Cap tread rubber

Abstract

To provide a cap tread rubber which is constituted of a plurality of rubbers, and in which the occurrence of crack between the rubbers is suppressed while eliciting effects based on physical properties of the rubbers and which enables improvement of wear resistance performance and low fuel consumption performance.SOLUTION: In a cap tread rubber consisted of a first tread rubber and a second tread rubber, a tire contact patch is configured in such a manner that each of the first tread rubber and the second tread rubber extends continuously in a tire circumferential direction. The first tread rubber is constituted by combining 50 parts by mass-70 parts by mass of a filler containing 40 mass% or more of silica with 100 parts by mass of a diene rubber containing 60 mass% or more of an isoprene rubber. The second tread rubber is constituted by combining 40 parts by mass or more of carbon black with 100 parts by mass of rubber constituents. In addition, a foaming rate in a state of an unvulcanized extrudate is 6% or lower.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a cap tread rubber that mainly constitutes the tread portion of a heavy-duty tire. [Background technology]

[0002] Tires (heavy-duty tires) used on vehicles such as trucks and buses are subject to a larger load than passenger car tires, so excellent wear resistance is important. On the other hand, with increasing interest in global environmental issues, environmental regulations for heavy-duty tires are also becoming stricter, and further improvement in fuel efficiency is required. For this reason, for example, Patent Document 1 proposes blending silica into a rubber component mainly made of natural rubber and blending a specific silane coupling agent to improve the dispersion of silica and improve fuel efficiency in addition to wear resistance.

[0003] On the other hand, attempts have been made to replace a part of the cap tread rubber constituting the tire contact surface with another rubber having different physical properties, thereby adding tire performance based on the physical properties of the other rubber. For example, in Patent Document 2, a so-called earth tread made of conductive rubber is provided in a part of the cap tread in the tire width direction, which penetrates the cap tread and is exposed to the tire contact surface. A tire equipped with such an earth tread can release static electricity generated on a vehicle while the vehicle is running from the belt layer to the road surface via the earth tread, thereby suppressing the charging of the vehicle.

[0004] However, when replacing a part of the cap tread rubber with another rubber having different physical properties as described above, if the adhesion between the rubber constituting the main part of the cap tread rubber and the other rubber cannot be ensured, cracks may occur at the interface between these rubbers, making the appearance abnormal and reducing the durability of the tire. Therefore, it has been difficult to ensure the excellent wear resistance required for heavy load tires while replacing a part of the cap tread rubber with another rubber having different physical properties (e.g., earth tread) in the above-mentioned heavy load tire. Therefore, in the cap tread rubber composed of multiple rubbers, measures are required to suppress the occurrence of cracks between these rubbers and improve wear resistance and fuel efficiency while exerting the effects based on the physical properties of each rubber. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-056068 A [Patent Document 2] JP 2019-018725 A Summary of the Invention [Problem to be solved by the invention]

[0006] The object of the present invention is to provide a cap tread rubber composed of multiple rubbers, which exhibits the effects based on the physical properties of each rubber, while suppressing the occurrence of cracks between the rubbers, and improving wear resistance and fuel efficiency. [Means for solving the problem]

[0007] The cap tread rubber of the present invention, which achieves the above-mentioned object, is a cap tread rubber consisting of a first tread rubber and a second tread rubber, the first tread rubber and the second tread rubber each constitute a tire contact surface so as to extend continuously in the tire circumferential direction, the first tread rubber is made by blending 50 to 70 parts by mass of a filler containing 40% or more by mass of silica with 100 parts by mass of a diene rubber containing 60% or more by mass of an isoprene rubber, and the second tread rubber is made by blending 40 parts by mass or more of carbon black with 100 parts by mass of a rubber component, and is characterized in that the foaming rate in an unvulcanized extrudate state is 6% or less. Effect of the Invention

[0008] The cap tread rubber of the present invention is composed of the first tread rubber and the second tread rubber as described above, and since these first tread rubber and the second tread rubber have the above-mentioned composition and physical properties, it is possible to ensure excellent abrasion resistance and fuel efficiency due to the first tread rubber, while exhibiting the static electricity suppression effect due to the second tread rubber, and to suppress the occurrence of cracks at the interface between these first tread rubber and the second tread rubber, thereby preventing abnormalities in the appearance of the tire.

