Tire

Abstract

To provide a tire having high grip performance retention after abrasion wear.SOLUTION: A tire comprises a tread rubber having a cap rubber and a base rubber. The cap rubber is formed from a rubber composition A which contains diene rubber, silica, and a softening agent with a molecular weight of 1000 or less that is liquid at 20°C. The base rubber is formed from a rubber composition B which contains diene rubber, and contains or does not contain a softening agent with a molecular weight of 1000 or less that is liquid at 20°C. The diene rubber in the rubber composition A contains modified styrene-butadiene rubber and unmodified styrene-butadiene rubber. When an amount of the softening agent in 100% by volume of the rubber composition A is represented by a (vol%), and an amount of the softening agent in 100% by volume of the rubber composition B is represented by b (vol%), 0≤a-b≤15.0 is satisfied.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a tire.

Background Art

[0002] Tires are required to have wet grip performance, which is the grip performance on a wet road surface, and silica having excellent performance is compounded in the tread rubber. Generally, the tread rubber of a tire has a two-layer structure (cap / base structure) of a cap rubber on the tread side of the tire and a base rubber disposed inside thereof. In recent years, as the requirement for wet grip performance has increased, rubber compositions containing a large amount of silica (highly filled silica compounds) have been widely used in cap rubbers. By increasing the amount of silica in this way, the wet grip performance is improved while the processability deteriorates. Therefore, in the case of highly filled silica compounds, the degree of deterioration of processability is suppressed by compounding a large amount of softening agent.

[0003] Patent Document 1 describes that by forming a cap rubber with a rubber composition containing a highly filled silica compound and reducing the styrene content of the styrene-butadiene rubber contained in the rubber composition, heat deterioration during high-speed driving is suppressed and wet grip performance is improved. Patent Document 1 also describes that by increasing the acetone extract, which is an index indicating the amount of softening agent in the cap rubber, wet grip performance can be stably exhibited.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a tread rubber having a cap / base structure, if the amount of softening agent in the cap rubber is large and the amount of softening agent in the base rubber is small, the softening agent in the cap rubber migrates to the base rubber due to the difference in component concentration of the softening agent. As a result, in the cap rubber, a concentration difference of the softening agent occurs between a portion near the surface layer and a portion near the interface with the base rubber. That is, when the softening agent easily migrates from the cap rubber to the base rubber, the softness of the rubber in the portion of the cap rubber near the base rubber decreases. Therefore, when the tread rubber wears to a stage near the portion close to the base rubber, the wet grip performance significantly decreases from when it was new. Thus, it is required to suppress as much as possible the decrease in wet grip performance due to such wear, that is, to increase the retention rate of the wet grip performance after wear.

[0006] Note that Patent Document 1 describes that the wet grip performance is improved by increasing the amount of softening agent in the cap rubber as described above. Patent Document 2 describes that in order to suppress the movement of the softening agent from the cap rubber to the base rubber, the softening agent content in the cap rubber is made less than the softening agent content in the base rubber, thereby suppressing the change in hardness over time. Patent Document 3 describes that by including a liquid polymer as a softening agent in the cap rubber, the migration of an antioxidant or wax to the base rubber is suppressed to improve the weather resistance. However, it has not been known that the retention rate of the wet grip performance after wear can be improved by setting a specific relationship between the amount of softening agent in the cap rubber and the amount of softening agent in the base rubber while using a specific diene rubber in the cap rubber.

[0007] An embodiment of the present invention aims to provide a tire having a high retention rate of wet grip performance after wear.

Means for Solving the Problems

[0008] The present invention includes the embodiments shown below. [1] A tire comprising a tread rubber having a cap rubber and a base rubber disposed inside the cap rubber, wherein the cap rubber is formed of a rubber composition A containing a diene rubber, silica, and a softening agent having a molecular weight of 1000 or less and being liquid at 20°C, the base rubber is formed of a rubber composition B containing a diene rubber and optionally containing a softening agent having a molecular weight of 1000 or less and being liquid at 20°C, the diene rubber of the rubber composition A includes a modified styrene-butadiene rubber and an unmodified styrene-butadiene rubber, and taking the amount of the softening agent in 100% by volume of the rubber composition A as a (volume %) and the amount of the softening agent in 100% by volume of the rubber composition B as b (volume %), a tire satisfying 0 ≦ a - b ≦ 15.0. [2] The tire according to [1], wherein the rubber composition A contains 105 to 200 parts by mass of silica with respect to 100 parts by mass of the diene rubber. [3] The tire according to [1] or [2], wherein 100 parts by mass of the diene rubber of the rubber composition B contains 50 to 100 parts by mass of natural rubber and / or synthetic isoprene rubber and 0 to 50 parts by mass of butadiene rubber. [4] The tire according to any one of [1] to [3], wherein the rubber composition A contains 3 to 50 parts by mass of a thermoplastic resin having a weight average molecular weight of 1000 to 30000 with respect to 100 parts by mass of the diene rubber. [Effects of the Invention]

