Tire

Abstract

To improve low heat generation while maintaining resistance to tread separation.SOLUTION: A tire is provided with a tread rubber including a cap tread and a base tread disposed on an inner side of the cap tread. The base tread is formed from a rubber composition containing a rubber component including 70 mass% or more of natural rubber and 35 to 60 pts.mass of a filler per 100 pts.mass of the rubber component, and having a loss factor tanδ of 0.045 or less as measured at 60°C under conditions of 10% static strain, 1% dynamic strain, and 10 Hz. The average peeling force is 300 N / mm or more when the rubber composition forming the cap tread and the rubber composition forming the base tread are bonded together and peeled at a speed of 50 mm / min.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to tires, and more particularly to tires having a tread rubber including a cap tread and a base tread. [Background technology]

[0002] With the increasing demand for improved fuel economy for automobiles, rubber compositions with excellent low heat buildup are also desired for tire treads. Tire treads are generally composed of a cap tread that comes into contact with the road surface and a base tread disposed on the inner periphery thereof, and it is known that a rubber composition with excellent low heat buildup is used for the base tread in order to improve the low heat buildup of the tire.

[0003] Tires also need to be strong enough to withstand running. However, if the amount of filler, which is a reinforcing material compounded in the rubber composition, is increased, although the strength is improved, the low heat buildup property tends to deteriorate.

[0004] Patent Document 1 discloses a rubber composition for a base tread having excellent fracture resistance and fuel economy, in which the dynamic modulus of elasticity, loss tangent, strength at break, and elongation at break satisfy a predetermined general formula.

[0005] Patent Document 2 discloses that durability and low heat buildup can be achieved by blending a predetermined amount of carbon black, silica, and a silane coupling agent into a rubber composition for a base tread, specifying the hardness of the rubber composition for the base tread and the hardness and storage modulus of the rubber composition for the cap tread, and further specifying the thicknesses of the base tread and the cap tread.

[0006] On the other hand, Patent Document 3 discloses that in a tread for an off-road tire, grip performance and mechanical strength are improved by blending a predetermined amount of carbon black and short fibers and specifying the orientation direction of the short fibers. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2018-058420 [Patent Document 2] Japanese Patent Publication No. 2022-148076 [Patent Document 3] Japanese Patent Application Publication No. 10-129214 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, tire treads are required to have low heat buildup and improved strength. For example, the demand for strength is particularly high for off-road tires, and it has been found that tires using conventional rubber compositions with excellent low heat buildup in the base tread have experienced separation near the interface between the cap tread and the base tread, particularly in the base tread area, during running tests in harsh terrain. Therefore, tread rubber is required to have improved low heat buildup while suppressing separation.

[0009] In view of the above, an object of an embodiment of the present invention is to provide a tire that can improve low heat buildup while maintaining tread separation resistance. [Means for solving the problem]

[0010] The present inventors have conducted extensive research in light of the above-mentioned problems and have found that by setting the loss factor tan δ of the rubber composition forming the base tread to 0.045 or less, and setting the average peel force in a state where the rubber composition forming the cap tread and the rubber composition forming the base tread are laminated together to 300 N / mm or more, separation originating from the base tread portion in the tire can be suppressed, thereby solving the above-mentioned problems.

[0011] The present invention includes the embodiments shown below. [1] A tire having a tread rubber including a cap tread and a base tread disposed radially inward of the cap tread, the base tread is formed from a rubber composition containing a rubber component including 70% by mass or more of natural rubber and 35 to 60 parts by mass of a filler per 100 parts by mass of the rubber component, and having a loss factor tanδ of 0.045 or less when measured under conditions of a temperature of 60°C, a static strain of 10%, a dynamic strain of 1% and a frequency of 10 Hz; A tire in which the average peel strength when the rubber composition forming the cap tread and the rubber composition forming the base tread are peeled off at a rate of 50 mm / min in a state where they are laminated together is 300 N / mm or more. [2] The tire according to [1], wherein the rubber composition forming the base tread has a 300% modulus of 12 to 20 MPa as measured in accordance with JIS K6251:2017. [3] The tire according to [1] or [2], wherein the rubber composition forming the base tread has a crescent tear strength of 40 N / mm or more as measured in accordance with JIS K6252-1:2015. [4] The filler of the rubber composition forming the base tread has a nitrogen adsorption specific surface area of ​​60 m 2 The tire according to any one of [1] to [3], containing carbon black of not more than 1 / g. [5] The tire according to any one of [1] to [4], wherein the filler in the rubber composition forming the base tread contains two or more types of carbon black. [6] The tire according to any one of [1] to [5], wherein the filler in the rubber composition forming the base tread contains silica. [7] The tire according to any one of [1] to [6], which is an off-road tire. [Effects of the Invention]