[0009] In the cap tread rubber of the present invention, the first tread rubber contains a guanidine vulcanization accelerator as a vulcanization accelerator, and the amount of the guanidine vulcanization accelerator is preferably 0.1 parts by mass or more per 100 parts by mass of the diene rubber. In this way, the first tread rubber contains a sufficient amount of a specific vulcanization accelerator, which is advantageous for improving wear resistance and fuel efficiency.

[0010] In the cap tread rubber of the present invention, the amount of silica in the first tread rubber is preferably 50 parts by mass or more per 100 parts by mass of the diene rubber. By allowing the first tread rubber to contain a sufficient amount of silica, it is possible to further improve fuel efficiency.

[0011] In the cap tread rubber of the present invention, the hardness of the second tread rubber is preferably not less than 62. When the second tread rubber has an appropriate hardness in this manner, it is advantageous for improving the abrasion resistance.

[0012] In the cap tread rubber of the present invention, it is preferable that the second tread rubber contains process oil and the amount of the process oil is 10 parts by mass or more per 100 parts by mass of the rubber component. By the second tread rubber containing a sufficient amount of process oil in this manner, the adhesion of the second tread rubber is improved, which is advantageous for improving tire durability.

[0013] The cap tread rubber of the present invention can be used in the cap tread layer of a tire. A tire using the cap tread rubber of the present invention in the cap tread layer ensures excellent wear resistance and fuel efficiency due to the first tread rubber, while exerting the static electricity suppression effect due to the second tread rubber, and can suppress the occurrence of cracks at the interface between the first tread rubber and the second tread rubber, thereby preventing abnormalities in the appearance of the tire. The tire to which the cap tread rubber of the present invention is applied is preferably a pneumatic tire, but may also be a non-pneumatic tire. In the case of a pneumatic tire, the inside can be filled with air, an inert gas such as nitrogen, or other gases.

[0014] In this case, it is preferable that the second tread rubber is locally disposed in a part of the first tread rubber in the tire width direction, and the cross-sectional area of ​​the second tread rubber in the tire meridian cross section is 0.1% to 3% of the cross-sectional area of ​​the cap tread layer. This allows the first tread rubber and the second tread rubber to be disposed in a well-balanced manner in the tire, and allows the first tread rubber to provide low fuel consumption performance and the second tread rubber to provide a high level of both static electricity suppression effect. [Brief description of the drawings]

[0015] [Figure 1] 1 is a meridian cross-sectional view showing an example of a tire in which the cap tread rubber of the present invention is used. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The configuration of the present invention will be described in detail below with reference to the accompanying drawings.

[0017] As shown in Fig. 1, a tire using the cap tread rubber of the present invention includes a tread portion 1, a pair of sidewall portions 2 arranged on both sides of the tread portion 1, and a pair of bead portions 3 arranged on the tire radial inner side of the sidewall portions 2. In Fig. 1, the symbol CL indicates the tire equator. Although not depicted because Fig. 1 is a meridian cross section, the tread portion 1, the sidewall portions 2, and the bead portions 3 each extend in the tire circumferential direction to form an annular shape, thereby forming a basic toroidal structure of a pneumatic tire. The following explanation using Fig. 1 is basically based on the meridian cross section shown in the figure, but each tire component extends in the tire circumferential direction to form an annular shape.

