenyl-p-phenylenediamine, N,N'-diphenyl-p-phenylenediamine, N,N'-di-2-naphthyl-p-phenylenediamine, and N-cyclohexyl-N'-phenyl-p-phenylenediamine, and quinoline-based antioxidants such as 2,2,4-trimethyl-1,2-dihydroquinoline polymer and 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline. These antioxidants may be used alone or in combination of two or more.

[0086] When an antioxidant is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, more preferably 1.0 part by mass or more, and even more preferably 1.5 parts by mass or more from the viewpoint of ozone crack resistance of the rubber, and is preferably 10.0 parts by mass or less, more preferably 5.0 parts by mass or less from the viewpoint of abrasion resistance and wet grip performance.

[0087] When stearic acid is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, from the viewpoint of processability, and is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, from the viewpoint of vulcanization rate.

[0088] When zinc oxide is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, more preferably 1.0 part by mass or more, and even more preferably 1.5 parts by mass or more from the viewpoint of processability, and is preferably 10.0 parts by mass or less, more preferably 5.0 parts by mass or less from the viewpoint of abrasion resistance.

[0089] As the vulcanizing agent, sulfur is preferably used, and examples of sulfur that can be used include powdered sulfur, oil-treated sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, and highly dispersible sulfur.

[0090] When sulfur is contained as a vulcanizing agent, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, more preferably 1.0 parts by mass or more, from the viewpoint of ensuring a sufficient vulcanization reaction and obtaining good grip performance and wear resistance. Furthermore, from the viewpoint of deterioration, the content is preferably 3.0 parts by mass or less, more preferably 2.5 parts by mass or less, and even more preferably 2.0 parts by mass or less. When oil-containing sulfur is used as the vulcanizing agent, the content of the vulcanizing agent is the total content of pure sulfur contained in the oil-containing sulfur.

[0091] Examples of vulcanizing agents other than sulfur include alkylphenol-sulfur chloride condensate, sodium 1,6-hexamethylene-dithiosulfate dihydrate, 1,6-bis(N,N'-dibenzylthiocarbamoyldithio)hexane, etc. These vulcanizing agents other than sulfur can be commercially available from Taoka Chemical Co., Ltd., Lanxess K.K., Flexis, etc.

[0092] Examples of vulcanization accelerators include sulfenamide-based, thiazole-based, thiuram-based, thiourea-based, guanidine-based, dithiocarbamic acid-based, aldehyde-amine-based or aldehyde-ammonia-based, imidazoline-based, and xanthate-based vulcanization accelerators. These vulcanization accelerators may be used alone or in combination of two or more. Among these, one or more vulcanization accelerators selected from the group consisting of sulfenamide-based, guanidine-based, and thiazole-based vulcanization accelerators are preferred.

[0093] Examples of sulfenamide vulcanization accelerators include N-tert-butyl-2-benzothiazolylsulfenamide (TBBS), N-cyclohexyl-2-benzothiazolylsulfenamide (CBS), N,N-dicyclohexyl-2-benzothiazolylsulfenamide (DCBS), etc. Among these, N-tert-butyl-2-benzothiazolylsulfenamide (TBBS) is preferred.

[0094] Examples of guanidine vulcanization accelerators include 1,3-diphenylguanidine (DPG), 1,3-di-o-tolylguanidine, 1-o-tolylbiguanide, di-o-tolylguanidine salt of dicatechol borate, 1,3-di-o-cumenylguanidine, 1,3-di-o-biphenylguanidine, 1,3-di-o-cumenyl-2-propionylguanidine, etc. Among these, 1,3-diphenylguanidine (DPG) is preferred.

[0095] Examples of the thiazole vulcanization accelerator include 2-mercaptobenzothiazole, cyclohexylamine salt of 2-mercaptobenzothiazole, di-2-benzothiazolyl disulfide, etc. Among these, 2-mercaptobenzothiazole is preferred.

[0096] When a vulcanization accelerator is contained, the content thereof per 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 2 parts by mass or more. The content thereof per 100 parts by mass of the rubber component is preferably 8 parts by mass or less, more preferably 7 parts by mass or less, and even more preferably 6 parts by mass or less. By setting the content of the vulcanization accelerator within the above range, breaking strength and elongation tend to be ensured.

