Heavy load retread pneumatic tire

Abstract

To provide a heavy load pneumatic tire which improves low fuel consumption performance and resistance performance against rib tearing.SOLUTION: A heavy load pneumatic tire is provided with a tread. The tread includes: a cap rubber layer 5 constituting a tread surface; and a base rubber layer 4 adjacent to tire radial direction inside of the cap rubber layer. The cap rubber layer and the base rubber layer are constituted by rubber compositions containing rubber constituent. Acetone extraction amount of the rubber composition constituting the base rubber layer is less than 4.0 mass%. When a modulus of the rubber composition constituting the base rubber layer at 23°C during 200% stretching is M200b (Mpa) and a modulus of the rubber composition constituting the cap rubber layer at 23°C during 200% stretching is M200c (Mpa), M200b and M200c satisfy the following expression (1): 0≤M200c-M200b≤5...(1).SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a pneumatic tire for heavy loads.

Background Art

[0002] As a method for improving the breaking strength of tires for trucks and buses, a technique for making carbon black finer or highly structured is known (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the above method of making carbon black finer or highly structured, it cannot be said that it is sufficient for improving the low fuel consumption performance and rib tear resistance performance of tires. In addition, due to the deterioration of processability associated with fining, the dispersibility of carbon black also deteriorates, and conversely, the wear resistance of the tire may deteriorate. Therefore, there is a limit to the conventional method of improving performance by improving carbon black.

[0005] In addition, due to the influence of recent environmental regulations, in tires for trucks and buses, there is an increasing demand for highly compatible performance not only in wear resistance performance but also in low fuel consumption performance, fracture resistance performance, etc.

[0006] An object of the present invention is to provide a pneumatic tire for heavy loads with improved low fuel consumption performance and rib tear resistance performance.

Means for Solving the Problems

[0007] The inventor of the present invention has found that the above problems can be solved by setting the difference in modulus at 200% elongation between the base rubber layer and the cap rubber layer within a predetermined range and setting the acetone extraction amount of the base rubber layer within a predetermined range, and has completed the present invention.

[0008] That is, the present invention is 〔1〕A pneumatic tire for heavy loads provided with a tread, wherein the tread has a cap rubber layer constituting a tread surface and a base rubber layer adjacent to the inner side in the tire radial direction of the cap rubber layer, and the cap rubber layer and the base rubber layer are composed of a rubber composition containing a rubber component, the acetone extraction amount of the rubber composition constituting the base rubber layer is less than 4.0% by mass, and the modulus at 200% elongation at 23°C of the rubber composition constituting the base rubber layer is M200b (MPa), and the modulus at 200% elongation at 23°C of the rubber composition constituting the cap rubber layer is M200c (MPa). When M200b and M200c satisfy the following formula (1), a pneumatic tire for heavy loads 0≦M200c - M200b≦5 ···(1) 〔2〕The pneumatic tire for heavy loads according to the above 〔1〕, wherein the tanδ (70°C tanδ) of the rubber composition constituting the base rubber layer at 70°C is less than 0.040. 〔3〕When the breaking strength at 23°C of the rubber composition constituting the base rubber layer is TB (MPa) and the breaking elongation at 23°C is EB (%), the tanδ (70°C tanδ), TB, and EB of the rubber composition constituting the base rubber layer at 70°C satisfy the following formula (2). The pneumatic tire for heavy loads according to the above 〔1〕 or 〔2〕 TB×EB / 70°C tanδ≧4.5×10 5 ···(2) 〔4〕The pneumatic tire for heavy loads according to any one of the above 〔1〕 to 〔3〕, wherein the rubber component constituting the base rubber layer contains butadiene rubber. 〔5〕The pneumatic tire for heavy loads according to any one of the above 〔1〕 to 〔4〕, wherein the thickness of the base rubber layer is 1 to 70% of the total thickness of the tread. 〔6〕The pneumatic heavy-duty tire according to any one of 〔1〕~〔5〕 above, comprising four or more belt layers on the inner side in the tire radial direction of the base rubber layer, and covering at least one of the outer surface in the tire radial direction of the outermost belt layer in the tire radial direction and the outer surface in the tire radial direction of the widest belt layer with a reinforcing rubber layer that ends without reaching the tire equatorial plane, and the value of tanδ(70℃tanδ) at 70°C of the rubber composition constituting the reinforcing rubber layer is larger than the value of tanδ(70℃tanδ) at 70°C of the rubber composition constituting the base rubber layer. 〔7〕The pneumatic heavy-duty tire according to any one of 〔1〕~〔6〕 above, wherein the width w of the reinforcing rubber layer is 10 to 40% of the tread contact width W of the tread. 〔8〕The pneumatic heavy-duty tire according to any one of 〔1〕~〔7〕 above, wherein the maximum thickness of the reinforcing rubber layer is 5 to 25% of the groove depth of the groove in the width direction existing at a position 1 / 4 of the tread contact width from the tire equatorial line. 〔9〕The pneumatic heavy-duty tire according to any one of 〔1〕~〔8〕 above, wherein the rubber composition constituting the base rubber layer contains glycerin fatty acid ester.