[0018] A carcass layer 4 is installed between a pair of left and right bead portions 3. This carcass layer 4 includes a plurality of reinforcing cords extending in the tire radial direction, and is folded back from the vehicle inner side to the vehicle outer side around a bead core 5 arranged in each bead portion 3. A bead filler 6 is arranged on the outer periphery of the bead core 5, and this bead filler 6 is wrapped by the main body and the folded back part of the carcass layer 4. Meanwhile, a plurality of belt layers 7 (two layers in FIG. 1) are embedded on the outer periphery side of the carcass layer 4 in the tread portion 1. Each belt layer 7 includes a plurality of reinforcing cords inclined with respect to the tire circumferential direction, and the reinforcing cords are arranged so as to cross each other between the layers. In these belt layers 7, the inclination angle of the reinforcing cords with respect to the tire circumferential direction is set to, for example, a range of 10° to 40°. Furthermore, a belt reinforcing layer 8 (two layers, a full cover 8a that covers the entire width of the belt layer 7 and an edge cover 8b that locally covers the end portion of the belt layer 7) is provided on the outer periphery side of the belt layer 7. The belt reinforcing layer 8 includes organic fiber cords oriented in the tire circumferential direction. In the belt reinforcing layer 8, the organic fiber cords are set at an angle of, for example, 0° to 5° with respect to the tire circumferential direction.

[0019] A tread rubber layer 11 is disposed on the outer peripheral side of the carcass layer 4 in the tread portion 1, a side rubber layer 12 is disposed on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the sidewall portion 2, and a rim cushion rubber layer 13 is disposed on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the bead portion 3. The tread rubber layer 11 has a structure in which two types of rubber layers with different physical properties (a cap tread layer 11C that constitutes the tread surface of the tread portion 1 and an under tread layer 11U disposed on its inner peripheral side) are laminated in the tire radial direction.

[0020] The cap tread rubber of the present invention is used for the above-mentioned cap tread layer 11C. Therefore, as long as the tread portion 1 (tread rubber layer 11) of the tire to which the present invention is applied is composed of the cap tread layer 11C and the under tread layer 11U, the basic structure of the other parts is not limited to the above-mentioned structure.

[0021] The cap tread rubber of the present invention is composed of a first tread rubber C1 and a second tread rubber C2. The first tread rubber C1 and the second tread rubber C2 each form a tire contact surface so as to extend continuously in the tire circumferential direction. In FIG. 1, the first tread rubber C1 forms a main part of the cap tread layer 11C, and the second tread rubber C2 (the shaded part in the figure) is locally arranged in a part of the first tread rubber C1 in the tire width direction. In particular, when the second tread rubber C2 is a rubber that functions as a so-called earth tread, the second tread rubber C2 is preferably provided so as to penetrate the cap tread layer 11C and reach the belt layer 7 or the belt reinforcing layer 8 from the tread surface, taking into consideration the path for discharging static electricity to the road surface.

[0022] The first tread rubber of the present invention constitutes the main part of the cap tread layer 11C as described above, and the rubber component is a diene rubber, which necessarily contains an isoprene rubber, and may optionally contain a butadiene rubber or a styrene-butadiene rubber.

[0023] Examples of the isoprene-based rubber used in the first tread rubber include various natural rubbers, epoxidized natural rubbers, and various synthetic polyisoprene rubbers. As the isoprene-based rubber, rubbers that are usually used in rubber compositions for tires can be used. The content of the isoprene-based rubber is 60% by mass or more, preferably 65% ​​by mass to 80% by mass, when the total diene-based rubber is taken as 100% by mass. By containing a specific amount of the isoprene-based rubber in this way, it is possible to achieve a good balance between wear resistance and fuel economy. If the content of the isoprene-based rubber is less than 60% by mass, chipping resistance is deteriorated.

[0024] A filler containing silica is always blended in the first tread rubber. The first tread rubber containing silica as a filler is advantageous in improving fuel economy performance. The filler always contains silica as described above, but other fillers can also be blended. Examples of other fillers include carbon black, calcium carbonate, magnesium carbonate, talc, clay, alumina, aluminum hydroxide, titanium oxide, and calcium sulfate. Of these, carbon black can be preferably used. These other fillers may be used alone or in combination of two or more.

[0025] The amount of the filler used in the first tread rubber is 50 to 70 parts by mass, preferably 55 to 70 parts by mass, based on 100 parts by mass of the diene rubber. If the amount of the filler is less than 50 parts by mass, the abrasion resistance is reduced. If the amount of the filler is more than 70 parts by mass, the heat generation property and processability are reduced.