[0012] According to an embodiment of the present invention, it is possible to improve low heat buildup while maintaining tread separation resistance. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a half cross-sectional view of a tire showing one embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a test piece used to measure an average peel force. DETAILED DESCRIPTION OF THE INVENTION

[0014] The tire according to this embodiment includes a tread rubber including a cap tread and a base tread disposed radially inward of the cap tread.

[0015] Fig. 1 is a half cross-sectional view of an example of a pneumatic tire 1. In Fig. 1, CL represents the tire equator. The tire 1 has an annular meridian cross-sectional shape that is symmetrical with respect to the tire equator CL and extends in the tire circumferential direction.

[0016] The pneumatic tire 1 includes a pair of bead portions 2 mounted on a rim, a pair of sidewalls 3 extending radially outward from the bead portions 2, and a tread 4 connecting the pair of sidewalls 3. An annular bead core 21 and a rubber bead filler 22 radially outward are embedded in the bead portions 2. One or more (two in the example of FIG. 1 ) carcass plies 5 including organic fiber cords arranged in the radial direction are toroidally laid between the pair of left and right bead cores 21, and both ends of the carcass ply 5 are anchored by the bead cores 21.

[0017] A belt 6 consisting of a plurality of cross belt plies (two in the example of FIG. 1) including steel cords is provided on the outer periphery of the carcass ply 5 in the tread 4, and a tread rubber 7 that comes into contact with the road surface is provided on the outer periphery of the belt 6. In addition, a sidewall rubber 8 is provided on the outer side of the carcass ply 5 in the sidewall 3.

[0018] The tread rubber 7 has a two-layer structure, a so-called cap / base structure, including a cap tread 71 on the tire tread side that comes into contact with the road surface, and a base tread 72 arranged radially inward of the cap tread 71. Grooves (not shown) are provided on the surface of the tread 4, which form a tread pattern with lands such as blocks and ribs.

[0019] The cap tread 71 and the base tread 72 are each formed of a rubber composition containing a diene rubber as a rubber component. Here, the rubber composition forming the cap tread 71 is referred to as "rubber composition A," and the rubber composition forming the base tread 72 is referred to as "rubber composition B."

[0020] In rubber composition B, the rubber component contains 70% by mass or more of natural rubber (NR). That is, 100% by mass of the rubber component contains 70 to 100% by mass of natural rubber, and may be natural rubber alone. The proportion of natural rubber is preferably 75% by mass or more, more preferably 80% by mass or more. The rubber component may contain other diene rubbers in addition to natural rubber. Examples of other diene rubbers include isoprene rubber (polyisoprene) (IR), butadiene rubber (polybutadiene) (BR), styrene-butadiene rubber (SBR), butadiene-isoprene copolymer rubber, and styrene-butadiene-isoprene copolymer rubber.

[0021] Here, diene rubber refers to rubber having repeating units corresponding to diene monomers with conjugated double bonds. Diene rubber also includes those whose terminals or main chains have been modified as needed (for example, terminal-modified BR and terminal-modified SBR).

[0022] In one embodiment, the rubber component of Rubber Composition B preferably contains 70 to 100 mass% natural rubber and 0 to 30 mass% butadiene rubber, more preferably 75 to 100 mass% natural rubber and 0 to 25 mass% butadiene rubber, and even more preferably 80 to 100 mass% natural rubber and 0 to 20 mass% butadiene rubber. In this case, the rubber component may be natural rubber alone, i.e., 100 mass% natural rubber and 0 mass% butadiene rubber.

[0023] The butadiene rubber is not particularly limited and may be a modified butadiene rubber whose terminals and / or main chain are modified, or an unmodified butadiene rubber. The modified butadiene rubber is a BR modified with a functional group introduced into the terminals and / or main chain. The functional group preferably contains an oxygen atom and / or a nitrogen atom, and examples thereof include at least one selected from the group consisting of an amino group, a hydroxy group, an alkoxy group, an epoxy group, and a carboxy group.