[0097] The rubber composition according to the present disclosure can be produced by a known method, for example, by kneading the above-described components using a rubber kneading device such as an open roll or an internal kneader (such as a Banbury mixer or kneader).

[0098] The kneading step includes, for example, a base kneading step in which compounding ingredients and additives other than the vulcanizing agent and vulcanization accelerator are kneaded, and a final kneading (F kneading) step in which the vulcanizing agent and vulcanization accelerator are added to the kneaded product obtained in the base kneading step and kneaded. Furthermore, the base kneading step can be divided into multiple steps as desired.

[0099] The kneading conditions are not particularly limited, but examples include a method in which the base kneading step involves kneading for 3 to 10 minutes at a discharge temperature of 150 to 170°C, and a method in which the final kneading step involves kneading for 1 to 5 minutes at 70 to 110°C. The vulcanization conditions are not particularly limited, but examples include a method in which vulcanization is carried out for 10 to 30 minutes at 150 to 200°C.

[0100] A tire having a tread including a cap rubber layer and a base rubber layer can be manufactured by a conventional method using the rubber composition described above. That is, an unvulcanized rubber composition obtained by blending the above components with a rubber component as needed is extruded using an extruder equipped with a die of a predetermined shape to match the shapes of the cap rubber layer and the base rubber layer, and the extruded composition is bonded together with other tire components on a tire building machine and molded by a conventional method to form an unvulcanized tire. The unvulcanized tire can then be heated and pressurized in a vulcanizer to manufacture the tire of the present disclosure. [Example]

[0101] Hereinafter, the present disclosure will be described based on examples, but the present disclosure is not limited to these examples.

[0102] The various chemicals used in the examples and comparative examples are listed below. NR:TSR20 BR1: UBEPOL BR150B (cis content: 97% by mass, Mw: 440,000) manufactured by Ube Industries, Ltd. BR2: BR1250H manufactured by Zeon Corporation (tin-modified BR, polymerized using lithium as an initiator, cis content: 42% by mass, Mw: 570,000) Carbon black 1: Carbon black produced according to Production Example 1 below (average particle size: 19 nm, CTAB: 150 μm) 2 / g, N2SA:155m 2 / g) Carbon black 2: Diablack N330 (average particle size: 31 nm, CTAB: 78 μm) manufactured by Mitsubishi Chemical Corporation 2 / g, N2SA:79m 2 / g) Carbon black 3: Diablack N220 (average particle size: 23 nm, CTAB: 110 m) manufactured by Mitsubishi Chemical Corporation 2 / g, N2SA: 114m 2 / g) Silica: Ultrasil VN3 manufactured by Evonik Degussa (average particle size: 18 nm, CTAB: 153 μm) 2 / g, N2SA:175m 2 / g) Wax: Ozoace 0355 manufactured by Nippon Seiro Co., Ltd. Antioxidant 1: Nocrac 6C (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine) manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Antioxidant 2: Nocrac RD (poly(2,2,4-trimethyl-1,2-dihydroquinoline)) manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Stearic acid: Camellia stearic acid beads manufactured by NOF Corporation Zinc oxide: Zinc oxide No. 1 manufactured by Mitsui Mining & Smelting Co., Ltd. Sulfur 1: HK-200-5 (powdered sulfur containing 5% oil) manufactured by Hosoi Chemical Industry Co., Ltd. Sulfur 2: SEIMI OT (insoluble sulfur containing 10% oil) manufactured by Nippon Kanretsu Kogyo Co., Ltd. Vulcanization accelerator: Noccela NS (N-tert-butyl-2-benzothiazolyl sulfenamide (TBBS)) manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0103] (Production Example 1) Carbon black 1 was produced using a carbon black reactor comprising, in order, a combustion zone having an inner diameter of 800 mm and a length of 1600 mm and equipped with an air inlet duct and a combustion burner, a feedstock introduction zone connected to the combustion zone and consisting of a narrow-diameter section having an inner diameter of 175 mm and a length of 1000 mm and having a feedstock nozzle penetrating from the periphery, and a rear reaction zone having an inner diameter of 400 mm and a length of 3000 mm and equipped with a quenching device. Heavy oil C was used as the fuel and creosote oil was used as the feedstock hydrocarbon, and various conditions were set.