[0009] According to an embodiment of the present invention, a tire having a high retention rate of wet grip performance after wear can be provided. [Brief Description of the Drawings]

[0010]

Figure 1

[0011] FIG. 1 shows a tire 1 according to an embodiment. The tire 1 is a pneumatic tire including a pair of bead parts 4 to be assembled to a rim, a pair of sidewalls 3 extending radially outward from the bead parts 4, and a tread 2 connecting between the pair of sidewalls 3. The tire 1 includes a carcass 6 formed by folding back and locking a carcass ply made of organic fiber cords arranged radially around bead cores 5 respectively embedded in the pair of bead parts 4, a belt 7 composed of two crossed belt plies made of steel cords arranged on the outer periphery of the carcass 6 in the tread 2, and a cap ply 8 made of organic fiber cords arranged on the outer periphery of the belt 7.

[0012] On the outer periphery of the cap ply 8 in the tread 2, a tread rubber 9 that contacts the road surface is provided. The tread rubber 9 has a two-layer structure including a cap rubber 91 on the tire tread side and a base rubber 92 arranged inside the cap rubber 91, and is a so-called cap / base structure.

[0013] The cap rubber 91 and the base rubber 92 are each formed of a rubber composition containing a diene-based rubber. Here, the rubber composition forming the cap rubber 91 is referred to as "rubber composition A", and the rubber composition forming the base rubber 92 is referred to as "rubber composition B".

[0014] The tread rubber according to this embodiment is such that the amount of softener in the cap rubber and the amount of softener in the base rubber satisfy a specific relationship. That is, taking the amount of softener in 100% by volume of the rubber composition A of the cap rubber as a (% by volume) and the amount of softener in 100% by volume of the rubber composition B of the base rubber as b (% by volume), the difference between the two (a - b) satisfies the following formula (1). Formula (1): 0 ≦ a - b ≦ 15.0

[0015] By setting the amount a (volume %) of the softening agent in the cap rubber to be equal to or greater than the amount b (volume %) of the softening agent in the base rubber and making the difference (a - b) 15.0 volume % or less, the migration of the softening agent from the cap rubber to the base rubber can be reduced, and the retention rate of the wet grip performance after wear can be increased. The difference (a - b) in the amount of the softening agent is preferably from 3.0 to 14.0 volume %, more preferably from 3.5 to 12.0 volume %, and still more preferably from 4.0 to 10.0 volume %.

[0016] In this specification, the amounts a and b (volume %) of the softening agent are calculated by the following formula. Amount of softening agent (volume %) = { (amount of softening agent (parts by mass) / specific gravity of softening agent) / (total amount of rubber composition (parts by mass) / compounding specific gravity of rubber composition)} × 100 Here, the compounding specific gravity of the rubber composition is the specific gravity of the rubber composition calculated from the amount (parts by mass) and specific gravity of each component. The specific gravity is based on water at 4°C under standard atmospheric pressure.

[0017] [Rubber Composition A] The rubber composition A that forms the cap rubber contains a diene rubber as a rubber component, silica, and a softening agent having a molecular weight of 1000 or less and being liquid at 20°C.

[0018] The diene rubber refers to a rubber having a repeating unit corresponding to a diene monomer having a conjugated double bond, and the main chain of the polymer contains a carbon-carbon double bond. As the diene rubber, usually, a solid diene rubber is used. In this specification, "solid" means having no fluidity at room temperature of 20°C.

[0019] In this embodiment, styrene-butadiene rubber (SBR) is used as the diene rubber of the rubber composition A. Specifically, a modified styrene-butadiene rubber (hereinafter also referred to as "modified SBR") and an unmodified styrene-butadiene rubber (hereinafter also referred to as "unmodified SBR") are used in combination. The modified SBR is advantageous for wet grip performance by improving the dispersibility of silica. On the other hand, when the modified SBR is contained in the rubber composition A, due to the influence of the polarity difference between the modified SBR and the softening agent, etc., the softening agent tends to migrate from the cap rubber to the base rubber. In contrast, in the case of unmodified SBR, such migration of the softening agent to the base rubber can be suppressed. Therefore, by using the modified SBR and the unmodified SBR in combination, in combination with the setting of the difference (a - b) in the amount of the softening agent described above, the retention rate of the wet grip performance after wear can be increased.

[0020] As the styrene-butadiene rubber, solution-polymerized styrene-butadiene rubber (SSBR) may be used, or emulsion-polymerized styrene-butadiene rubber (ESBR) may be used. The modified SBR is one in which its terminal and / or main chain is modified in such SBR, and the unmodified SBR is one that is not modified.

[0021] As the modified SBR, an SBR modified by a functional group is used by introducing a functional group that interacts with silica at the terminal and / or main chain. As the functional group, those containing an oxygen atom and / or a nitrogen atom are preferable, and examples include at least one selected from the group consisting of an amino group, a hydroxy group, an amide group, an alkoxy group, a silyl group, an alkoxysilyl group, an epoxy group, and a carboxy group. By using such a modified SBR containing a functional group, the dispersibility of silica can be improved.