[0024] The butadiene rubber may also be a high-cis butadiene rubber (high-cis BR) having a cis-1,4 bond content of 90% by mass or more (more preferably 95% by mass or more). Examples of high-cis BR include butadiene rubber polymerized using a cobalt catalyst or a neodymium catalyst. Here, the cis-1,4 bond content is 1 It is a value calculated from the integral ratio of the H-NMR spectrum.

[0025] Rubber composition B contains 35 to 60 parts by mass of filler per 100 parts by mass of the rubber component. When the amount of filler is 35 parts by mass or more, the strength of the base tread is maintained, and tread separation resistance is easily maintained. When the amount of filler is 60 parts by mass or less, low heat buildup is easily improved. The amount of filler in rubber composition B is preferably 40 to 55 parts by mass, more preferably 42 to 52 parts by mass, per 100 parts by mass of the rubber component.

[0026] As the filler, carbon black and / or silica, which are reinforcing fillers, are preferably used. In rubber composition B, the filler is a filler having a nitrogen adsorption specific surface area (N2SA) of 60 m from the viewpoint of low heat buildup. 2 It is preferable that the carbon black (X) contains carbon black having an N2SA of 20 to 60 m / g or less (hereinafter referred to as carbon black (X)). 2 / g, more preferably 20 to 50m 2 / g, more preferably 25 to 45m 2 / g.

[0027] In this specification, the nitrogen adsorption specific surface area (N2SA) of carbon black is measured in accordance with JIS K6217-2:2017 Method A.

[0028] In the rubber composition B, the content of the carbon black (X) is not particularly limited, but is preferably 10 to 45 parts by mass, more preferably 15 to 40 parts by mass, and even more preferably 20 to 30 parts by mass, per 100 parts by mass of the rubber component.

[0029] In the rubber composition B, the filler preferably contains two or more types of carbon black. As the two or more types of carbon black, for example, 2 Two or more kinds of carbon black (X) having an N2SA of 60m / g or less may be used. 2 / g (hereinafter referred to as carbon black (Y)), or a combination of carbon black (X) and carbon black (Y) may be used. By using two or more carbon blacks with different N2SAs in combination, it becomes easier to achieve both low heat buildup and strength.

[0030] The N2SA of carbon black (Y) is not particularly limited, but is preferably 60m 2 / g over 100m 2 / g or less, and more preferably 65 to 100m 2 / g, more preferably 70 to 95m 2 / g.

[0031] In the rubber composition B, the filler preferably contains silica. As the silica, wet silica such as wet precipitation silica and wet gelation silica is preferable. The nitrogen adsorption specific surface area (BET) of the silica is not particularly limited, and is, for example, 100 to 300 m 2 / g is also acceptable, 150-250m 2 / g is also acceptable, 180-220m 2 / g is also acceptable.

[0032] In this specification, the nitrogen adsorption specific surface area of ​​silica is the BET specific surface area measured in accordance with the BET method described in JIS K6430:2008.

[0033] In rubber composition B, the content of silica is not particularly limited, but is preferably 1 to 30 parts by mass, more preferably 2 to 25 parts by mass, and even more preferably 3 to 20 parts by mass, per 100 parts by mass of the rubber component.

[0034] In one embodiment, the filler of rubber composition B may contain, relative to 100 parts by mass of the rubber component, 10 to 45 parts by mass of carbon black (X) and 2 to 35 parts by mass of carbon black (Y) and / or silica, 15 to 40 parts by mass of carbon black (X) and 10 to 30 parts by mass of carbon black (Y) and / or silica, or 20 to 30 parts by mass of carbon black (X) and 15 to 25 parts by mass of carbon black (Y) and / or silica.

[0035] In addition to the above components, rubber composition B may contain various additives that are generally used in rubber compositions, such as a silane coupling agent, wax, an antioxidant, zinc oxide, stearic acid, oil, a vulcanizing agent, and a vulcanization accelerator.