[0104] Examples and Comparative Examples According to the formulations shown in Tables 1 and 2, the chemicals other than sulfur and the vulcanization accelerator were kneaded for 5 minutes using a 1.7 L internal Banbury mixer until the discharge temperature reached 170°C, yielding a kneaded mixture. The resulting kneaded mixture was then re-mixed (remilled) for 4 minutes using the same Banbury mixer at a discharge temperature of 150°C. Next, sulfur and the vulcanization accelerator were added to the resulting kneaded mixture using a two-screw open roll mill, and the mixture was kneaded for 4 minutes until the temperature reached 105°C, yielding an unvulcanized rubber composition. The resulting unvulcanized rubber composition was press-vulcanized at 170°C for 12 minutes to produce a vulcanized rubber composition for testing.

[0105] Furthermore, the unvulcanized rubber composition was extruded into the shape of a cap rubber layer and a base rubber layer using an extruder equipped with a nozzle of a predetermined shape, and these were laminated together with other tire components to form an unvulcanized tire, which was then press-vulcanized to produce the test tires (12R22.5, truck and bus tires) listed in Table 3.

[0106] The test vulcanized rubber compositions and test tires obtained were evaluated as follows. The evaluation results are shown in Tables 1 to 3.

[0107] <Measurement of acetone extractables (AE amount)> Each rubber test piece made from the vulcanized rubber compositions B1 to B12 for the base rubber layer was immersed in acetone for 24 hours to extract the soluble components. The mass of each test piece was measured before and after extraction, and the amount of acetone extracted was calculated using the following formula. Acetone extractable amount (%) = {(mass of rubber test piece before extraction - mass of rubber test piece after extraction) / (mass of rubber test piece before extraction)} × 100

[0108] <Viscoelasticity test> The vulcanized rubber compositions B1 to B12 for the base rubber layer were formed into sheet-shaped vulcanized rubber test pieces, and the loss tangent (tanδ) was measured at a temperature of 70°C, an initial strain of 5%, a dynamic strain of ±1%, and a frequency of 10 Hz using a viscoelasticity spectrometer RSA-G2 manufactured by TA Instruments in accordance with JIS K 6394:2007. The reciprocal value of the 70°C tanδ of the base rubber layer was expressed as an index (fuel economy performance index), with Comparative Example 1 being set at 100. A higher index indicates better fuel economy performance. (Fuel efficiency index)=(70°C tan δ of the base rubber layer of Comparative Example 1) / (70°C tan δ of the base rubber layer of each test tire)×100

[0109] <Tensile test> Dumbbell-shaped No. 7 test pieces were prepared from the vulcanized rubber composition A1 for the cap rubber layer and the vulcanized rubber compositions B1 to B12 for the base rubber layers. Tensile tests were conducted in accordance with JIS K 6251:2017 at a temperature of 23°C and a tensile speed of 3.3 mm / s to measure the modulus (MPa) at 200% elongation, the strength at break (TB) (MPa), and the elongation at break (EB) (%).

[0110] <Durability test> The rib tear resistance performance of each test tire was evaluated using a test apparatus 52 shown in FIG. 2 and described in JP 2020-26257 A. FIG. 3 is a plan view of the protrusion jig and reference surface of the test apparatus 52. The protrusion jig 56 prepared had the external shape shown in FIG. 4, was made of steel, and had an upper surface 56a with a length L of 200 mm, a width W of 50 mm, a height H of the protrusion 72 of 150 mm, and an inclination angle θ of the upper surface 56a of 3°. This protrusion jig 56 was attached to a base member 58 of the test apparatus 52.

[0111] (1) Tire assembly Each test tire was mounted on a regular rim to form a tire assembly 51.

[0112] (2) Pretreatment process The tire assembly 51 was placed in a dry heat oven at a temperature of 90° C. and heated for 10 days, after which the tire assembly 51 was removed from the oven and allowed to return to room temperature.

[0113] (3) Main treatment process The pre-treated tire assembly 51 was attached to a testing device 52. At this time, the tire assembly 51 and the protruding jig 56 were aligned so that only a shoulder rib (not shown) of the tread portion 1 (a rib formed between the axially outermost circumferential groove of the tire and the ground-contact edge of the tread) was in contact with an upper surface 56a of the protruding jig 56. Thereafter, the tire assembly 51 (shaft 66) was lowered at a speed of 50.0 mm / min, and the descent was continued until each test tire slid off the protruding jig 56.

