TIRES

Abstract

Pneumatic tires (50), including: a tread section (1) extending in a circumferential direction of the tire and having a ring shape; and a plurality of main grooves (10) formed in the tread section (1) which run in a circumferential direction of the tire, wherein the tread section (1) comprises a protector tread rubber (11A) arranged at least on an outer side in the tire radial direction and a base rubber (11B) arranged on an inner side of the protector tread rubber (11A) in the tire radial direction, wherein a total thickness (TOGa) of the protector tread rubber (11A) and the base rubber (11B) and a thickness (UTGa) of the base rubber (11B) satisfy a relationship 0.20 ≤ UTGa / TOGa ≤ 0.40 in a ground contact area defined by a pair of the main grooves (10) located on both outermost sides in the tire width direction in the tread section (1), where the hardness UTHs of the base rubber (11B) is in the range of 62 or more and 67 or less, the hardness UTHs of the base rubber (11B) and the hardness CapHs of the outermost side satisfy the relationship 0.90 ≤ CapHs / UTHs ≤ 1.20 and a tan δ (60°C) of the base rubber (11B) is less than 0.06, where the total thickness (TOGa) of the protector tread rubber (11A) and the base rubber (11B), the thickness (UTGa) of the base rubber (11B) and a tread width (TW) of the tread section (1) satisfy a relationship 0.0025 ≤ (UTGa / TOGa) / TW ≤ 0.0040.

Description

Technical field

[0001] The present invention relates to a pneumatic tire which includes a tread section in which a protector tread rubber and a base rubber are layered. State of the art

[0002] In recent years, the rolling resistance coefficient (RRC) of pneumatic tires has been reduced to improve vehicle fuel efficiency. This type of pneumatic tire uses a layered protective tread rubber and a base rubber to form the tread section, and a technique has been proposed in which the rolling resistance coefficient is reduced by relatively increasing the thickness of the base rubber (see, for example, Patent Document 1). Patent Document 2 describes a pneumatic tire with a tread rubber layer that has a two-layer structure consisting of a top layer and a base layer, thereby reducing rolling resistance while maintaining wear resistance. List of literature on patent literature Patent document 1: JP 6158467 B Patent document 2: JP 2007- 137 411 A Brief description of the invention: Technical problem

[0003] However, in conventional configurations the hardness of the base rubber is lower (softer) than that of the protector tread rubber, and there is a problem that simply increasing the thickness of the base rubber can worsen steering stability.

[0004] The present invention was made in view of the above problems, and one object of the present invention is to provide a pneumatic tire in which the coefficient of rolling resistance is reduced while maintaining steering stability. Solution to the problem

[0005] To solve the problem described above and to fulfill the objective, a pneumatic tire according to one embodiment of the present invention includes: a tread section extending in the circumferential direction of the tire and having an annular shape, and a plurality of main grooves formed in the tread section and extending in the circumferential direction of the tire. The tread section includes: a protective tread rubber arranged at least on an outer side in the radial direction of the tire, and a base rubber arranged on an inner side in the radial direction of the protective tread rubber.The combined thickness (TOGa) of the protector tread rubber and the base rubber, and the thickness (UTGa) of the base rubber, satisfy a relationship of 0.20 ≤ UTGa / TOGa ≤ 0.40 in a ground contact area defined by a pair of the main grooves located on both outermost sides (in the tire width direction) of the tread section. The hardness (UTHs) of the base rubber is in the range of 62 or higher and 67 or lower. The hardness (UTHs) of the base rubber and the hardness (CapHs) of the protector tread rubber satisfy a relationship of 0.90 ≤ CapHs / UTHs ≤ 1.20. The tan δ (60°C) of the base rubber is less than 0.06.

[0006] In the pneumatic tire described above, the base rubber preferably contains an amine-based aging inhibitor of 2.0 phr or more.

[0007] Furthermore, the protective tread rubber in the pneumatic tire described above preferably contains an amine-based aging inhibitor of 2.0 phr or more.

[0008] Furthermore, in the pneumatic tire described above, a CPM content of the amine-based aging inhibitor of the protector tread rubber and a UTM content of the amine-based aging inhibitor of the base rubber preferably fulfill a relationship 0.5≤(UTM / CPM)≤1.5.