[0036] Silane coupling agents are preferably used when silica is used as a filler. Examples of silane coupling agents include bis(3-triethoxysilylpropyl)tetrasulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(4-triethoxysilylbutyl)tetrasulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(2-trimethoxysilylethyl)tetrasulfide, bis(2-triethoxysilylethyl)trisulfide, bis(4-trimethoxysilylbutyl)trisulfide, bis(3-triethoxysilylpropyl)disulfide, bis(2-triethoxysilylethyl)disulfide, bis(4-triethoxysilylbutyl)disulfide, bis(3-triethoxysilylpropyl)disulfide, Examples of suitable silane coupling agents include sulfide-based silane coupling agents such as bis(2-trimethoxysilylpropyl)disulfide, bis(2-trimethoxysilylethyl)disulfide, bis(4-trimethoxysilylbutyl)disulfide, 3-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyltetrasulfide, 2-triethoxysilylethyl-N,N-dimethylthiocarbamoyltetrasulfide, and 3-triethoxysilylpropyl methacrylate monosulfide; and mercapto-based silane coupling agents such as 3-mercaptopropyltrimethoxysilane, 2-mercaptoethyltriethoxysilane, and 3-octanoylthio-1-propyltriethoxysilane. These may be used alone or in combination of two or more. The amount of silane coupling agent is not particularly limited and may be, for example, 2 to 20 parts by mass or 5 to 15 parts by mass per 100 parts by mass of silica.

[0037] The content of the wax is not particularly limited, and may be, for example, 0 to 10 parts by mass, 0.5 to 5 parts by mass, or 1 to 4 parts by mass per 100 parts by mass of the rubber component.

[0038] Examples of the antioxidant include various antioxidants such as amine-ketone, aromatic secondary amine, monophenol, bisphenol, and benzimidazole antioxidants, any of which may be used alone or in combination of two or more. The content of the antioxidant is not particularly limited, and may be, for example, 0 to 10 parts by mass, 0.5 to 5 parts by mass, or 1 to 4 parts by mass per 100 parts by mass of the rubber component.

[0039] The content of zinc oxide is not particularly limited, and may be, for example, 0 to 10 parts by mass, 0.5 to 7 parts by mass, or 1 to 5 parts by mass per 100 parts by mass of the rubber component.

[0040] The content of stearic acid is not particularly limited, and may be, for example, 0 to 10 parts by mass, 0.5 to 5 parts by mass, or 1 to 4 parts by mass per 100 parts by mass of the rubber component.

[0041] The vulcanizing agent is preferably sulfur, and examples thereof include powdered sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, highly dispersible sulfur, etc. The amount of sulfur is not particularly limited, and may be 0.1 to 5 parts by mass, 0.5 to 4 parts by mass, or 1 to 3 parts by mass per 100 parts by mass of the rubber component.

[0042] Examples of the vulcanization accelerator include various vulcanization accelerators such as sulfenamide-based, guanidine-based, thiuram-based, and thiazole-based accelerators, and any one of them can be used alone or in combination of two or more. The content of the vulcanization accelerator is not particularly limited, and may be 0.1 to 5 parts by mass, 0.5 to 4 parts by mass, or 1 to 3 parts by mass per 100 parts by mass of the rubber component.

[0043] In this embodiment, the formulation of the rubber composition A that forms the cap tread 71 is not particularly limited, and a rubber composition that is normally used for cap treads of pneumatic tires can be applied. For example, the rubber composition A may contain a rubber component containing a diene rubber, and carbon black and / or silica.

[0044] In rubber composition A, the diene rubber as the rubber component is not particularly limited. In one embodiment, the rubber component preferably contains at least one selected from the group consisting of natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR), and more preferably contains natural rubber and styrene-butadiene rubber. The rubber component of rubber composition A may contain, for example, 30 to 90 parts by mass of natural rubber and 10 to 70 parts by mass of styrene-butadiene rubber, 50 to 80 parts by mass of natural rubber and 20 to 50 parts by mass of styrene-butadiene rubber, or 60 to 75 parts by mass of natural rubber and 25 to 40 parts by mass of styrene-butadiene rubber.

[0045] In the rubber composition A, it is preferable to use the above carbon black (Y) as the carbon black. In addition, it is preferable to use silica having a nitrogen adsorption specific surface area (BET) of, for example, 100 to 300 m 2 / g, more preferably 150 to 250m 2 / g, more preferably 180 to 220m 2 / g. In the rubber composition A, it is preferable to use a combination of carbon black (Y) and silica as the filler. The amount of carbon black and / or silica is not particularly limited, and may be, for example, 30 to 100 parts by mass or 40 to 80 parts by mass per 100 parts by mass of the rubber component.