[0022] The diene rubber of rubber composition A may be only modified SBR and unmodified SBR, but may also contain other diene rubbers together with these. Examples of other diene rubbers include natural rubber (NR), synthetic isoprene rubber (IR), butadiene rubber (BR), nitrile rubber (NBR), chloroprene rubber (CR), and the like. Among these, as other diene rubbers, it is preferably at least one selected from the group consisting of NR, IR, and BR.

[0023] 100 parts by mass of the diene rubber of rubber composition A may contain 10 to 90 parts by mass of modified SBR and 10 to 90 parts by mass of unmodified SBR. Preferably, it contains 20 to 80 parts by mass of modified SBR and 20 to 80 parts by mass of unmodified SBR, and more preferably contains 30 to 70 parts by mass of modified SBR and 30 to 70 parts by mass of unmodified SBR.

[0024] In one embodiment, 100 parts by mass of the diene rubber of rubber composition A preferably contains 10 to 50 parts by mass of modified SBR, 20 to 60 parts by mass of unmodified SBR, and 10 to 50 parts by mass of BR, and more preferably contains 20 to 40 parts by mass of modified SBR, 30 to 50 parts by mass of unmodified SBR, and 20 to 40 parts by mass of BR. In other embodiments, 100 parts by mass of the diene rubber of rubber composition A preferably contains 40 to 70 parts by mass of modified SBR, 25 to 55 parts by mass of unmodified SBR, and 5 to 25 parts by mass of NR and / or IR, and more preferably contains 45 to 65 parts by mass of modified SBR, 30 to 50 parts by mass of unmodified SBR, and 5 to 20 parts by mass of NR and / or IR.

[0025] Silica is compounded in rubber composition A as a reinforcing filler. Examples of silica include wet silica and dry silica. Preferably, wet silica such as wet precipitated silica and wet gelation silica is used.

[0026] The nitrogen adsorption specific surface area (BET) of the silica is not particularly limited, and may be, for example, 100 to 300 m 2 / g, may be 150 to 250 m 2 / g, may be 160 to 220 m2 It may also be / g. 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.

[0027] The silica content is preferably 105 to 200 parts by mass with respect to 100 parts by mass of the diene rubber, and the wet grip performance can be improved. The silica content is more preferably 110 to 180 parts by mass, more preferably 115 to 150 parts by mass, and still more preferably 120 to 140 parts by mass with respect to 100 parts by mass of the diene rubber.

[0028] In the rubber composition A, as the reinforcing filler, silica alone may be used, or carbon black may be used together with silica. The content of carbon black is not particularly limited. For example, it may be 1 to 30 parts by mass, 2 to 20 parts by mass, or 3 to 10 parts by mass with respect to 100 parts by mass of the diene rubber.

[0029] The rubber composition A preferably contains a silane coupling agent. By containing the silane coupling agent, the dispersibility of silica is improved. Therefore, even when the amount of the softening agent is reduced in the rubber composition A to satisfy the above formula (1), it is easier to maintain the processability. The content of the silane coupling agent is preferably 2 to 20 parts by mass, more preferably 5 to 15 parts by mass with respect to 100 parts by mass of silica.

[0030] Examples of the silane coupling agent include sulfide silane coupling agents such as bis(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)disulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(4-triethoxysilylbutyl)disulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, and bis(2-trimethoxysilylethyl)disulfide; mercapto silane coupling agents such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropyldimethylmethoxysilane, and mercaptoethyltriethoxysilane; and thioester group-containing silane coupling agents such as 3-octanoylthio-1-propyltriethoxysilane, 3-propionylthiopropyltrimethoxysilane, 3-hexanoylthio-1-propyltriethoxysilane, and 3-octanoylthio-1-propyltrimethoxysilane. These can be used alone or in combination of two or more thereof.

[0031] The rubber composition A is compounded with a softening agent. The softening agent is a compounding agent for imparting softness to the compounded rubber and enhancing plasticity and processability. In the present specification, those having a molecular weight of 1000 or less and being liquid at normal temperature are used. Therefore, for example, those containing sulfur such as a silane coupling agent are not included in the softening agent even if they are liquid and have a low molecular weight at normal temperature. In the present specification, "being liquid at normal temperature" means having fluidity at 20°C. Further, the molecular weight means the weight average molecular weight (Mw) when the softening agent is a polymer.

[0032] Specific examples of the softening agent include polyolefin oils, naphthenic oils, paraffin oils, Distillate Aromatic Extracts (DAE) oils, Medium Extracted Solvates (MES) oils, Treated Distillate Aromatic Extracts (TDAE) oils, Residual Aromatic Extracts (RAE) oils, Treated Residual Aromatic Extracts (TRAE) oils, Safety Residual Aromatic Extracts (SRAE) oils, mineral oils, vegetable oils, ether plasticizers, ester plasticizers, phosphate ester plasticizers, sulfonate plasticizers, liquid diene polymers, and mixtures of any two or more of these. Among these, hydrocarbon oils such as polyolefin oils, naphthenic oils, paraffin oils, aromatic oils, and mineral oils are preferred.