[0009] Furthermore, in the pneumatic tire described above, the total thickness TOGa of the protector tread rubber and the base rubber, the thickness UTGa of the base rubber and a tread width TW of the tread section satisfy a relationship 0.0025 ≤ (UTGa / TOGa) / TW ≤ 0.0040 according to the invention.

[0010] Furthermore, in the pneumatic tire described above, the tan δ (60°C) of the protector tread rubber is preferably 0.10 or more and 0.30 or less.

[0011] Furthermore, in the pneumatic tire described above, the average groove depth GD of the main groove and the thickness CPGa of the protector tread rubber preferably satisfy a relationship 1.0 ≤ (GD / CPGa) ≤ 1.3.

[0012] Furthermore, in the pneumatic tire described above, preferably 50 parts by mass of carbon black with a nitrogen adsorption-specific surface area N2SA of 70 m² are used. 2 / g or less per 100 parts by mass of a rubber component comprising 50% or more by mass of natural rubber and 35% or more by mass of terminally modified butadiene rubber and 50% or less by mass of butadiene rubber, mixed into the base rubber, and having a modulus of recoil elasticity of the base rubber at 40°C preferably being 80% or more.

[0013] Furthermore, the pneumatic tire described above is preferably a summer tire or an all-season tire. Advantageous effects of the invention

[0014] In the pneumatic tire according to one embodiment of the present invention, the total thickness TOGa of the protector tread rubber and the base rubber, and the thickness UTGa of the base rubber in the ground contact area, defined by a pair of main grooves located on both outermost sides in the tread section (in the direction of tire width), satisfy the relationship 0.20 ≤ UTGa / TOGa ≤ 0.40. The hardness UTHs of the base rubber is in the range of 62 or more and 67 or less. The hardness UTHs of the base rubber and the hardness CapHs of the protector tread rubber satisfy the relationship 0.90 ≤ CapHs / UTHs ≤ 1.20. The tan δ (60°C) of the base rubber is less than 0.06. As a result, rolling resistance can be reduced while maintaining steering stability. The total thickness TOGa of the protector tread rubber and the base rubber, the thickness UTGa of the base rubber and a tread width TW of the tread section satisfy a relationship 0.0025≤(UTGa / TOGa) / TW≤0.0040. Brief description of the drawings Fig. Figure 1 is a meridian cross-sectional view illustrating a pneumatic tire according to the present embodiment. Fig. Figure 2 is an enlarged cross-sectional view showing a main section of the pneumatic tire. Fig. 1 illustrates. Fig. Figure 3 is a table showing the results of performance tests of pneumatic tires according to the present embodiment. Description of embodiments

[0015] Embodiments of the present invention are described in detail below with reference to the drawings. In the embodiments described below, identical or similar components to those of other embodiments have identical reference numerals, and descriptions of these components are either simplified or omitted. The present invention is not limited by the embodiments. Components of the embodiments include elements that are essentially identical or that can be interchanged and easily devised by a person skilled in the art.

[0016] Fig. Figure 1 is a meridian cross-sectional view illustrating a pneumatic tire according to the present embodiment. Fig. Figure 2 is an enlarged cross-sectional view showing a main section of the pneumatic tire. Fig. 1 illustrates. In Fig. 1. “Meridian cross-section” refers to a cross-section of the tire along a plane that includes a tire axis of rotation (not illustrated). The reference symbol CL also indicates an equatorial plane of the tire and refers to a plane passing through the center of the tire in the direction of the tire axis of rotation and perpendicular to the tire axis of rotation. Furthermore, the tire width direction refers to a direction parallel to the tire axis of rotation; the inner side in the tire width direction refers to the side facing the tire equatorial plane CL in the tire width direction, and the outer side in the tire width direction refers to the side away from the tire equatorial plane CL in the tire width direction.The tire radial direction refers to a direction perpendicular to the tire's axis of rotation, the inner side in the tire radial direction refers to the side in the direction of the axis of rotation in the tire radial direction, and the outer side in the tire radial direction refers to the side away from the axis of rotation in the tire radial direction.

[0017] The pneumatic tire according to the present embodiment is directed towards a tire that is a so-called summer tire or an all-season tire, and does not include a studless tire (winter tire). Furthermore, the pneumatic tire according to the present embodiment is mounted on a vehicle generally referred to as a passenger car or a small passenger car, and is particularly suitable for a vehicle such as a so-called subcompact or compact car (a vehicle of the A-segment).