[0046] In addition to the above components, various additives generally used in rubber compositions, such as a silane coupling agent, wax, antioxidant, zinc oxide, stearic acid, oil, vulcanizing agent, vulcanization accelerator, etc., can be blended into the rubber composition A. Details and contents of these additives are as described above for the rubber composition B.

[0047] The rubber compositions A and B can be prepared by kneading in a conventional manner using a commonly used mixer such as a Banbury mixer, kneader, roll, etc. That is, for example, in the first mixing stage, additives other than the vulcanizing agent and vulcanization accelerator are added and mixed with the rubber component, and then in the final mixing stage, the vulcanizing agent and vulcanization accelerator are added and mixed with the resulting mixture to prepare the rubber composition.

[0048] In this embodiment, the rubber composition B forming the base tread has a loss factor tanδ of 0.045 or less, measured under conditions of a temperature of 60°C, a static strain of 10%, a dynamic strain of 1%, and a frequency of 10 Hz. A loss factor of 0.045 or less can improve low heat buildup properties. The lower the loss factor, the less likely it is to generate heat and the better the low heat buildup properties, so there is no particular lower limit, but it is usually 0.025 or more. The loss factor of rubber composition B is more preferably 0.025 to 0.044, and even more preferably 0.025 to 0.043.

[0049] The loss factor tanδ of rubber composition B is the loss tangent in a tensile mode measured on a vulcanized rubber sample in accordance with JIS K6394:2007 under conditions of a temperature of 60°C, a static strain of 10%, a dynamic strain of 1%, and a frequency of 10 Hz.

[0050] Rubber composition B preferably has a 300% modulus (M300) of 12 to 20 MPa as measured in accordance with JIS K6251:2017. Having a 300% modulus of 12 MPa or more reduces stress concentration when deformed by force, making it easier to improve average peel force and tread separation resistance. The 300% modulus of rubber composition B is preferably 14 to 20 MPa, more preferably 16 to 20 MPa.

[0051] The 300% modulus of rubber composition B is the tensile stress at 300% elongation measured at a temperature of 23°C for a vulcanized rubber sample in accordance with JIS K6251:2017.

[0052] The rubber composition B preferably has a crescent tear strength of 40 N / mm or more as measured in accordance with JIS K6252-1:2015. A tear strength of 40 N / mm or more improves cut resistance, making it easier to improve the average peel force and tread separation resistance. Since a higher tear strength is preferable, the upper limit is not particularly limited, but is usually 100 N / mm or less.

[0053] The tear strength of rubber composition B is the maximum force required to tear a crescent-shaped test piece measured at 23°C in accordance with JIS K6252-1:2015 for a vulcanized rubber sample, divided by the thickness of the test piece.

[0054] In this embodiment, the average peel force when rubber composition A and rubber composition B are laminated together and peeled at a rate of 50 mm / min is 300 N / mm or more. Having an average peel force of 300 N / mm or more can improve tread separation resistance. The higher the average peel force, the better the tread separation resistance, so there is no particular upper limit, but it is usually 600 N / mm or less. The average peel force is preferably 320 to 600 N / mm, and more preferably 350 to 550 N / mm. Here, the average peel force is the average value of the peel force per mm width of the test piece, and the measurement method is as described in the Examples section.

[0055] As described above, separation of the tread rubber occurs starting from the base tread portion, and therefore, in order to increase the average peel force, for example, it is possible to increase the ratio of natural rubber in rubber composition B, to compound carbon black (Y) and / or silica and increase the amount of carbon black (Y) and / or silica compounded, or to increase the total amount of filler.

[0056] The method for manufacturing the tire according to the present embodiment is not particularly limited. For example, rubber compositions A and B are each molded into a predetermined shape by extrusion processing according to a conventional method, to obtain an unvulcanized tread rubber member in which a cap tread rubber member and a base tread rubber member are laminated. The tread rubber member is combined with other tire components to produce an unvulcanized tire (green tire). Thereafter, a tire can be manufactured by vulcanizing and molding at, for example, 140°C to 180°C.