Advantages of the Invention

[0009] According to the present invention, there is provided a pneumatic heavy-duty tire with improved low fuel consumption performance and rib tear resistance performance.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0011] The pneumatic heavy-duty tire according to the present disclosure is a pneumatic heavy-duty tire provided with a tread, wherein the tread has a cap rubber layer constituting a tread surface and a base rubber layer adjacent to the inner side in the tire radial direction of the cap rubber layer, the cap rubber layer and the base rubber layer are composed of a rubber composition containing a rubber component, the acetone extraction amount of the rubber composition constituting the base rubber layer is less than 4.0% by mass, and when the modulus at 200% elongation at 23°C of the rubber composition constituting the base rubber layer is M200b (MPa) and the modulus at 200% elongation at 23°C of the rubber composition constituting the cap rubber layer is M200c (MPa), M200b and M200c satisfy the following formula (1). 0≦M200c - M200b≦5 ···(1)

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

[0013] When the modulus balance between the base rubber layer and the cap rubber layer deteriorates, there is a risk of causing tread damage such as peeling at the interface between the cap rubber layer and the base rubber layer or cracks in the base tread rubber.

[0014] Therefore, by setting the difference in the modulus at 200% elongation of the base rubber layer and the cap rubber layer within a predetermined range and setting the acetone extraction amount of the base rubber layer within a predetermined range, it is considered that the modulus balance between the base rubber layer and the cap rubber layer can be optimized, heat generation of the tread rubber can be suppressed, and the rib tear resistance performance can be improved.

[0015] Furthermore, it is considered that the low fuel consumption performance and the rib tear resistance performance can be improved by the cooperation of the structure of the reinforcing rubber layer described later and the physical properties of the rubber compositions constituting the respective layers of the tread.

[0016] It is preferable that the tanδ (tanδ at 70°C) of the rubber composition constituting the base rubber layer is less than 0.040 at 70°C.

[0017] When the breaking strength at 23°C of the rubber composition constituting the base rubber layer is TB (MPa) and the elongation at break at 23°C is EB (%), it is preferable that the tanδ (tanδ at 70°C), TB, and EB of the rubber composition constituting the base rubber layer satisfy the following formula (2). TB × EB / 70°C 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 on the inner side in the tire radial direction of the base rubber layer, and at least one of the outer surface in the tire radial direction of the outermost belt layer in the tire radial direction and the outer surface in the tire radial direction 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 tanδ (tanδ at 70°C) of the rubber composition constituting the reinforcing rubber layer is larger than the value of tanδ (tanδ 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 the groove in the width direction existing 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 glycerin fatty acid ester.

[0024] A pneumatic heavy-duty tire containing 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 exemplification for explaining the present disclosure and is not intended to limit the technical scope of the present invention only to this description scope. In this specification, when a numerical range is indicated using "~", it is assumed to include the numerical values at both ends thereof.

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

[0026] In FIG. 1, 1 indicates a tread portion, 2 indicates a carcass composed of one or more carcass plies extending toroidally between one bead core and the other bead core, and 3 indicates a belt composed of five laminated belt layers 3a to 3e disposed on the outer side in the tire radial direction of the carcass 2. And, a base rubber layer 4 and a cap rubber layer 5 are laminated on the outer side in the tire radial direction of the belt 3.

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

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

[0029] The width w of the reinforcing rubber layer 7 is preferably 10 to 40% of the tread contact width W of the tread, more preferably 15 to 30%. Also, the maximum thickness t of the reinforcing rubber layer 7 is preferably 5 to 25% of the groove depth of the widthwise groove existing at a position 1 / 4 of the tread contact width from the tire equator line, more preferably 7 to 20%. According to these, while effectively preventing the progress of fracture into the base rubber 4, the base rubber layer 4 with low heat generation can fully exhibit its original function, and high durability can be imparted to the tread rubber 6. In the present disclosure, the "tread contact width" means the tread contact width, which is a straight-line distance parallel to the tire axis between the outermost grounding positions (grounding ends) in the tire axial direction when the tire is mounted on an application rim, filled with a specified air pressure, and placed vertically with a camber angle of zero degrees with respect to a flat plate and a load corresponding to the maximum load capacity is applied.

[0030] In the present disclosure, "tanδ at 70°C" refers to the loss tangent tanδ 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 in accordance with JIS K 6394:2007.

[0031] From the viewpoint of low fuel consumption performance, the tanδ at 70°C 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] Note that the tanδ at 70°C of the base rubber layer 4 can be appropriately adjusted according to the types and compounding amounts of the rubber component, filler, silane coupling agent, oil, glycerin fatty acid ester, etc. described later.

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

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

[0035] The modulus M200b at 200% elongation of the rubber composition constituting the base rubber layer 4 is preferably 4.0 MPa or more, more preferably 4.5 MPa or more, still more preferably 5.0 MPa or more, and particularly preferably 5.5 MPa or more, from the viewpoints of suppressing deterioration of fuel efficiency due to rubber distortion, suppressing deterioration of handling stability performance, and suppressing uneven wear. On the other hand, M200b is preferably 15.0 MPa or less, more preferably 14.0 MPa or less, still more preferably 13.0 MPa or less, and particularly preferably 12.0 MPa or less. When M200b exceeds 15.0 MPa, it is difficult for external force to escape, and there is a concern that crack propagation may occur at the interface due to the concentration of input at the interface between the cap rubber layer 5 and the base rubber layer 4.