[0033] In rubber composition A, the content of the softening agent is not particularly limited as long as the above formula (1) is satisfied. For example, the amount (parts by mass) of the softening agent may be 10 to 55 parts by mass, preferably 15 to 50 parts by mass, more preferably 20 to 45 parts by mass, based on 100 parts by mass of the diene rubber. Also, the amount a (volume %) of the softening agent may be, for example, 5.0 to 22.0 volume %, preferably 7.0 to 20.0 volume %, more preferably 8.0 to 19.5 volume %, and still more preferably 8.5 to 17.0 volume %. Note that when an oil-extended rubber is used as the diene rubber, the amount of oil contained in the oil-extended rubber is also included in the amount of the softening agent.

[0034] Rubber composition A preferably contains a thermoplastic resin having a weight average molecular weight (Mw) of 1000 to 30000. The thermoplastic resin is in a solid state at normal temperature and is plasticized by heating during the kneading of rubber composition A. Therefore, even when the amount of the softening agent in rubber composition A is reduced to satisfy the above formula (1), a decrease in processability can be suppressed.

[0035] The weight average molecular weight of the thermoplastic resin is more preferably greater than 1000 and not more than 20000, more preferably 1100 to 10000, more preferably 1500 to 8000, and still more preferably 2000 to 5000.

[0036] In this specification, the weight average molecular weight (Mw) is determined in terms of polystyrene by measurement using gel permeation chromatography (GPC). Specifically, in the following examples, a solution prepared by dissolving 10 mg of a sample in 5 mL of tetrahydrofuran was used as the measurement target. After filtering the measurement target, using "Nexera" manufactured by Shimadzu Corporation, at a temperature of 40 °C and a flow rate of 1.0 mL / min, it was passed through columns (manufactured by Agilent Technologies, PLgel GUARD 5μm 50×7.5mm + PLgel 50Å 5μm 300×7.5mm + PLgel 100Å 5μm 300×7.5mm + PLgel 500Å 5μm 300×7.5mm), detected with a differential refractive index detector, and the molecular weight was calculated in terms of polystyrene using commercially available standard polystyrene.

[0037] Specific examples of the thermoplastic resin include terpene resins, styrene resins, petroleum resins, rosin resins, coumarone resins, etc., and any one or two or more of these may be used in combination. Among these, from the viewpoint of compatibility with diene rubbers, resins having aromatic units (structural units having an aromatic ring) such as styrene resins are preferred.

[0038] The terpene resin is a resin obtained by polymerizing terpene monomers such as α-pinene, β-pinene, limonene, and dipentene. Examples thereof include polyterpene resins produced using only terpene monomers, terpene phenol resins, and aromatic-modified terpene resins. As the polyterpene resin, for example, an α-pinene / β-pinene mixed resin is preferred.

[0039] Examples of styrene resins include polystyrene, α-methylstyrene homopolymer, styrene / α-methylstyrene copolymer, styrene monomer / aliphatic monomer copolymer, α-methylstyrene / aliphatic monomer copolymer, styrene monomer / α-methylstyrene / aliphatic monomer copolymer, and the like.

[0040] Examples of petroleum resins include aliphatic petroleum resins (C5 petroleum resins), aromatic petroleum resins (C9 petroleum resins), and aliphatic / aromatic copolymeric petroleum resins (C5 / C9 petroleum resins). C5 petroleum resins are resins obtained by cationically polymerizing unsaturated monomers such as isoprene and cyclopentadiene, which are petroleum fractions (C5 fractions) corresponding to 4 to 5 carbon atoms, and may be hydrogenated. C9 petroleum resins are resins obtained by cationically polymerizing monomers such as vinyltoluene, alkylstyrene, and indene, which are petroleum fractions (C9 fractions) corresponding to 8 to 10 carbon atoms, and may be hydrogenated. C5 / C9 petroleum resins are resins obtained by copolymerizing a C5 fraction and a C9 fraction by cationic polymerization, and may be hydrogenated.

[0041] Examples of rosin resins include natural resin rosin and rosin-modified resins (e.g., rosin-modified maleic acid resins) obtained by modifying it by hydrogenation, disproportionation, dimerization, esterification, and the like.

[0042] Coumarone resins are resins mainly composed of coumarone, and examples thereof include coumarone resins, coumarone-indene resins, and copolymer resins mainly composed of coumarone, indene, and styrene.

[0043] The softening point of the thermoplastic resin is not particularly limited, and may be, for example, 40 to 160°C, 60 to 140°C, or 80 to 130°C. The softening point of the thermoplastic resin is measured using a ring and ball softening point measuring device in accordance with JIS K6220-1:2001.

[0044] In rubber composition A, the content of the above thermoplastic resin is preferably 3 to 50 parts by mass, more preferably 5 to 40 parts by mass, still more preferably 8 to 35 parts by mass, and even more preferably 10 to 30 parts by mass, based on 100 parts by mass of the diene rubber.