[0057] The type of tire according to the present embodiment is not particularly limited, and examples thereof include pneumatic tires of various sizes and for various uses, such as tires for passenger cars, large tires for trucks and buses, etc. Preferably, the tire is an off-road tire suitable for traveling on rough ground such as sandy or rocky areas, and more preferably, an off-road pneumatic radial tire having a block tread pattern. [Example]

[0058] Examples of the present invention will be shown below, but the present invention is not limited to these examples.

[0059] [Preparation of Rubber Composition B] Using a Banbury mixer, first, in the first mixing stage, compounding ingredients excluding sulfur and vulcanization accelerator were added to the rubber component and kneaded (discharge temperature = 155°C) according to the formulation (parts by mass) shown in Tables 1 and 2 below. Next, in the final mixing stage, sulfur and vulcanization accelerator were added to the obtained kneaded mixture and kneaded (discharge temperature = 90°C) to prepare rubber composition B for base tread. Details of each component in Tables 1 and 2 are as follows.

[0060] Natural rubber: RSS#3 Butadiene rubber 1: High-cis BR (cis-1,4 bond content 97% by mass) polymerized using a Co catalyst, "UBEPOL BR150B" manufactured by UBE Corporation Butadiene rubber 2: High-cis BR (cis-1,4 bond content 95% by mass) polymerized using a neodymium catalyst, "BR730" manufactured by ENEOS Material Corporation Butadiene rubber 3: Terminally modified BR, "Nipol BR1250H" manufactured by Nippon Zeon Co., Ltd.

[0061] Carbon black 1: N2SA=27m 2 / g, "Seast V" manufactured by Tokai Carbon Co., Ltd. Carbon Black 2: N2SA=42m 2 / g, "Seat SO" manufactured by Tokai Carbon Co., Ltd. Carbon Black 3: N2SA=84m 2 / g, "Seat 300" manufactured by Tokai Carbon Co., Ltd. Carbon Black 4: N2SA=93m 2 / g, "Seast KH" manufactured by Tokai Carbon Co., Ltd. Silica: Tosoh Silica Corporation's "Nipsil AQ" (nitrogen adsorption specific surface area 205 m 2 / g) Silane coupling agent: sulfide type, "Si75" manufactured by Evonik Japan

[0062] Wax: "OZOACE 1722" manufactured by Nippon Seiro Co., Ltd. Anti-aging agent 1: "Nocrac 6C" manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Anti-aging agent 2: "ANTIOXIDANT TMQ" manufactured by Kemai Chemical Co., Ltd. Zinc oxide: "Zinc oxide type 3" manufactured by Mitsui Mining & Smelting Co., Ltd. Stearic acid: Kao Corporation's "Lunac S20"

[0063] Sulfur: 5% oil-treated powdered sulfur manufactured by Tsurumi Chemical Industry Co., Ltd. Vulcanization accelerator: TBBS, Sansera NS-G manufactured by Sanshin Chemical Industry Co., Ltd.

[0064] [Table 1]

[0065] [Table 2]

[0066] [Preparation of Rubber Composition A] Using a Banbury mixer, first, in the first mixing stage, compounding ingredients excluding sulfur and vulcanization accelerator were added to the rubber component and kneaded (discharge temperature = 155°C) according to the formulation shown in Table 3 below. Next, in the final mixing stage, sulfur and vulcanization accelerator were added to the obtained kneaded mixture and kneaded (discharge temperature = 90°C) to prepare rubber composition A for cap tread.

[0067] [Table 3]

[0068] [Measurement and Evaluation] (tanδ@60℃, low heat buildup, hardness, M300, tear strength) Each rubber composition B obtained was vulcanized at 150°C for 30 minutes to prepare vulcanized rubber samples of a predetermined shape. The resulting rubber samples were used to measure and evaluate the loss coefficient (tan δ@60°C), low heat buildup, hardness, M300 (300% modulus), and tear strength. The measurement and evaluation methods are as follows. The results are shown in Tables 1 and 2.

[0069] Tan δ@60°C: Measured in accordance with JIS K6394:2007. Specifically, the loss factor tan δ was measured for a rubber sample measuring 5 mm wide x 40 mm long x 2 mm thick using a viscoelasticity measuring device manufactured by GABO under the following conditions (tensile mode): temperature 60°C, static strain 10%, dynamic strain 1%, and frequency 10 Hz.