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

[0037] 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, it is considered possible to suppress damage to the tread such as peeling at the interface between the base rubber layer 4 and the cap rubber layer 5 and cracks in the base rubber layer 4.

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

[0039] Note that the modulus, TB, and EB at 200% elongation of each rubber layer can be appropriately adjusted according to the types and compounding amounts of the rubber components, fillers, silane coupling agents, softening agents, etc., which will be described later.

[0040] In the present disclosure, the acetone extraction amount is an index of the concentration of organic low-molecular compounds in the plasticizer contained in the vulcanized rubber composition. The acetone extraction amount can be determined by immersing each vulcanized rubber test piece in acetone for 24 hours in accordance with JIS K 6229-3:2015 to extract the soluble components, measuring the mass of each test piece before and after extraction, and using the following formula. Acetone extraction 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 extraction 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, still more preferably less than 3.7% by mass, and particularly preferably less than 3.6% by mass. By setting the difference in the acetone extraction amount within the above range, a high-dimensional balance between low fuel consumption performance and fracture resistance performance can be achieved.

[0042] From the viewpoint of the balance between the durability and low fuel consumption of the tire, the 70°C tanδ, the breaking strength TB (MPa) at 23°C, and the elongation at break EB (%) at 23°C of the rubber composition constituting the base rubber layer 4 preferably satisfy the following formula (2). TB × EB / 70°C tanδ ≧ 4.5 × 10 5 ···(2)

[0043] TB × EB / 70°C tanδ is preferably 4.6 × 10 5 or more, more preferably 4.7 × 10 5 or more, still more preferably 4.8 × 10 5 or more, particularly preferably 4.9 × 10 5 or more.

[0044] [Rubber composition] The heavy-load pneumatic tire of the present disclosure can improve low fuel consumption performance and rib tear resistance performance by the cooperation of the structure of the reinforcing rubber layer described above and the physical properties of the rubber compositions constituting the respective layers of the tread.

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

[0046] (Isoprene rubber) As the isoprene rubber, for example, those commonly used in the tire industry such as isoprene rubber (IR) and natural rubber can be used. Natural rubber includes, in addition to unmodified natural rubber (NR), modified natural rubbers such as epoxidized natural rubber (ENR), hydrogenated natural rubber (HNR), deproteinized natural rubber (DPNR), high-purity natural rubber, and grafted natural rubber. These isoprene rubbers may be used alone or in combination of two or more.

[0047] NR is not particularly limited, and those commonly used in the tire industry can be used. For example, SIR20, RSS#3, TSR20, etc. can be mentioned.

[0048] The content of the isoprene rubber in the rubber component is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, particularly preferably 80% by mass or more, and may be a rubber component consisting only of isoprene rubber.

[0049] (BR) BR is not particularly limited. For example, BR with a cis content (cis-1,4 bond content) of 90% by mass or more (high-cis BR), rare-earth-based butadiene rubber synthesized using a rare-earth element-based catalyst (rare-earth-based BR), BR containing syndiotactic polybutadiene crystals (SPB-containing BR), modified BR (high-cis modified BR, low-cis modified BR), etc., which are common in the tire industry, can be used. From the perspective of reinforcement with fillers, modified BR is preferably used as BR. In addition, SPB-containing BR can be used to ensure strength when the filler is reduced.

[0050] Examples of high-cis BR include those manufactured by Nippon Zeon Co., Ltd., those manufactured by Ube Industries, Ltd., those manufactured by JSR Corporation, etc. By containing high-cis BR, the low-temperature properties and abrasion resistance can be improved. The cis content of high-cis BR is preferably 90% by mass or more, more preferably 95% by mass or more, still 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 spectrum analysis.

[0051] Rare-earth-based BR is synthesized using a rare-earth element-based catalyst, and the vinyl bond content (1,2-bond butadiene unit amount) is preferably 1.8 mol% or less, more preferably 1.0 mol% or less, still more preferably 0.8 mol% or less, and the cis content (cis-1,4 bond content) is preferably 95% by mass or more, more preferably 96% by mass or more, still more preferably 97% by mass or more. Examples of rare-earth-based BR that can be used include those manufactured by Lanxess Co., Ltd.

[0052] SPB-containing BR includes those in which the 1,2-syndiotactic polybutadiene crystals are not simply dispersed in BR but are dispersed after chemically bonding to BR. Examples of such SPB-containing BR that can be used include those manufactured by Ube Industries, Ltd.

[0053] As the modified BR, terminally modified BR coupled with tin, terminally modified BR having an alkoxysilyl group and / or an amino group are preferably used. Also, the modified BR may be either non-hydrogenated or hydrogenated. As such modified BR, those manufactured by Nippon Zeon Co., Ltd., Asahi Kasei Chemicals Corporation, etc. can be used.