[0114] (4) Evaluation process After this treatment process, the tire assembly 51 was removed from the testing device 52, and the portion of the tire 2 that had been pressed against the protruding jig 56 was visually observed. Tires in which no rib tear was confirmed were marked with "+", and tires in which rib tear was confirmed were marked with "-".

[0115] [Table 1]

[0116] [Table 2]

[0117] [Table 3]

[0118] The results in Tables 1 to 3 show that the heavy-duty pneumatic tire of the present disclosure, in which the difference in modulus between the base rubber layer and the cap rubber layer at 200% stretch is within a predetermined range and the acetone extractable amount of the base rubber layer is within a predetermined range, has improved fuel efficiency and rib tear resistance. [Explanation of symbols]

[0119] 1 Tread section 2. Carcass 3 Belt 3a, 3b, 3c, 3d, 3e belt layer 4 Base rubber layer 5 Cap rubber layer 6 Tread rubber E Tire equatorial plane w Reinforcement rubber layer width W Tread width t Maximum thickness of reinforcement rubber layer 51 Tire assembly 52 Test Equipment 56 Protrusion jig 56a Upper surface of protrusion jig 58 Base material 58a Upper surface of base member (reference surface) 62 Slide bar 64 Vertical movement section 66 axes 71 Pedestal 72 Protrusion 74 Inner edge of protrusion jig

Claims

1. A heavy-duty pneumatic tire having a tread, the tread has a cap rubber layer constituting a tread surface and a base rubber layer adjacent to the cap rubber layer on the radially inner side thereof, the cap rubber layer and the base rubber layer being made of a rubber composition containing a rubber component and sulfur, the acetone extractable amount of the rubber composition constituting the base rubber layer is less than 4.0% by mass, A heavy-duty pneumatic tire, wherein M200b (MPa) is the modulus of a rubber composition constituting the base rubber layer when stretched 200% at 23°C, and M200c (MPa) is the modulus of a rubber composition constituting the cap rubber layer when stretched 200% at 23°C, and M200b and M200c satisfy the following formula (1): 0≦M200c−M200b≦4 (1)

2. 2. The heavy-duty pneumatic tire according to claim 1, wherein the maximum value of M200c-M200b in the formula (1) is 3 or less.

3. A heavy-duty pneumatic tire having a tread, the tread has a cap rubber layer constituting a tread surface and a base rubber layer adjacent to the cap rubber layer on the radially inner side thereof, the cap rubber layer and the base rubber layer being made of a rubber composition containing a rubber component and sulfur, the acetone extractable amount of the rubber composition constituting the base rubber layer is less than 3.9% by mass, A heavy-duty pneumatic tire, wherein M200b (MPa) is the modulus of a rubber composition constituting the base rubber layer when stretched 200% at 23°C, and M200c (MPa) is the modulus of a rubber composition constituting the cap rubber layer when stretched 200% at 23°C, and M200b and M200c satisfy the following formula (1): 0≦M200c−M200b≦5 (1′)

4. The heavy-duty pneumatic tire according to claim 1 or 2, wherein the rubber composition of the base rubber layer has an acetone extractable amount of less than 3.9% by mass.

5. 5. The heavy-duty pneumatic tire according to claim 3, wherein the rubber composition of the base rubber layer has an acetone extractable amount of less than 3.8% by mass.

6. 6. The heavy-duty pneumatic tire according to claim 5, wherein the acetone extractable amount of the rubber composition of the base rubber layer is less than 3.7% by mass.

7. 7. The heavy-duty pneumatic tire according to claim 6, wherein the acetone extractable amount of the rubber composition of the base rubber layer is less than 3.6% by mass.

8. The heavy-duty pneumatic tire according to any one of claims 1 to 7, wherein the rubber composition constituting the base rubber layer has a tan δ at 70°C (70°C tan δ) of less than 0.

040.

9. The heavy-duty pneumatic tire according to claim 8, wherein the rubber composition constituting the base rubber layer has a tan δ at 70°C (70°C tan δ) of less than 0.

039.

10. The heavy-duty pneumatic tire according to claim 9, wherein the rubber composition constituting the base rubber layer has a tan δ at 70°C (70°C tan δ) of less than 0.