[0070] Low heat buildup: The tan δ@60°C measured above was expressed as an index, with the value of Comparative Example 1 being 100. The smaller the index, the smaller the tan δ, and therefore the less likely it is to generate heat, indicating excellent low heat buildup.

[0071] Hardness: The hardness of a 12.5 mm thick rubber sample was measured at 23°C in accordance with the Durometer Type A method of JIS K6253-3:2012.

[0072] M300: In accordance with JIS K6251:2017, a 2mm thick dumbbell-shaped No. 3 rubber sample was tested at a tensile speed of 500mm / min (measurement temperature 23°C) using a Shimadzu tensile testing machine to determine the tensile stress at 300% elongation.

[0073] Tear strength: Measured in accordance with JIS K6252-1:2015. Specifically, a rubber sample (2 mm thick) was punched into a crescent shape and cut with a 0.50±0.08 mm notch in the center of the indentation. Testing was carried out at a pulling rate of 500 mm / min (measurement temperature 23°C) using a Shimadzu tensile testing machine. The maximum tear force until the rubber sample broke was read and divided by the thickness of the rubber sample to determine the tear strength.

[0074] Rubber composition A prepared according to the formulation shown in Table 3 was vulcanized at 150°C for 30 minutes, and the M300 of the rubber sample was measured using the same method as for rubber composition B. As a result, the M300 of rubber composition A was 9.2 MPa.

[0075] (average peel force) The average peel strength was measured for a test piece obtained by laminating rubber composition B to rubber composition A. The measurement method is as follows. The results are shown in Tables 1 and 2.

[0076] An unvulcanized rubber sheet of rubber composition A and an unvulcanized rubber sheet of rubber composition B were overlapped, a PET film sandwiched between them, and reinforcing layers were placed on both the front and back of the unvulcanized rubber sheet. The resulting mixture was vulcanized at 150°C for 30 minutes to produce a test specimen 10 with the cross-sectional shape shown in Figure 2. As shown in Figure 2, test specimen 10 is made by laminating a 130 mm long x 3 mm thick rubber sheet 11 made of rubber composition A and a 130 mm long x 3 mm thick rubber sheet 12 made of rubber composition B together. A PET film 13 is sandwiched between the two sheets extending 25 mm from one end in the longitudinal direction, providing a gripping portion 16 (non-adhesive portion) for use in a peel test. Reinforcing layers 14 and 15 are vulcanization-adhered to both the front and back of test specimen 10.

[0077] The reinforcing layers 14, 15 are sheets for reinforcing the rubber sheets 11, 12 so that they can be peeled off without being cut during a peel test, and are made of topping material containing organic fiber cords that are generally used in carcass plies. The topping material is made of organic fiber cords that are parallel to the longitudinal direction of the test piece 10 and coated with rubber. In this example, polyester fiber cords were used as the organic fiber cords.

[0078] After vulcanization, the test piece 10 was cut into a strip shape with a width of 10 mm. The gripping portion of the obtained strip-shaped test piece 10 was held by an "Autograph DCS500" manufactured by Shimadzu Corporation, and the test piece 10 was pulled up and down at a peeling speed of 50 mm / min so that the test piece 10 was in a T-shape, and the test piece 10 was peeled off for 80 mm or more in the longitudinal direction.

[0079] The average peel strength was calculated as follows: all peak values ​​of peel strength from the start of peeling to a peel length of 80 mm were detected. The detection sensitivity was set to 1 / 100 of the full scale. The peel strength is defined by the following formula: Peel strength (N / mm) = Peel force (N) / width of test piece (mm) Of all the detected peak values, the peak values ​​from the start of peeling to a length equivalent to 5% of the peel length (i.e., 4 mm) were excluded. Therefore, the total peel length was defined as the time from 4 mm peeling to 80 mm peeling, and this total peel length was divided equally into 5 sections, and the average of all peak values ​​of peel strength in each section was calculated. The average value of these was calculated from the average peak values ​​calculated for each section. The obtained value was the average of all peak values ​​over the total peel length, and this was defined as the average peel strength (N / mm).