[0054] As the terminally modified BR coupled with tin, it is obtained by polymerizing 1,3-butadiene with a lithium initiator and then adding a tin compound, and preferably the terminal of the modified BR molecule is bonded by a tin-carbon bond. Examples of the lithium initiator include lithium-based compounds such as alkyllithium, aryllithium, vinyllithium, organotinlithium, and organonitrogenlithium compounds, and lithium metal. By using the lithium initiator as the initiator of the modified BR, a modified BR with a high vinyl and low cis content can be produced. Examples of the tin compound 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, p-tributyltin styrene, etc. 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 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. Further, from the viewpoint of crosslinking uniformity, etc., it is preferably 2,000,000 or less, and more preferably 1,000,000 or less. Note that Mw can be determined by standard polystyrene conversion based on the measured value 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] The content in the rubber component when containing BR 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 low fuel consumption performance. Further, 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) There is no particular limitation on SBR, and examples include solution-polymerized SBR (S-SBR), emulsion-polymerized SBR (E-SBR), and modified SBRs thereof (modified S-SBR, modified E-SBR). Examples of modified SBR include SBR in which the terminal and / or main chain is modified, and modified SBR coupled with tin, silicon compounds, etc. (condensates, those having a branched structure, etc.). Among them, E-SBR is preferred in that it can well improve low fuel consumption performance and abrasion resistance performance. These SBRs may be used alone or in combination of two or more.

[0059] The content in the rubber component when containing SBR is not particularly limited, and for example, it can be 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, 20% by mass or less.

[0060] (Other rubber components) As the rubber component according to the present disclosure, rubber components other than the above-mentioned isoprene rubber, SBR, and BR may be contained. As other rubber components, crosslinkable rubber components generally used in the tire industry can be used. For example, 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), hydrin rubber, etc. may be mentioned. 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. Further, the filler may be a filler consisting only of carbon black, or a filler consisting only of carbon black and silica.

[0062] (Silica) By blending silica into the rubber composition for a tread according to the present disclosure, low fuel consumption performance, fracture resistance performance, and abrasion resistance performance can be improved. The silica is not particularly limited, and for example, silica prepared by a dry method (anhydrous silica), silica prepared by a wet method (hydrous silica), etc., which are common in the tire industry, can be used. Among them, hydrous silica prepared by a wet method is preferable because it has many silanol groups. These silicas may be used alone or in combination of two or more.

[0063] The average particle size of the silica is preferably 50 nm or less, more preferably 40 nm or less, and even more preferably 30 nm or less. Also, the average particle size of the 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 the silica is within the above range, the effects of the present disclosure tend to be exhibited more favorably. The average particle size of the silica is the number average particle size and is measured as the average value for any 100 particles by a transmission electron microscope.

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

[0065] The nitrogen adsorption specific surface area (N2SA) of the silica is 80 m 2 / g or more is preferable, 110 m 2 / g or more is more preferable, 140 m 2 / g or more is even more preferable, and 170 m 2 / g or more is particularly preferable. Also, the N2SA of the silica is preferably 350 m 2 / g or less, more preferably 300 m 2 / g or less, and even more preferably 250 m 2 / g or less. When the N2SA of the silica is within the above range, the effects of the present disclosure tend to be exhibited more favorably. The N2SA of the silica in this specification is a value measured by the BET method in accordance with ASTM D3037-93.

[0066] The content of silica relative to 100 parts by mass of the rubber component is preferably 20 parts by mass or less, more preferably 18 parts by mass or less, still more preferably 15 parts by mass or less, and particularly preferably 10 parts by mass or less from the viewpoint of low fuel consumption performance. On the other hand, from the viewpoint of elongation at break, the content is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and still more preferably 5 parts by mass or more.

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

[0068] The average particle diameter of the carbon black is preferably 90 nm or less, more preferably 70 nm or less, and still more preferably 50 nm or less. Also, the average particle diameter of the carbon black is preferably 13 nm or more, more preferably 15 nm or more, and still more preferably 17 nm or more. When the average particle diameter of the carbon black is within the above range, the effects of the present disclosure tend to be exhibited more favorably. The average particle diameter of the carbon black is the number average particle diameter and is measured as the average value of any 100 particles by a transmission electron microscope.

[0069] The cetyltrimethylammonium bromide (CTAB) specific surface area of the carbon black is preferably 80 m 2 / g or more, more preferably 90 m 2 / g or more, and still more preferably 100 m 2 / g or more. Also, the CTAB specific surface area of the carbon black is more preferably 170 m 2 / g or less, more preferably 165 m 2 / g or less, and still more preferably 160 m 2 / g or less. When the CTAB specific surface area of the carbon black is within the above range, the effects of the present disclosure tend to be exhibited more favorably. 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 preferably 40 m 2 / g or more, more preferably 60 m 2 / g or more, still more preferably 80 m 2 / g or more from the viewpoint of fracture strength. From the viewpoint of processability, it is preferably 200 m 2 / g or less, more preferably 150 m 2 / g or less, still more preferably 130 m 2 / g or less. The N2SA of carbon black in this specification is a value measured in accordance with Method A of JIS K 6217-2 "Basic properties of carbon black for rubber - Part 2: Method for determining specific surface area - Nitrogen adsorption method - Single point method".