[0045] In rubber composition A, the total amount of the above softening agent and thermoplastic resin is not particularly limited, but is preferably 25 to 70 parts by mass, more preferably 30 to 65 parts by mass, still more preferably 35 to 60 parts by mass, and even more preferably 40 to 55 parts by mass, based on 100 parts by mass of the diene rubber.

[0046] In addition to the above components, rubber composition A may be blended with various additives generally used in rubber compositions, such as zinc oxide, stearic acid, anti-aging agent, wax, vulcanizing agent, vulcanization accelerator, etc., as optional components.

[0047] The content of zinc oxide is not particularly limited. For example, it may be 0 to 10 parts by mass, 0.5 to 5 parts by mass, or 1 to 4 parts by mass based on 100 parts by mass of the diene rubber.

[0048] The content of stearic acid is not particularly limited. For example, it may be 0 to 10 parts by mass, 0.5 to 5 parts by mass, or 1 to 4 parts by mass based on 100 parts by mass of the diene rubber.

[0049] Examples of the anti-aging agent include various anti-aging agents such as amine-ketone type, aromatic secondary amine type, monophenol type, bisphenol type, benzimidazole type, etc., and any one kind or a combination of two or more kinds can be used. The content of the anti-aging agent is not particularly limited. For example, it may be 0 to 10 parts by mass, or 1 to 5 parts by mass based on 100 parts by mass of the diene rubber.

[0050] The content of wax is not particularly limited. For example, it may be 0 to 10 parts by mass, 0.5 to 5 parts by mass, or 1 to 4 parts by mass based on 100 parts by mass of the diene rubber.

[0051] As the vulcanizing agent, sulfur is preferably used. The content of the vulcanizing agent is not particularly limited, but may be 0.1 to 10 parts by mass, may be 0.5 to 5 parts by mass, or may be 1 to 3 parts by mass with respect to 100 parts by mass of the diene rubber.

[0052] 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, but is preferably 0.1 to 7 parts by mass, more preferably 0.5 to 5 parts by mass, and may be 1 to 4 parts by mass with respect to 100 parts by mass of the diene rubber.

[0053] [Rubber Composition B] The rubber composition B that forms the base rubber contains a diene rubber as a rubber component. Examples of the diene rubber of the rubber composition B include natural rubber (NR), synthetic isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), and the like. Among these, it is preferably at least one selected from the group consisting of NR, IR, BR, and SBR.

[0054] In one embodiment, 100 parts by mass of the diene rubber of the rubber composition B preferably contains 50 to 100 parts by mass of NR and / or IR and 0 to 50 parts by mass of BR (this includes the aspect without BR). More preferably, 100 parts by mass of the diene rubber contains 55 to 100 parts by mass of NR and / or IR and 0 to 45 parts by mass of BR, and may contain 60 to 80 parts by mass of NR and / or IR and 20 to 40 parts by mass of BR.

[0055] The rubber composition B may or may not contain a softening agent having a molecular weight of 1000 or less and being liquid at 20°C. Preferably, the rubber composition B contains 1 part by mass or more of the softening agent with respect to 100 parts by mass of the diene rubber. In the rubber composition B, the content of the softening agent can be adjusted within a range that satisfies the above formula (1). For example, the amount (parts by mass) of the softening agent is preferably 2 to 30 parts by mass, more preferably 3 to 20 parts by mass, still more preferably 4 to 15 parts by mass, and even more preferably 5 to 10 parts by mass with respect to 100 parts by mass of the diene rubber. Also, as the amount b (volume %) of the softening agent, for example, it may be 0 to 15.0 volume %, preferably 1.0 to 12.0 volume %, more preferably 2.0 to 10.0 volume %, and even more preferably 3.0 to 8.0 volume %.

[0056] Regarding details such as the definition and specific examples of the softening agent in the rubber composition B, they are the same as those of the rubber composition A. Therefore, as the softening agent, hydrocarbon oils such as polyolefin-based oils, naphthene-based oils, paraffin-based oils, aroma-based oils, and mineral oils are preferable. The softening agent of the rubber composition A and the softening agent of the rubber composition B may be the same or different.

[0057] A reinforcing filler is compounded into the rubber composition B. As the reinforcing filler of the rubber composition B, carbon black and / or silica is used, and preferably carbon black is used. The content of the reinforcing filler is not particularly limited. For example, it may be 10 to 80 parts by mass, 15 to 60 parts by mass, or 20 to 50 parts by mass with respect to 100 parts by mass of the diene rubber. The content of carbon black is also not particularly limited. For example, it may be 10 to 80 parts by mass, 15 to 60 parts by mass, or 20 to 50 parts by mass with respect to 100 parts by mass of the diene rubber.

[0058] The carbon black is not particularly limited, and various known varieties can be used. Specifically, SAF grade (N100 series), ISAF grade (N200 series), HAF grade (N300 series), FEF grade (N500 series), GPF grade (N600 series) (all ASTM grades) can be mentioned. These carbon blacks of each grade can be used alone or in combination of two or more.