[0026] The silica necessarily contained in the filler used in the first tread rubber may be any silica normally used in rubber compositions for tires. For example, wet silica (hydrated silicic acid), dry silica (anhydrous silicic acid), calcium silicate, aluminum silicate, etc. may be mentioned, and these may be used alone or in combination of two or more kinds.

[0027] The content of silica in the filler used in the first tread rubber is 40% by mass or more, preferably 45% by mass to 99% by mass, when the entire filler is 100% by mass. If the content of silica is less than 40% by mass, the effect of improving fuel economy is limited. In addition, the compounding amount of silica alone is preferably 20 parts by mass or more, more preferably 50 parts by mass or more, per 100 parts by mass of the diene rubber constituting the first tread rubber. In this way, the first tread rubber contains a sufficient amount of silica, so that the fuel economy can be further improved. If the compounding amount of silica is less than 20 parts by mass, the effect of improving fuel economy is limited.

[0028] The CTAB adsorption specific surface area of ​​the silica used in the first tread rubber is not particularly limited, but is preferably 120 m 2 / g or more, more preferably 140m 2 / g~230m 2 / g, more preferably 140m 2 / g~175m 2 / g. The CTAB adsorption specific surface area of ​​silica is 120 m 2 When the specific surface area is less than 1 / g, the effect of improving the wear resistance and low rolling resistance may not be sufficiently obtained. In this specification, the CTAB adsorption specific surface area of ​​silica is a value measured according to ISO 5794.

[0029] In the first tread rubber, it is preferable to compound a silane coupling agent together with silica. By compounding the silane coupling agent, the dispersibility of silica in the rubber component can be improved, and abrasion resistance and fuel economy performance can be improved in a well-balanced manner.

[0030] The type of silane coupling agent is not particularly limited as long as it can be used in a rubber composition containing silica. Examples of the silane coupling agent include sulfur-containing silane coupling agents such as bis-(3-triethoxysilylpropyl) tetrasulfide, bis(3-triethoxysilylpropyl) disulfide, 3-trimethoxysilylpropyl benzothiazole tetrasulfide, γ-mercaptopropyl triethoxysilane, and 3-octanoylthiopropyl triethoxysilane.

[0031] When the silane coupling agent is blended in the first tread rubber, the blending amount is not particularly limited, but is preferably 1% by mass to 20% by mass, more preferably 5% by mass to 15% by mass, based on the blending amount of the silica. If the blending amount of the silane coupling agent is less than 1% by mass of the silica blending amount, the dispersion of the silica cannot be sufficiently improved. If the blending amount of the silane coupling agent is more than 20% by mass of the silica blending amount, the silane coupling agent condenses with itself, and the desired tensile strength of the rubber composition cannot be obtained.

[0032] The filler used in the first tread rubber necessarily contains silica as described above, but preferably further contains carbon black (sometimes referred to as "first carbon black" to distinguish it from the carbon black compounded in the second tread rubber described later). When the first carbon black is used in combination, its nitrogen adsorption specific surface area N2SA is preferably 65 m 2 / g or more, more preferably 65m 2 / g~120m 2 / g, more preferably 65m 2 / g~100m 2 / g. In this way, the use of carbon black with an appropriate particle size is advantageous in improving wear resistance. The nitrogen adsorption specific surface area N2SA of the first carbon black is 65 m 2 If the nitrogen adsorption specific surface area N2SA of carbon black is less than 1 / g, the effect of improving abrasion resistance cannot be expected to be sufficient. The nitrogen adsorption specific surface area N2SA of carbon black is measured in accordance with JIS K6217-2.

[0033] When the first carbon black is used in combination, the amount of the first carbon black is preferably 1% by mass to 55% by mass, more preferably 5% by mass to 50% by mass, when the total amount of the filler used in the first tread rubber is taken as 100% by mass. If the amount of the first carbon black is less than 1% by mass, the rubber cannot be blackened. If the amount of the first carbon black is more than 55% by mass, there is a risk that fuel economy will decrease.