[0071] The content of carbon black with respect to 100 parts by mass of the rubber component is preferably 20 parts by mass or more, more preferably 25 parts by mass or more, still more preferably 30 parts by mass or more from the viewpoint of low fuel consumption performance. Also, from the viewpoint of low fuel consumption performance, the content is preferably 60 parts by mass or less, more preferably 55 parts by mass or less, still more preferably 50 parts by mass or less, and particularly preferably 45 parts by mass or less.

[0072] As fillers other than silica and carbon black, for example, aluminum hydroxide, calcium carbonate, alumina, clay, talc, etc., those common in the tire industry can be used.

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

[0074] From the perspective of wear resistance performance, the total content of silica and carbon black with respect to 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. Also, from the perspective of suppressing the deterioration of low fuel consumption performance and wear resistance performance, it 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. For example, 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; silane coupling agents having an amino group such as 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, and 3-(2-aminoethyl)aminopropyltriethoxysilane; glycidoxy-based silane coupling agents such as γ-glycidoxypropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; nitro-based silane coupling agents such as 3-nitropropyltrimethoxysilane and 3-nitropropyltriethoxysilane; chloro-based silane coupling agents such as 3-chloropropyltrimethoxysilane and 3-chloropropyltriethoxysilane, etc. are mentioned, and 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 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, still more preferably 9.0 parts by mass or more, and particularly preferably 9.5 parts by mass or more from the viewpoint of enhancing the dispersibility of silica. Also, from the viewpoint of preventing a decrease in wear resistance performance, it is preferably 18 parts by mass or less, more preferably 16 parts by mass or less, still more preferably 14 parts by mass or less, and particularly preferably 12 parts by mass or less.

[0077] <Other compounding agents> In the rubber composition for treads according to the present disclosure, in addition to the above components, compounding agents generally used in the conventional tire industry, such as oils, waxes, glycerin fatty acid esters, processing aids, anti-aging agents, stearic acid, zinc oxide, vulcanizing agents such as sulfur, vulcanization accelerators, etc. can be appropriately contained.

[0078] Examples of the oil include process oils, vegetable oils, animal oils, etc. Examples of the process oil include paraffinic process oils, naphthenic process oils, aromatic process oils, etc. Also, a process oil with a low content of polycyclic aromatic compound (PCA) can be used for environmental measures. Examples of the low-PCA-content process oil include mild extraction solvent solvate (MES), treated distillate aromatic extract (TDAE), heavy naphthenic oil, etc.

[0079] When contained, the content relative to 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 still more preferably 4 parts by mass or less from the viewpoint of the balance between low fuel consumption performance and breaking strength. In this specification, the oil content also includes the amount of oil contained in the oil-extended rubber.

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

[0081] Glycerin fatty acid ester is a compound in which a fatty acid is ester-bonded to at least one of the three OH groups of glycerin, and is classified into glycerin fatty acid monoester, glycerin fatty acid diester, and glycerin fatty acid triester according to the number of fatty acids. The fatty acid constituting the glycerin fatty acid ester is preferably a fatty acid having 8 to 28 carbon atoms, more preferably 8 to 22 carbon atoms, still more preferably 10 to 18 carbon atoms, and particularly preferably 12 to 18 carbon atoms. Also, the fatty acid may be saturated, unsaturated, straight-chain, or branched-chain, but a straight-chain saturated fatty acid is preferred. Specific examples of the fatty acid include capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, isostearic acid, oleic acid, linoleic acid, etc.

[0082] When contained, the content relative to 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 still more preferably 1.0 part by mass or more. Also, the content is preferably 10.0 parts by mass or less, more preferably 8.0 parts by mass or less, and still 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 the low fuel consumption performance and wear 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, mixtures of fatty acid metal salts and fatty acid amides, and the like. These processing aids may be used alone or in combination of two or more. As the processing aids, those commercially available from, for example, Schill+Seilacher, Performance Additives, etc. can be used.

[0084] When containing a processing aid, the content thereof relative to 100 parts by mass of the rubber component is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, from the viewpoint of exerting the effect of improving processability. Further, from the viewpoints of abrasion resistance and fracture strength, it is preferably 10 parts by mass or less, more preferably 8 parts by mass or less.

[0085] The anti-aging agent is not particularly limited, and examples thereof include amine-based, quinoline-based, quinone-based, phenol-based, imidazole-based compounds, and anti-aging agents such as metal carbamates. Phenylenediamine-based anti-aging agents 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, N-cyclohexyl-N'-phenyl-p-phenylenediamine, and quinoline-based anti-aging agents such as 2,2,4-trimethyl-1,2-dihydroquinoline polymer, 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline are preferred. These anti-aging agents may be used alone or in combination of two or more.