[0059] In addition to the above components, the rubber composition B may be blended with various additives generally used in rubber compositions, such as zinc oxide, stearic acid, anti-aging agent, wax, vulcanizing agent, vulcanization accelerator, etc., as optional components. Specific examples and contents thereof are the same as those of the rubber composition A, and the description is omitted.

[0060] [Method for preparing rubber composition] The above rubber compositions A and B can be prepared by kneading according to a conventional method using a mixer such as a Banbury mixer, kneader, roll, etc. usually used. That is, for example, in the first mixing stage (non-pro kneading process), additives other than the vulcanizing agent and vulcanization accelerator are added and mixed to the diene rubber. Then, the vulcanizing agent and vulcanization accelerator are added and mixed to the obtained mixture in the final mixing stage (pro kneading process). Thereby, the unvulcanized rubber compositions A and B can be respectively prepared.

[0061] [Tire] The pneumatic tire as one embodiment can be manufactured as follows. An unvulcanized cap rubber member is produced by a rubber extruder using the rubber composition A. Also, an unvulcanized base rubber member is produced by a rubber extruder using the rubber composition B. These unvulcanized cap rubber member and base rubber member are laminated to produce an unvulcanized tread rubber member, and in combination with other tire members, an unvulcanized tire (green tire) is formed. Then, using the unvulcanized tire, a pneumatic tire can be manufactured by vulcanization molding at, for example, 140 to 180 °C according to a conventional method.

[0062] The type of tire according to the embodiment is not particularly limited, and examples thereof include pneumatic tires for various applications and various sizes, such as passenger car tires, large tires for trucks and buses, and the like.

Examples

[0063] Examples are shown below, but the present invention is not limited to these examples.

[0064] Each component used in the examples and comparative examples is as follows. ·BR: "BR150B" manufactured by UBE Industries, Ltd. (specific gravity: 0.91, Tg: -104°C) ·SBR1: Unmodified ESBR, "SBR1723" manufactured by ENEOS MATERIALS Co., Ltd. (37.5 parts by mass of oil-extended product with respect to 100 parts by mass of rubber content, specific gravity: 0.93 as oil-extended product (specific gravity of oil is 0.94), Tg: -53°C) ·SBR2: Alkoxyl group and amino group-terminated modified solution-polymerized SBR, "HPR355" manufactured by ENEOS MATERIALS Co., Ltd. (specific gravity: 0.93, Tg: -24°C) ·NR: RSS#3 (specific gravity: 0.92)

[0065] ·Carbon black: HAF, "Seast 3" manufactured by Tokai Carbon Co., Ltd. (specific gravity: 1.8) ·Silica: "Ultrasil VN3" manufactured by Evonik Industries AG (BET: 180 m 2 / g, specific gravity: 1.95) ·Silane coupling agent 1: Bis(3-triethoxysilylpropyl)tetrasulfide, "Si69" manufactured by Evonik Industries AG (specific gravity: 1.1) ·Silane coupling agent 2: 3-octanoylthio-1-propyltriethoxysilane, "NXT" manufactured by Momentive Performance Materials Inc. (specific gravity: 0.97) ·Softening agent: Process oil, weight average molecular weight: 700, "Process NC-140" manufactured by ENEOS Corporation (specific gravity: 0.94)

[0066] ·Zinc oxide: "Zinc Oxide No. 2" manufactured by Mitsui Mining & Smelting Co., Ltd. (specific gravity: 5.6) · Stearic acid: "Lunac S-20" manufactured by Kao Corporation (specific gravity: 0.85) · Antioxidant: "Nocrack 6C" manufactured by Ouchi Shinsei Chemical Industry Co., Ltd. (specific gravity: 1.09) · Wax: "OZOACE0355" manufactured by Nippon Seiro Co., Ltd. (specific gravity: 0.9) · Resin 1: α-pinene / β-pinene mixed resin, "SYLVATRAXX4150" manufactured by Clayton (weight average molecular weight: 2110, softening point: 115°C, specific gravity: 0.98) · Resin 2: Styrene resin, "SYLVATRAXX4401" manufactured by Clayton (weight average molecular weight: 1200, softening point: 85°C, specific gravity: 1.06) · Resin 3: Styrene resin, "FTR2120" manufactured by Mitsui Chemicals, Inc. (weight average molecular weight: 2600, softening point: 125°C, specific gravity: 1.07)

[0067] · Sulfur: "Powdered sulfur" manufactured by Tsurumi Chemical Industry Co., Ltd. (specific gravity: 2) · Vulcanization accelerator CZ: "Soxinol CZ" manufactured by Sumitomo Chemical Co., Ltd. (specific gravity: 1.29) · Vulcanization accelerator DPG: "Noxeller D" manufactured by Ouchi Shinsei Chemical Co., Ltd. (specific gravity: 1.2)

[0068] The evaluation methods in the examples and comparative examples are as follows. (1) Initial wet grip performance Four prototype tires before wear were mounted on a passenger car and driven on a road surface sprinkled with water at a water depth of 2 - 3 mm, and the friction coefficient was measured at a speed of 100 km / h to evaluate the wet grip performance. In Table 2, the friction coefficient of Comparative Example 1 was set as 100 and shown in index form, and in Table 3, the friction coefficient of Comparative Example 11 was set as 100 and shown in index form. The larger the numerical value, the higher and better the wet grip performance at the time of new product.