038.

11. The heavy-duty pneumatic tire according to claim 10, wherein the rubber composition constituting the base rubber layer has a tan δ at 70°C (70°C tan δ) of less than 0.

037.

12. 12. The heavy-duty pneumatic tire according to claim 1, wherein, when the strength at break of the rubber composition constituting the base rubber layer at 23°C is TB (MPa) and the elongation at break at 23°C is EB (%), the tan δ at 70°C (70°C tan δ), TB, and EB of the rubber composition constituting the base rubber layer satisfy the following formula (2): TB×EB / 70℃tanδ≧4.5×10 5 ・・・(2)

13. The heavy-duty pneumatic tire according to claim 12, wherein the minimum value of TB×EB / 70° C. tan δ in the formula (2) is 4.6 or more.

14. The heavy-duty pneumatic tire according to claim 13, wherein the minimum value of TB×EB / 70° C. tan δ in the formula (2) is 4.7 or more.

15. The heavy-duty pneumatic tire according to claim 14, wherein the minimum value of TB×EB / 70° C. tan δ in the formula (2) is 4.8 or more.

16. The heavy-duty pneumatic tire according to claim 15, wherein the minimum value of TB×EB / 70° C. tan δ in the formula (2) is 4.9 or more.

17. The heavy-duty pneumatic tire according to any one of claims 1 to 16, wherein the rubber composition constituting the base rubber layer contains 5 parts by mass or more of silica per 100 parts by mass of the rubber component.

18. The heavy-duty pneumatic tire according to any one of claims 1 to 17, wherein a rubber component constituting the base rubber layer contains butadiene rubber.

19. The heavy-duty pneumatic tire according to any one of claims 1 to 18, wherein the thickness of the base rubber layer is 1 to 70% of the total thickness of the tread.

20. Four or more belt layers are provided on the radially inner side of the base rubber layer, At least one of an outer surface in the radial direction of the tire of the outermost belt layer in the radial direction of the tire and an outer surface in the radial direction of the widest belt layer is covered with a reinforcing rubber layer that ends without reaching the tire equatorial plane, The heavy load pneumatic tire according to any one of claims 1 to 19, wherein the value of tan δ (70 ° C. tan δ) at 70 ° C. of the rubber composition constituting the reinforcing rubber layer is greater than the value of tan δ (70 ° C. tan δ) at 70 ° C. of the rubber composition constituting the base rubber layer.

21. 21. The heavy-duty pneumatic tire according to claim 20, wherein the width w of the reinforcing rubber layer is 10 to 40% of the tread width W of the tread.

22. Four or more belt layers are provided on the radially inner side of the base rubber layer, At least one of an outer surface in the radial direction of the tire of the outermost belt layer in the radial direction of the tire and an outer surface in the radial direction of the widest belt layer is covered with a reinforcing rubber layer that ends without reaching the tire equatorial plane, 22. The heavy-duty pneumatic tire according to claim 21, wherein the width w of the reinforcing rubber layer is 15 to 30% of the tread width W of the tread.

23. Four or more belt layers are provided on the radially inner side of the base rubber layer, At least one of an outer surface in the radial direction of the tire of the outermost belt layer in the radial direction of the tire and an outer surface in the radial direction of the widest belt layer is covered with a reinforcing rubber layer that ends without reaching the tire equatorial plane, The heavy load pneumatic tire according to any one of claims 1 to 22, wherein the maximum thickness of the reinforcing rubber layer is 5 to 25% of the groove depth of a widthwise groove located at a position 1 / 4 of the tread width from the tire equator line.

24. Four or more belt layers are provided on the radially inner side of the base rubber layer, At least one of an outer surface in the radial direction of the tire of the outermost belt layer in the radial direction of the tire and an outer surface in the radial direction of the widest belt layer is covered with a reinforcing rubber layer that ends without reaching the tire equatorial plane, The heavy load pneumatic tire according to any one of claims 1 to 23, wherein the maximum thickness of the reinforcing rubber layer is 7 to 20% of the groove depth of a width direction groove located at a position 1 / 4 of the tread width from the tire equator line.

25. The heavy duty pneumatic tire according to any one of claims 1 to 24, wherein the rubber composition constituting the base rubber layer contains a glycerin fatty acid ester.

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