[0086] When containing an anti-aging agent, the content thereof relative to 100 parts by mass of the rubber component is preferably 0.5 part by mass or more, more preferably 1.0 part by mass or more, further preferably 1.5 part by mass or more, from the viewpoint of ozone crack resistance of the rubber. Further, from the viewpoints of abrasion resistance performance and wet grip performance, it is preferably 10.0 parts by mass or less, more preferably 5.0 parts by mass or less.

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

[0088] When contained, the content of zinc oxide with respect to 100 parts by mass of the rubber component is preferably 0.5 part by mass or more, more preferably 1.0 part by mass or more, still more preferably 1.5 part by mass or more, from the viewpoint of processability. Also, from the viewpoint of abrasion resistance performance, it is preferably 10.0 parts by mass or less, more preferably 5.0 parts by mass or less.

[0089] Sulfur is preferably used as the vulcanizing agent. As sulfur, powdered sulfur, oil-treated sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, highly dispersed sulfur, etc. can be used.

[0090] When the vulcanizing agent contains sulfur, the content with respect to 100 parts by mass of the rubber component is preferably 0.5 part by mass or more, more preferably 1.0 part by mass or more, from the viewpoint of ensuring a sufficient vulcanization reaction and obtaining good grip performance and abrasion resistance performance. Also, from the viewpoint of deterioration, it is preferably 3.0 parts by mass or less, more preferably 2.5 parts by mass or less, still more preferably 2.0 parts by mass or less. Note that when using oil-containing sulfur as the vulcanizing agent, the content of the vulcanizing agent is the total content of the pure sulfur component contained in the oil-containing sulfur.

[0091] Examples of vulcanizing agents other than sulfur include alkylphenol sulfur chloride condensates, sodium 1,6-hexamethylene-dithiolsulfate dihydrate, 1,6-bis(N,N'-dibenzylthiocarbamoyldithio)hexane, etc. Commercially available products from companies such as Tago Chemical Industry Co., Ltd., Rancess Co., Ltd., and Flexsys can be used as these vulcanizing agents other than sulfur.

[0092] Examples of the vulcanization accelerator include sulfenamide-based, thiazole-based, thiuram-based, thiourea-based, guanidine-based, dithiocarbamic acid-based, aldehyde-amine-based or aldehyde-ammonia-based, imidazoline-based, or xanthate-based vulcanization accelerators, etc. These vulcanization accelerators may be used alone or in combination of two or more. Among them, 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 the sulfenamide-based vulcanization accelerator include N-tert-butyl-2-benzothiazolylsulfenamide (TBBS), N-cyclohexyl-2-benzothiazolylsulfenamide (CBS), N,N-dicyclohexyl-2-benzothiazolylsulfenamide (DCBS), etc. Among them, N-tert-butyl-2-benzothiazolylsulfenamide (TBBS) is preferred.

[0094] Examples of the guanidine-based vulcanization accelerator include 1,3-diphenylguanidine (DPG), 1,3-di-o-tolylguanidine, 1-o-tolylbiguanide, di-o-tolylguanidine salt of dicatecholborate, 1,3-di-o-cumenylguanidine, 1,3-di-o-biphenylguanidine, 1,3-di-o-cumenyl-2-propionylguanidine, etc. Among them, 1,3-diphenylguanidine (DPG) is preferred.

[0095] Examples of the thiazole-based vulcanization accelerator include 2-mercaptobenzothiazole, cyclohexylamine salt of 2-mercaptobenzothiazole, di-2-benzothiazolyldisulfide, etc. Among them, 2-mercaptobenzothiazole is preferred.

[0096] When containing a vulcanization accelerator, the content with respect to 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 2 parts by mass or more. Further, the content of the vulcanization accelerator with respect to 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, the 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, it can be produced by kneading each of the above components using a rubber kneading device such as an open roll, a closed kneader (Banbury mixer, kneader, etc.).

[0098] The kneading process includes, for example, a base kneading process of kneading compounding agents and additives other than the vulcanizing agent and the vulcanization accelerator, and a final kneading (F kneading) process of adding the vulcanizing agent and the vulcanization accelerator to the kneaded product obtained in the base kneading process and kneading them. Further, the base kneading process can be divided into a plurality of processes if desired.

[0099] The kneading conditions are not particularly limited. For example, in the base kneading process, kneading is performed at a discharge temperature of 150 to 170 ° C for 3 to 10 minutes, and in the final kneading process, kneading is performed at 70 to 110 ° C for 1 to 5 minutes. The vulcanization conditions are not particularly limited, and for example, a method of vulcanizing at 150 to 200 ° C for 10 to 30 minutes can be mentioned.

[0100] A tire provided with a tread including a cap rubber layer and a base rubber layer can be produced by a normal method using the above rubber composition. That is, an unvulcanized rubber composition in which the above components are blended with the rubber component as necessary 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 bonded together with other tire members on a tire molding machine and molded by a normal method to form an unvulcanized tire, and this unvulcanized tire is heated and pressurized in a vulcanizer to produce the tire of the present disclosure.