[0069] (2) Wet grip performance after wear Using a tire grinding device ("Kenma-kun" manufactured by AIS Co., Ltd.), the tread surface of the prototype tire was cut so that the tread thickness from the tread groove bottom was 2 mm ± 0.4 mm, and a worn tire was produced. That is, the worn tire has the cap rubber shaved so that the groove depth of the tread is 2 mm. Using four worn tires, the same evaluation as the above initial wet grip performance was performed. In Table 2, the wear coefficient of Comparative Example 1 and in Table 3, the friction coefficient of Comparative Example 11 were each set to 100 and shown in exponential form. The larger the numerical value, the higher and better the wet grip performance after wear.

[0070] (3) Retention rate of wet grip performance after wear Taking the friction coefficient in the above initial wet grip performance evaluation as the "initial friction coefficient" and the friction coefficient in the wet grip performance evaluation after wear as the "post-wear friction coefficient", the retention rate of wet grip performance after wear (retention rate) was calculated by the following formula. The higher the numerical value, the smaller the decrease in wet grip performance after wear compared to the initial wet grip performance, indicating good performance. Retention rate (%) = (Post-wear friction coefficient / Initial friction coefficient) × 100

[0071] [Preparation of rubber composition B for base rubber] Using a Banbury mixer, according to the formulation (parts by mass) shown in Table 1 below, first, in the first mixing stage, compounding agents excluding sulfur and vulcanization accelerators were added to the diene rubber and kneaded (discharge temperature = 160 °C). Then, sulfur and vulcanization accelerators were added to the obtained kneaded product and kneaded in the final mixing stage (discharge temperature = 90 °C) to prepare BASE(1) to (4) as rubber compositions B for base rubber.

[0072] In Table 1, the softening agent amount b (volume %) was calculated by the following formula. The same applies to the softening agent amount a (volume %) in Tables 2 and 3. Softening agent amount (volume %) = {(Softening agent amount (parts by mass) / Softening agent specific gravity) / (Total formulation amount of rubber composition (parts by mass) / Rubber composition formulation specific gravity)} × 100 Here, the compounding proportion of the rubber composition is the specific gravity of the rubber composition calculated from the amounts (parts by mass) and specific gravities of the respective components, and is shown as "compounding proportion" in Tables 1 to 3. The specific gravity is based on water at 4°C under normal atmospheric pressure as the reference substance in the usual manner.

[0073] [Table 1]

[0074] [First Experimental Example] (Preparation of Rubber Composition A for Cap Rubber) Using a Banbury mixer, according to the formulation (parts by mass) shown in Table 2 below, first, in the first mixing stage, compounding agents except sulfur and vulcanization accelerators were added to the diene rubber and kneaded (discharge temperature = 160°C). Next, sulfur and vulcanization accelerators were added to the obtained kneaded product in the final mixing stage and kneaded (discharge temperature = 90°C) to prepare Rubber Composition A for cap rubber.

[0075] Regarding the amount of "SBR1" in Table 2, the figures in parentheses are the parts by mass of oil (softening agent) as the oil-extended portion, and the remainder is the amount of rubber component. Also, the compounding amount of the "softening agent" in Table 2 does not include the oil amount of the oil-extended portion of SBR1. The same applies to Table 3 below.

[0076] (Manufacture of Tire) According to Table 2, using Rubber Composition A for cap rubber and Rubber Composition B for base rubber respectively, a pneumatic radial tire (tire size: 215 / 45ZR17) was manufactured by vulcanization molding according to a conventional method. At that time, regarding the thickness of each layer of the cap rubber and the base rubber, the thickness of the cap rubber at the tread center rib was set to 13 mm, and the thickness of the base rubber was set to 2 mm. Regarding the obtained prototype tire, the initial wet grip performance, the wet grip performance after wear, and the retention rate of the wet grip performance after wear were evaluated. The results are shown in Table 2.

[0077] [Table 2]

[0078] Comparative Example 1 is an example where the difference (a - b) in the amount of softening agent between the cap rubber and the base rubber exceeds 15.0% by volume, and the wet grip performance retention rate after wear is low, being 67%. Comparative Example 2 is an example where the difference (a - b) in the amount of softening agent is even larger than that in Comparative Example 1. In Comparative Example 2, compared with Comparative Example 1, since the concentration of the softening agent between the cap rubber and the base rubber is high, the migration of the softening agent to the base rubber is large, and the wet grip performance retention rate after wear further deteriorated.

[0079] In Comparative Example 3, the difference (a - b) in the amount of softening agent is 15.0% by volume or less, but only modified SBR is used as the SBR of the cap rubber and is not used in combination with unmodified SBR. Therefore, although the initial wet grip performance is excellent, the wet grip performance retention rate after wear is inferior to that of Comparative Example 1.