[0034] In the first tread rubber, it is preferable to compound a guanidine-based vulcanization accelerator as a vulcanization accelerator. Examples of the guanidine-based vulcanization accelerator include diphenyl guanidine, di(o-tolyl)guanidine, and o-tolyl bigianide. The compounding amount of the guanidine-based vulcanization accelerator is preferably 0.1 parts by mass or more, more preferably 0.4 to 1.5 parts by mass, per 100 parts by mass of the diene rubber. In this way, the first tread rubber contains a sufficient amount of a specific vulcanization accelerator, which is advantageous for improving wear resistance and fuel efficiency. If the compounding amount of the guanidine-based vulcanization accelerator is less than 0.1 parts by mass, it becomes difficult to ensure an appropriate vulcanization speed.

[0035] The second tread rubber forms the tire ground contact surface together with the above-mentioned first tread rubber, but is different from the first tread rubber constituting the main part of the cap tread rubber, and is provided partially in a part of the tire width direction in order to add a specific function (for example, antistatic effect) to the cap tread rubber. In such a second tread rubber, the type of rubber component is not particularly limited, and diene rubbers generally used in rubber compositions for tires, such as natural rubber, butadiene rubber, isoprene rubber, styrene-butadiene rubber, acrylonitrile-butadiene rubber, butyl rubber, and halogenated butyl rubber, can be used. These diene rubbers may be terminal-modified rubbers. These diene rubbers can be used alone or as an arbitrary blend. The rubber component constituting the second tread rubber may be appropriately selected depending on the function to be added by the second tread rubber, and in the case of the present invention, natural rubber, butadiene rubber, isoprene rubber, styrene-butadiene rubber, and the like can be suitably used among the above-mentioned various rubbers.

[0036] In the second tread rubber, carbon black (sometimes referred to as "second carbon black" to distinguish it from the carbon black blended in the first tread rubber) is always blended as a filler. By blending carbon black in this way, abrasion resistance can be improved. The blending amount of the second carbon black is 40 parts by mass or more, preferably 45 to 65 parts by mass, based on the rubber component constituting the second tread rubber. If the blending amount of the second carbon black is less than 40 parts by mass, abrasion resistance decreases.

[0037] The second tread rubber may contain silica as a filler. However, when the second tread rubber contains silica, the compounding amount is preferably 15 parts by mass or less, more preferably 10 parts by mass or less, based on the above-mentioned rubber component. If the compounding amount of silica exceeds 15 parts by mass, the antistatic effect is reduced.

[0038] The second tread rubber may contain other fillers in addition to the above-mentioned carbon black and silica. Examples of the other fillers include calcium carbonate, magnesium carbonate, talc, clay, alumina, aluminum hydroxide, titanium oxide, and calcium sulfate. These other fillers may be used alone or in combination of two or more.

[0039] The type of carbon black used in the second tread rubber is not particularly limited, but the nitrogen adsorption specific surface area N2SA is preferably 65 m 2 / g or more, more preferably 65m 2 / g~120m 2 / g, more preferably 65m 2 / g~100m 2 / g. Using carbon black with an appropriate particle size is advantageous for improving wear resistance. The nitrogen adsorption specific surface area (N2SA) of carbon black is preferably 65 m 2 If the nitrogen adsorption specific surface area N2SA of carbon black is less than 1 / g, the effect of improving abrasion resistance cannot be expected to be sufficient. The nitrogen adsorption specific surface area N2SA of carbon black is measured in accordance with JIS K6217-2.

[0040] It is preferable to compound process oil in the second tread rubber. By compounding process oil, the adhesion of the second tread rubber to the first tread rubber is improved, which is advantageous for improving tire durability. The compounded amount of process oil is 10 parts by mass or more, preferably 15 parts by mass to 35 parts by mass, per 100 parts by mass of the rubber component constituting the second tread rubber. If the compounded amount of process oil is less than 10 parts by mass, the effect of improving the adhesion described above cannot be expected to be sufficient.