Examples

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

[0102] Hereinafter, various chemicals used in the examples and comparative examples are collectively shown. NR: TSR20 BR1: UBEPOL BR150B manufactured by Ube Industries, Ltd. (cis content: 97% by mass, Mw: 440,000) BR2: BR1250H manufactured by Nippon Zeon Co., Ltd. (tin-modified BR, polymerized using lithium as an initiator, cis content: 42% by mass, Mw: 570,000) Carbon Black 1: Carbon black manufactured according to the following Production Example 1 (average particle diameter: 19 nm, CTAB: 150 m 2 / g, N2SA: 155 m 2 / g) Carbon Black 2: Diablack N330 manufactured by Mitsubishi Chemical Corporation (average particle diameter: 31 nm, CTAB: 78 m 2 / g, N2SA: 79 m 2 / g) Carbon Black 3: Diablack N220 manufactured by Mitsubishi Chemical Corporation (average particle diameter: 23 nm, CTAB: 110 m 2 / g, N2SA: 114 m 2 / g) Silica: Ultrasil VN3 manufactured by Evonik Degussa GmbH (average particle diameter: 18 nm, CTAB: 153 m 2 / g, N2SA: 175 m 2 / g) Wax: Oz Ace 0355 manufactured by Nippon Seiro Co., Ltd. Antioxidant 1: Nocrack 6C (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine) manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Antioxidant 2: Nocrack RD (poly(2,2,4-trimethyl-1,2-dihydroquinoline)) manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Stearic acid: Tsubaki Bead Stearic Acid manufactured by NOF Corporation Zinc oxide: Zinc White 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 Dry Distillation Industry Co., Ltd. Vulcanization accelerator: Nocceler NS (N-tert-butyl-2-benzothiazolylsulfenamide (TBBS)) manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0103] (Production Example 1) A carbon black reactor was used, which consisted of a combustion zone with an inner diameter of 800 mm and a length of 1600 mm equipped with an air introduction duct and a combustion burner, a raw material introduction zone consisting of a narrow-diameter part with an inner diameter of 175 mm and a length of 1000 mm connected from the combustion zone and penetratingly connected to raw material nozzles from the periphery, and a rear reaction zone with an inner diameter of 400 mm and a length of 3000 mm equipped with a quenching device. C heavy oil was used as the fuel and creosote oil was used as the raw material hydrocarbon. Each condition was set to produce Carbon Black 1.

[0104] (Examples and Comparative Examples) According to the compounding formulations shown in Table 1 and Table 2, using a 1.7 L sealed Banbury mixer, chemicals other than sulfur and the vulcanization accelerator were kneaded for 5 minutes until the discharge temperature reached 170 °C to obtain a kneaded product. Further, the obtained kneaded product was kneaded again (remilled) for 4 minutes at a discharge temperature of 150 °C using the Banbury mixer. Next, using a twin-screw open roll, sulfur and the vulcanization accelerator were added to the obtained kneaded product and kneaded for 4 minutes until the temperature reached 105 °C to obtain an unvulcanized rubber composition. The obtained unvulcanized rubber composition was press-vulcanized at 170 °C for 12 minutes to prepare a test vulcanized rubber composition.

[0105] Also, the unvulcanized rubber composition was extruded into the shapes of a cap rubber layer and a base rubber layer using an extruder equipped with a die of a predetermined shape, laminated together with other tire members to form an unvulcanized tire, and press-vulcanized to manufacture the test tires (12R22.5, truck and bus tires) described in Table 3.

[0106] The following evaluations were performed on the obtained vulcanized rubber composition for testing and the test tire. The evaluation results are shown in Tables 1 to 3.

[0107] <Measurement of Acetone Extraction Amount (AE Amount)> Each rubber test piece prepared 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 before and after extraction was measured, and the acetone extraction amount was determined by the following calculation formula. Acetone extraction 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 made into sheet-shaped vulcanized rubber test pieces, and in accordance with JIS K 6394:2007, using a viscoelastic spectrometer RSA-G2 manufactured by TA Instruments, at a temperature of 70°C, an initial strain of 5%, a dynamic strain of ±1%, and a frequency of 10 Hz, the loss tangent (tanδ) was measured. Also, the value of the reciprocal of the 70°C tanδ of the base rubber layer was expressed exponentially with Comparative Example 1 as 100 (low fuel consumption performance index). The larger the index, the better the low fuel consumption performance. (Low fuel consumption performance index) = (70°C tanδ of the base rubber layer of Comparative Example 1) / (70°C tanδ 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 layer, respectively. In accordance with JIS K 6251:2017, a tensile test was carried out under the condition of a tensile speed of 3.3 mm / second in a 23°C atmosphere, and the modulus (MPa) at 200% elongation, the strength at break (TB) (MPa), and the elongation at break (EB) (%) were measured.

[0110] <Endurance Test> The cut resistance performance of each test tire was evaluated using the test apparatus 52 shown in FIG. 2 described in Japanese Patent Application Laid-Open No. 2020-26257. FIG. 3 is a plan view of the protrusion jig and the reference plane of the test apparatus 52. As the protrusion jig 56, a protrusion jig 56 having the external shape shown in FIG. 4, made of steel, with the length L of the upper surface 56a being 200 mm, the width W being 50 mm, the height H of the protrusion 72 being 150 mm, and the inclination angle θ of the upper surface 56a being 3° was prepared. This protrusion jig 56 was attached to the base member 58 of the test apparatus 52.