[0018] As in Fig. As illustrated in Figure 1, a pneumatic tire 50 includes: a tread section 1 extending in the circumferential direction of the tire and having a ring shape, a pair of sidewall sections 2, 2 arranged on both sides of the tread section 1, and a pair of bead sections 3, 3 arranged on an inner side of the sidewall sections 2 in the radial direction of the tire.

[0019] At least one carcass layer 4 is positioned between the pair of bead sections 3, 3. The carcass layer 4 encloses a plurality of reinforcing cords extending in the tire radial direction and is folded from the inside to the outside of the tire around bead cores 5 located in the respective bead sections 3. A bead filler 6, having a triangular cross-sectional shape and made of a rubber compound, is positioned on the outer circumference of the bead core 5.

[0020] On the other hand, a plurality of belt layers 7 are embedded on the outer circumferential side of the carcass layer 4 in the tread section 1. The belt layers 7 enclose a plurality of reinforcing cords inclined with respect to the tire's circumferential direction, and the reinforcing cords are arranged such that they overlap between the layers. In the belt layers 7, the angle of inclination of the reinforcing cords with respect to the tire's circumferential direction is set within a range of, for example, not less than 10° and not more than 40°. Steel cords are preferably used as the reinforcing cord threads of the belt layers 7.To improve durability at high speeds, at least one belt cover layer 8, formed by arranging reinforcing cords at an angle of, for example, no more than 5° with respect to the tire's circumferential direction, is arranged on an outer circumferential side of the belt layers 7. Organic fiber cord threads such as nylon and aramid are preferably used as the reinforcing cord threads of the belt cover layer 8.

[0021] It should be noted that the tire internal structure described above is a typical example of a pneumatic tire, but the pneumatic tire is not limited to this.

[0022] The pneumatic tire described above has a multitude of main grooves 10 (four main grooves in Fig. 1), extending in the circumferential direction of the tire, are formed in tread section 1. The main grooves 10 are grooves that are each provided with wear indicators (not illustrated) at predetermined intervals in the circumferential direction of the tire. The main grooves 10 include: two central main grooves 10A, located on the inner side in the direction of tire width, with the tire equatorial plane CL positioned between them, and two shoulder main grooves 10B, each located on the outer side in the direction of tire width of the central main groove 10A. The shoulder main groove 10B corresponds to a main groove located on the outermost side in the direction of tire width. In a case where it is not necessary to distinguish the central main groove 10A from the shoulder main groove 10B, the central main groove 10A and the shoulder main groove 10B are simply referred to as the main groove 10.Furthermore, lug grooves extending in the direction of the tire width are formed as 10 different grooves from the main grooves in tread section 1.

[0023] The two central main grooves 10A and the two shoulder main grooves 10B are formed in the tread section 1, and thus a plurality of rib sections 20 (five rib sections in Fig. 1) defined therein. In particular, the rib sections 20 include: a middle rib section 20A extending circumferentially between the pair of middle main grooves 10A, 10A; second rib sections 20B, each extending circumferentially between the middle main groove 10A and the shoulder main groove 10B; and shoulder rib sections 20C, each located on the radially outer side of the shoulder main groove 10B and extending circumferentially. In a case where the middle rib section 20A, the second rib section 20B, and the shoulder rib section 20C are not distinguished from one another, the middle rib section 20A, the second rib section 20B, and the shoulder rib section 20C are simply referred to as the rib section 20.

[0024] In the pneumatic tire 50 described above, a tread rubber layer 11 is arranged on the outer side of the carcass layer 4, the belt layers 7, and the belt cover layer 8 in the tread section 1. A sidewall rubber layer 12 is arranged on the outer side of the carcass layer 4 in the sidewall section 2. A rim cushion rubber layer 13 is arranged on the outer side of the carcass layer 4 in the bead section 3. Additionally, an inner liner layer 14 is arranged along the carcass layer 4 on the inner sidewall of the tire.