[0041] The second tread rubber has an expansion rate of 6% or less, preferably 1% to 5% in the unvulcanized extrudate state. By having such an expansion rate, it is possible to suppress the occurrence of cracks at the interface between the first tread rubber and the second tread rubber, and prevent abnormalities in the appearance of the tire. If the expansion rate in the unvulcanized extrudate state exceeds 6%, the occurrence rate of cracks between the rubbers increases, and the occurrence rate of abnormalities in the appearance increases. The expansion rate in the unvulcanized extrudate state can be set by adjusting, for example, the blending amount of the plasticizer component and the type and blending amount of carbon black in the above-mentioned blending. The expansion rate in the unvulcanized extrudate state is a change rate [unit: %] calculated from the following formula (1) using the specific gravities d1 and d2 before and after the pressure, cut out from the cap tread rubber (unvulcanized extrudate state) to prepare a sample, measure the specific gravity d1 of the sample, and further measure the specific gravity d2 after applying a pressure of 0.6 MPa to the sample for 30 seconds. (d2-d1) / d1×100 (1)

[0042] The second tread rubber further has a hardness of preferably 62 or more, more preferably 62 to 68. The second tread rubber having a suitable hardness is advantageous in improving the abrasion resistance of the entire cap tread. If the hardness is less than 62, the abrasion resistance decreases. The hardness of the second tread rubber can be set by adjusting the blending amount of carbon black in the above blending. The hardness of the second tread rubber is a value measured at a temperature of 20°C using a durometer type A in accordance with JIS K6253.

[0043] In the tire, the second tread rubber is provided locally in a part of the cap tread layer as described above. In this case, the cross-sectional area of ​​the second tread rubber in the tire meridian section is preferably set to 0.1% to 3%, more preferably 0.5% to 2% of the cross-sectional area of ​​the cap tread layer. This allows the first tread rubber and the second tread rubber to be arranged in a well-balanced manner in the tire (cap tread layer), and it is possible to highly achieve both the low fuel consumption performance provided by the first tread rubber and the effect added by the second tread rubber (for example, the static electricity suppression effect).

[0044] The cap tread rubber of the present invention can be used for the cap tread layer 11C of the tire as described above. The tire using the cap tread rubber of the present invention for the cap tread layer ensures excellent wear resistance and fuel efficiency due to the first tread rubber, while exerting the static electricity suppression effect due to the second tread rubber, and suppresses the occurrence of cracks at the interface between the first tread rubber and the second tread rubber, thereby preventing the occurrence of abnormalities in the appearance of the tire.

[0045] The present invention will be further described below with reference to examples, but the scope of the present invention is not limited to these examples. EXAMPLES

[0046] Cap tread rubbers (Standard Example 1, Comparative Examples 1 to 6, Examples 1 to 7) consisting of a first tread rubber and a second tread rubber having the formulation and physical properties shown in Table 1 were extrusion molded, and test tires (tire size: 11R22.5) pneumatic tires having a cap tread layer composed of these cap tread rubbers and the basic structure shown in Figure 1 were manufactured.

[0047] In Table 1, the foaming ratio of the second tread rubber is a rate of change (unit: %) calculated by cutting out the second tread rubber from the cap tread rubber (unvulcanized extrudate state) to prepare a sample, measuring the specific gravity d1 of the sample, and measuring the specific gravity d2 after applying a pressure of 0.6 MPa to the sample for 30 seconds. The hardness of the second tread rubber was measured at a temperature of 20°C using a durometer type A in accordance with JIS K6253. The ratio of the cross-sectional area of ​​the second tread rubber is the ratio of the cross-sectional area of ​​the second tread rubber to the cross-sectional area of ​​the cap tread layer in the meridian cross section shown in Figure 1.

[0048] Using each test tire, the crack resistance, fuel economy performance, abrasion resistance, and antistatic effect were evaluated by the methods described below.

[0049] Crack resistance Each test tire was mounted on a wheel with a rim size of 22.5 x 9.0, and the air pressure was set to 230 kPa before being mounted on a test vehicle. After driving 50,000 km on a dry road surface, the number of cracks that occurred at the interface between the first tread rubber and the second tread rubber was measured. The evaluation results were expressed as an index using the reciprocal of the measured value, with standard example 1 being set at 100. The higher the index value, the fewer the number of cracks and the better the crack resistance.