[0080] On the other hand, in Examples 1 to 6, the difference (a - b) in the amount of softening agent between the cap rubber and the base rubber is 15.0% by volume or less, and the wet grip performance retention rate after wear is higher than that of Comparative Example 1. As shown in Examples 1 to 3, the smaller the difference (a - b) in the amount of softening agent, the higher and better the wet grip performance retention rate after wear.

[0081] Regarding the thermoplastic resin used in combination with the softening agent, styrenic resins (Resins 2, 3) have higher compatibility with SBR than terpene resins (Resin 1), so the migration to the base rubber is less. Therefore, in Examples 4 to 6, the wet grip performance retention rate after wear is superior to that of Example 3. Also, in Example 6 using Resin 3 with a higher molecular weight among styrenic resins, the wet grip performance retention rate after wear is superior to that of Example 5 using Resin 2.

[0082] [Second Experimental Example] (Preparation of Rubber Composition A for Cap Rubber) Using a Banbury mixer, according to the formulation (parts by mass) shown in Table 3 below, first, in the first mixing stage, compounding agents other than sulfur and vulcanization accelerators were added to the diene rubber and kneaded (discharge temperature = 160 °C). Next, sulfur and a vulcanization accelerator were added to the obtained kneaded product in the final mixing stage and kneaded (discharge temperature = 90 °C) to prepare a rubber composition A for the cap rubber.

[0083] (Manufacture of Tires) According to Table 3, using the rubber composition A for the cap rubber and the rubber composition B for the base rubber, a pneumatic radial tire was manufactured in the same manner as in the first experimental example. For the obtained prototype tire, the initial wet grip performance, the wet grip performance after wear, and the retention rate of the wet grip performance after wear were evaluated. The results are shown in Table 3.

[0084]

Table 3

[0085] Comparative Example 11 is an example where the difference (a - b) in the amount of softening agent between the cap rubber and the base rubber exceeds 15.0% by volume, and the retention rate of the wet grip performance after wear is low, at 66%. In Comparative Example 12, the difference (a - b) in the amount of this softening agent is larger than that in Comparative Example 11, and the retention rate of the wet grip performance after wear further deteriorates. In Comparative Example 13, since modified SBR and unmodified SBR are not used in combination in the cap rubber, although the initial wet grip performance is excellent, the retention rate of the wet grip performance after wear is inferior.

[0086] On the other hand, in Examples 11 to 15, the difference (a - b) in the amount of softening agent between the cap rubber and the base rubber is 15.0% by volume or less, and the retention rate of the wet grip performance after wear is higher than that in Comparative Example 11. As shown in Examples 11 to 13, the smaller the difference (a - b) in the amount of softening agent, the better the retention rate of the wet grip performance after wear.

[0087] Example 14 is an example in which the diene rubber of the base rubber is only natural rubber, and the wet grip performance retention rate after wear was as high as that of Example 13. Example 15 is an example in which BR is contained in a large amount as the diene rubber of the base rubber. In Example 15, since the majority of the diene rubber is BR, the softening agent is easily incorporated into the base rubber because BR having high molecular mobility becomes the sea phase. Therefore, although the wet grip performance retention rate after wear was excellent compared to Comparative Example 11, it was lower than that of Example 11.

[0088] In addition, the various numerical ranges described in the specification can be arbitrarily combined with their upper limit values and lower limit values, and all of these combinations are described in the specification as preferred numerical ranges. Also, the description of the numerical range of "X to Y" means X or more and Y or less.

Explanation of Reference Numerals

[0089] 1... Tire, 2... Tread, 3... Sidewall, 4... Bead portion, 5... Bead core, 6... Carcass, 7... Belt, 8... Cap ply, 9... Tread rubber, 91... Cap rubber, 92... Base rubber

Claims

1. A tire comprising a tread rubber having a cap rubber and a base rubber disposed inside the cap rubber, wherein the cap rubber is formed of a rubber composition A containing a diene rubber, silica, and a softening agent having a molecular weight of 1000 or less and being liquid at 20°C, the base rubber is formed of a rubber composition B containing a diene rubber and optionally containing a softening agent having a molecular weight of 1000 or less and being liquid at 20°C, the diene rubber in the rubber composition A includes a modified styrene-butadiene rubber and an unmodified styrene-butadiene rubber, a tire satisfying 0 ≦ a - b ≦ 15.0, where a (volume %) is the amount of the softening agent in 100 volume % of the rubber composition A and b (volume %) is the amount of the softening agent in 100 volume % of the rubber composition B.

2. The tire according to claim 1, wherein the rubber composition A contains 105 to 200 parts by mass of silica with respect to 100 parts by mass of the diene rubber.

3. The tire according to claim 1 or 2, wherein 100 parts by mass of the diene rubber in the rubber composition B contains 50 to 100 parts by mass of natural rubber and / or synthetic isoprene rubber and 0 to 50 parts by mass of butadiene rubber.

4. The tire according to claim 1 or 2, wherein the rubber composition A contains 3 to 50 parts by mass of a thermoplastic resin having a weight average molecular weight of 1000 to 30000 with respect to 100 parts by mass of the diene rubber.

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