[0080] (Tire evaluation: tread separation resistance) Using rubber composition A for the cap tread and rubber composition B for the base tread, a test tire (off-road tire, tire size: 265 / 70R18, tire internal pressure: 230 kPa, maximum load: 2400 kg) was manufactured according to a conventional method. The test tire was mounted on a vehicle and driven around a gravel road with a lap length of 200 m at a speed of 50 km / h. The tread surface was checked after 40 laps to see if there were any cracks from the bottom of the groove to the block. If there were no cracks of 5 mm or more, the tire was rated "P (pass)," and if there were any cracks of 5 mm or more, the tire was rated "F (fail)." The results are shown in Tables 1 and 2.

[0081] As shown in Table 2, Comparative Example 1, which is a standard example, had tread separation resistance, but the loss factor tan δ of rubber composition B was high and the tire was poor in low heat buildup. In Comparative Example 2, the loss factor tan δ of rubber composition B was reduced compared to Comparative Example 1, resulting in a low heat buildup in the base tread, but the 300% modulus and average peel force were small and the tire was poor in tread separation resistance. In Comparative Example 3, the total filler amount of rubber composition B was increased to give the base tread a high hardness suitable for off-road use, and the average peel force was large and the tire was excellent in tread separation resistance, but the loss factor tan δ of rubber composition B was large and the tire was poor in low heat buildup.

[0082] In Comparative Example 4, the total filler amount of rubber composition B was within the specified range, and the average release force was high, resulting in excellent tread separation resistance, but the loss factor tan δ was high and the tire had poor low heat buildup. In Comparative Example 5, the total filler amount of rubber composition B was within the specified range, but the proportion of natural rubber was low, resulting in low tear strength and average release force, resulting in poor tread separation resistance, and also a high loss factor tan δ compared to Comparative Example 1, resulting in poor low heat buildup. In Comparative Example 6, the loss factor tan δ of rubber composition B was small, resulting in excellent low heat buildup, but the tear strength and average release force were low, resulting in poor tread separation resistance.

[0083] In contrast, in Examples 1 to 8, the loss factor tanδ of rubber composition B was 0.045 or less, indicating excellent low heat buildup. Furthermore, rubber composition B had a high 300% modulus and tear strength, indicating excellent strength. Furthermore, the average peel strength between rubber composition A and rubber composition B was 300 N / mm or more, indicating excellent tread separation resistance. Therefore, in Examples 1 to 8, low heat buildup was improved while maintaining tread separation resistance.

[0084] The various numerical ranges described in this specification can be arbitrarily combined with their upper and lower limits, and all such combinations are considered to be preferred numerical ranges described in this specification. Furthermore, a numerical range described as "X to Y" means from X to Y. [Explanation of symbols]

[0085] 1...pneumatic tire, 4...tread, 7...tread rubber, 71...cap tread, 72...base tread

Claims

1. A tire having a tread rubber including a cap tread and a base tread disposed radially inward of the cap tread, the base tread is formed from a rubber composition containing a rubber component including 70% by mass or more of natural rubber and 35 to 60 parts by mass of a filler per 100 parts by mass of the rubber component, the rubber composition having a loss factor tanδ of 0.045 or less as measured under conditions of a temperature of 60°C, a static strain of 10%, a dynamic strain of 1% and a frequency of 10 Hz; A tire in which the average peel strength when the rubber composition forming the cap tread and the rubber composition forming the base tread are peeled off at a speed of 50 mm / min in a state where they are laminated together is 300 N / mm or more.

2. The tire according to claim 1, wherein the rubber composition forming the base tread has a 300% modulus measured in accordance with JIS K6251:2017 of 12 to 20 MPa.

3. The tire according to claim 1 or 2, wherein the rubber composition forming the base tread has a crescent tear strength of 40 N / mm or more as measured in accordance with JIS K6252-1:2015.

4. The filler of the rubber composition forming the base tread has a nitrogen adsorption specific surface area of ​​60 m ２ 3. The tire of claim 1 or 2, comprising carbon black in an amount of 0.1g or less.

5. The tire according to claim 1 or 2, wherein the filler in the rubber composition forming the base tread comprises two or more types of carbon black.

6. 3. The tire according to claim 1, wherein the filler of the rubber composition forming the base tread comprises silica.

7. 3. The tire according to claim 1, which is an off-road tire.

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