[0050] Low fuel consumption Each test tire was mounted on a wheel with a rim size of 22.5 x 9.0, and the reciprocal of the rolling resistance coefficient of the test tire was calculated using a drum testing machine with a drum diameter of 1707 mm in accordance with ISO28580 under conditions of a load of 28.76 kN, air pressure of 900 kPa, and a speed of 60 km / h. The evaluation results were expressed as an index using the reciprocal of the measured value, with standard example 1 being 100. The higher the index value, the lower the rolling resistance and the better the fuel efficiency performance.

[0051] Abrasion resistance For each cap tread rubber, the amount of wear was measured using a Lambourn abrasion tester (manufactured by Iwamoto Seisakusho) in accordance with JIS K6264 under conditions of temperature 20°C, load 39N, slip rate 30%, and time 4 minutes. The results were expressed as an index using the reciprocal of the measured value, with the standard example being set at 100. The higher the index, the smaller the amount of wear and the better the abrasion resistance.

[0052] Effect of suppressing static electricity The electrical resistance of each cap tread rubber was measured in accordance with JIS K6911. The evaluation results were as follows: 8 When the electrical resistance is less than 10 Ω, the static electricity suppression effect is "effective" and when the electrical resistance is less than 10 8 When the resistance was Ω or more, the static charge suppression effect was shown as "none."

[0053] [Table 1]

[0054] The types of raw materials used in Table 1 are shown below. [First tread rubber] NR: Natural rubber, RSS No.3 SBR: Styrene butadiene rubber, Zeon Nipol 1502 CB: Carbon black, Cabot Japan Co., Ltd. Show Black N339 (nitrogen adsorption specific surface area N2SA: 90 m 2 / g) Silica: Solvay Zeosil 1165MP (CTAB adsorption specific surface area: 156 m 2 / g) Silane coupling agent: bis(3-triethoxysilylpropyl)tetrasulfide, Si69 manufactured by Evonik Degussa Japan Co., Ltd. Zinc oxide: Three types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. Wax: Sannock manufactured by Ouchi Shinko Chemical Co., Ltd. Stearic acid: NOF Corporation YR stearic acid Anti-aging agent: Santoflex 6PPD manufactured by Solutia Europe Vulcanization accelerator 1: Sulfenamide vulcanization accelerator, Noccela CZ-G manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Vulcanization accelerator 2: Guanidine-based vulcanization accelerator, Sancerer D manufactured by Sanshin Chemical Industry Co., Ltd. Sulfur: Oil-treated sulfur manufactured by Karuizawa Refinery

[0055] [Second tread rubber] NR: Natural rubber, RSS No.3 SBR: Styrene butadiene rubber, Zeon Nipol 1502 CB: Carbon black, Cabot Japan Co., Ltd. Show Black N339 (nitrogen adsorption specific surface area N2SA: 90 m 2 / g) Process oil: Shell Lubricants Japan Co., Ltd. Extract No. 4 S Zinc oxide: Three types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. Wax: Sannock manufactured by Ouchi Shinko Chemical Co., Ltd. Stearic acid: NOF Corporation YR stearic acid Anti-aging agent: Santoflex 6PPD manufactured by Solutia Europe Vulcanization accelerator: Noccela CZ-G manufactured by Ouchi Shinko Chemical Industry Co., Ltd. Sulfur: Oil-treated sulfur manufactured by Karuizawa Refinery

[0056] As is clear from Table 1, in comparison with Reference Example 1, Examples 1 to 7 were able to improve the crack resistance, fuel efficiency, and abrasion resistance while also providing an antistatic effect.

[0057] On the other hand, in Comparative Examples 1 and 2, since the second tread rubber was not used, it was not possible to add the static electricity suppression effect. In Comparative Example 3, since the foaming rate of the second tread rubber was high, the crack resistance was reduced. In Comparative Example 4, since the amount of carbon black contained in the second tread rubber was small, the crack resistance and abrasion resistance were reduced. In Comparative Example 5, since the compounding amount of isoprene-based rubber in the first tread rubber was small, the crack resistance was reduced. In Comparative Example 6, since the compounding amount of filler in the first tread rubber was large, the crack resistance was deteriorated.