[0111] (1) Tire assembly Each test tire was incorporated into a standard 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. Then, the tire assembly 51 was taken out of the oven and returned to room temperature.

[0113] (3) Main treatment process The pretreated tire assembly 51 was attached to the test apparatus 52. At this time, the tire assembly 51 and the protrusion jig 56 were aligned so that only the shoulder rib (the rib formed between the outermost circumferential groove in the tire axial direction and the tread grounding end) of the tread portion 1, which is not shown, contacted the upper surface 56a of the protrusion jig 56. Then, the tire assembly 51 (shaft 66) was lowered at a speed of 50.0 mm / min, and the lowering was continued until each test tire slipped off the protrusion jig 56.

[0114] (4) Evaluation process The tire assembly 51 that had completed the main treatment process was removed from the test apparatus 52, and the portion pressed against the protrusion jig 56 of the tire 2 was visually observed. Tires in which the occurrence of rib tear was not confirmed were marked as "+", and tires in which the occurrence of rib tear was confirmed were marked as "-".

[0115]

Table 1

[0116]

Table 2

[0117]

Table 3

[0118] From the results of Table 1 to Table 3, it can be seen that the pneumatic tire for heavy loads of the present disclosure, with the difference in modulus at 200% elongation of the base rubber layer and the cap rubber layer within a predetermined range and the acetone extraction amount of the base rubber layer within a predetermined range, has improved low fuel consumption performance and rib tear resistance performance.

Description of Signs

[0119] 1 Tread portion 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 Reinforcing rubber layer width W Tread contact width t Maximum thickness of reinforcing rubber layer 51 Tire assembly 52 Test device 56 Projection jig 56a Upper surface of projection jig 58 Base member 58a Upper surface (reference surface) of base member 62 Slide bar 64 Vertical movement part 66 Shaft 71 Pedestal 72 Protrusion 74 Inner edge of projection jig

Claims

1. A pneumatic tire for heavy loads having a tread, wherein the tread has a cap rubber layer forming a tread surface and a base rubber layer adjacent to the inner side in the tire radial direction of the cap rubber layer, and the cap rubber layer and the base rubber layer are composed of a rubber composition containing a rubber component; the acetone extraction amount of the rubber composition constituting the base rubber layer is less than 4.0% by mass; when the modulus at 200% elongation at 23°C of the rubber composition constituting the base rubber layer is M200b (MPa) and the modulus at 200% elongation at 23°C of the rubber composition constituting the cap rubber layer is M200c (MPa), a pneumatic tire for heavy loads in which M200b and M200c satisfy the following formula (1). 0 ≦ M200c - M200b ≦ 5... (1)

2. The pneumatic tire for heavy loads according to claim 1, wherein the rubber composition constituting the base rubber layer contains silica.

3. The pneumatic tire for heavy loads according to claim 1 or 2, wherein the rubber composition constituting the base rubber layer contains 5 parts by mass or more of silica with respect to 100 parts by mass of the rubber component.

4. The pneumatic tire for heavy loads according to any one of claims 1 to 3, wherein tanδ (70°C tanδ) of the rubber composition constituting the base rubber layer at 70°C is less than 0.

040.

5. When the breaking strength at 23°C of the rubber composition constituting the base rubber layer is TB (MPa) and the elongation at break at 23°C is EB (%), tanδ (70°C tanδ), TB and EB of the rubber composition constituting the base rubber layer at 70°C satisfy the following formula (2). The pneumatic tire for heavy loads according to any one of claims 1 to 4. TB × EB / tanδ at 70°C ≥ 4.5 × 10 5 ・・・(2)

6. The pneumatic tire for heavy loads according to any one of claims 1 to 5, wherein the rubber component constituting the base rubber layer includes butadiene rubber.

7. The pneumatic tire for heavy loads according to any one of claims 1 to 6, wherein the thickness of the base rubber layer is 1 to 70% of the total thickness of the tread.

8. Four or more belt layers are provided on the inner side in the tire radial direction of the base rubber layer, at least one of the outer surface in the tire radial direction of the outermost belt layer in the tire radial direction and the outer surface in the tire radial direction of the widest belt layer is covered with a reinforcing rubber layer that ends without reaching the tire equatorial plane. The value of tanδ (tanδ at 70°C) of the rubber composition constituting the reinforcing rubber layer is greater than the value of tanδ (tanδ at 70°C) of the rubber composition constituting the base rubber layer, the heavy load pneumatic tire according to any one of claims 1 to 7.

9. The width w of the reinforcing rubber layer is 10 to 40% of the tread contact width W of the tread, the heavy load pneumatic tire according to claim 8.

10. The maximum thickness of the reinforcing rubber layer is 5 to 25% of the groove depth of the width direction groove existing at the position of 1 / 4 of the tread contact width from the tire equator line, the heavy load pneumatic tire according to claim 8 or 9.

11. The rubber composition constituting the base rubber layer contains glycerin fatty acid ester, the heavy load pneumatic tire according to any one of claims 1 to 10.

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