[0058] The present disclosure includes the following inventions. Invention [1] A cap tread rubber comprising a first tread rubber and a second tread rubber, the first tread rubber and the second tread rubber each forming a tire contact surface so as to extend continuously in the tire circumferential direction, the first tread rubber being made by blending 50 to 70 parts by mass of a filler containing 40% or more by mass of silica with 100 parts by mass of diene rubber containing 60% or more by mass of isoprene rubber, the second tread rubber being made by blending 40 parts by mass or more of carbon black with 100 parts by mass of rubber component, and the cap tread rubber being characterized in that the foaming rate in an unvulcanized extrudate state is 6% or less. Invention [2] The cap tread rubber according to invention [1], characterized in that the first tread rubber contains a guanidine-based vulcanization accelerator as a vulcanization accelerator, and the amount of the guanidine-based vulcanization accelerator is 0.1 parts by mass or more per 100 parts by mass of the diene rubber. Invention [3] The cap tread rubber according to invention [1] or [2], characterized in that the amount of silica in the first tread rubber is 50 parts by mass or more per 100 parts by mass of the diene rubber. Invention [4] The cap tread rubber according to any one of inventions [1] to [3], characterized in that the second tread rubber has a hardness of 62 or more. Invention [5] The cap tread rubber according to any one of inventions [1] to [4], characterized in that the second tread rubber contains process oil and the blending amount of the process oil is 10 parts by mass or more per 100 parts by mass of the rubber component. Invention [6] A tire characterized in that the cap tread rubber according to any one of inventions [1] to [5] is used in a cap tread layer. Invention [7] The tire according to invention [6], characterized in that the second tread rubber is locally arranged in a part of the first tread rubber in the tire width direction, and the cross-sectional area of ​​the second tread rubber in the tire meridian cross section is 0.1% to 3% of the cross-sectional area of ​​the cap tread layer. [Explanation of symbols]

[0059] 1 Tread section 2 Sidewall 3 Bead section 4 Carcass layer 5 Bead Core 6 Bead Filler 7 Belt layer 8 Belt reinforcement layer 11 Tread rubber layer 11U Undertread 11C Cap Tread C1 First tread rubber C2 Second tread rubber 12 Side rubber layer 13 Rim cushion rubber layer CL Tire Equator

Claims

1. A cap tread rubber consisting of a first tread rubber and a second tread rubber, the first tread rubber and the second tread rubber each constitute a tire contact surface so as to extend continuously in a tire circumferential direction, The first tread rubber is obtained by blending 50 to 70 parts by mass of a filler containing 40% or more by mass of silica with 100 parts by mass of a diene rubber containing 60% or more by mass of an isoprene rubber, The second tread rubber is a cap tread rubber characterized in that it is made by blending 40 parts by mass or more of carbon black with respect to 100 parts by mass of a rubber component, and has a foaming rate of 6% or less in an unvulcanized extrudate state.

2. The cap tread rubber according to claim 1, characterized in that the first tread rubber contains a guanidine-based vulcanization accelerator as a vulcanization accelerator, and the amount of the guanidine-based vulcanization accelerator mixed is 0.1 parts by mass or more per 100 parts by mass of the diene rubber.

3. The cap tread rubber according to claim 1 or 2, characterized in that the compounding amount of said silica in said first tread rubber is 50 parts by mass or more per 100 parts by mass of said diene rubber.

4. 3. The cap tread rubber according to claim 1, wherein the second tread rubber has a hardness of 62 or more.

5. 3. The cap tread rubber according to claim 1, wherein the second tread rubber contains process oil, and the compounding amount of the process oil is 10 parts by mass or more per 100 parts by mass of the rubber component.

6. A tire comprising a cap tread layer made of the cap tread rubber according to claim 1 or 2.

7. The tire according to claim 6, characterized in that the second tread rubber is locally disposed in a part of the first tread rubber in the tire width direction, and a cross-sectional area of ​​the second tread rubber in a tire meridian cross section is 0.1% to 3% of a cross-sectional area of ​​the cap tread layer.

Images