[0025] As in Fig. As illustrated in Figure 2, the tread rubber layer 11 comprises a multilayer structure of at least two layers and includes a protector tread rubber 11A, located on the outermost side in the tire radial direction, and a base rubber 11B adjacent to the protector tread rubber 11A on the inner side in the tire radial direction. The protector tread rubber 11A is made of a rubber material that exhibits excellent road contact properties and weather resistance and is exposed to a surface (also referred to as the tread surface or road contact surface) 1A of the tread section 1, thus coming into contact with the road surface during driving. Furthermore, various grooves, such as the main grooves 10 of the tread section 1 and the lug grooves, are formed primarily in the protector tread rubber 11A.The base rubber 11B is arranged between the protector tread rubber 11A and the belt layers 7 to form a base section of the tread rubber layer 11.

[0026] Furthermore, for pneumatic tires used as summer or all-season tires, a configuration is being investigated in which steering stability and a reduction in the rolling resistance coefficient are achieved in a compatible manner for the purpose of improving a vehicle's fuel efficiency. In the present configuration, the rolling resistance coefficient is reduced while ensuring good steering stability by improving the thickness (strength), hardness, and tan δ value (loss factor) of the base rubber 11B in the tread rubber layer 11.

[0027] In particular, in the pneumatic tire 50 described above, the total thickness TOGa of the protector tread rubber 11A and the base rubber 11B, and the thickness UTGa of the base rubber 11B, satisfy the relationship 0.20 ≤ UTGa / TOGa ≤ 0.40. The total thickness TOGa is the sum (CPGa + UTGa = TOGa) of the thickness CPGa of the protector tread rubber 11A and the thickness UTGa of the base rubber 11B. Accordingly, in the present configuration, the total thickness TOGa and the thickness CPGa of the protector tread rubber 11A satisfy 0.60 ≤ CaGa / TOGa ≤ 0.80.

[0028] As described above, the tread rubber layer 11 can achieve a reduction in the rolling resistance coefficient by adjusting the thickness UTGa of the base rubber 11B relative to the overall thickness TOGa, so that it is relatively thick. It should be noted that the thickness of each rubber is the average thickness measured in a ground contact area between the two main shoulder grooves 10B, 10B of the tread section 1, specifically in a section central in the direction of tire width (a 25% area from the center to both outer edges in the direction of width) of each rib section 20.

[0029] The ground contact area is the area defined by ground contact edges T located at both ends in the tire's width direction, and is the area where the tread surface of tread section 1 of the pneumatic tire 50 comes into contact with a dry, flat road surface when the pneumatic tire 50 is mounted on a specified rim, inflated to a specified internal pressure, and loaded with 70% of a specified load. Here, "specified rim" refers to a "standard rim" as defined by the Japan Automobile Tyre Manufacturers Association Inc. (JATMA), a "design rim" as defined by the Tire and Rim Association, Inc. (TRA), or a "measuring rim" as defined by the European Tyre and Rim Technical Organisation (ETRTO).Furthermore, specified internal pressure refers to a "maximum air pressure" as defined by JATMA, a maximum value in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" as defined by TRA, or "TIRE PRESSURES*" as defined by ETRTO. Additionally, "specified load" refers to a "maximum load capacity" as defined by JATMA, the maximum value in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" as defined by TRA, or "LOAD CAPACITY" as defined by ETRTO.

[0030] Furthermore, in the pneumatic tire 50 described above, the base rubber 11B has a hardness UTHs in the range of 62 or higher and 67 or lower. Additionally, the hardness UTHs of the base rubber 11B and the hardness CapHs of the protective tread rubber 11A satisfy the relationship 0.90 ≤ CapHs / UTHs ≤ 1.20. Here, hardness is the durometer hardness, measured according to JIS-K6253 using a Type A durometer at a temperature of 23°C, and is also referred to as JIS hardness. In this configuration, the base rubber 11B has a relatively high hardness (medium hardness), and the base rubber 11B and the protective tread rubber 11A are formulated to have the same hardness. As a result, the stiffness of the tread section 1 can be ensured, and good steering stability can be guaranteed.

[0031] Furthermore, in the pneumatic tire 50 described above, the tan δ (60°C) of the base rubber 11B is set to a value that is smaller than the tan δ (60°C) of the protective tread rubber 11A. Specifically, the tan δ (60°C) of the base rubber 11B is set to be greater than 0 and less than 0.06, and the tan δ (60°C) of the protective tread rubber 11A is set to 0.10 or more and less than 0.30. Here, the tan δ (60°C) refers to a loss factor (loss modulus / storage modulus) at 60°C and is an indicator for evaluating the elasticity and viscosity properties of the rubber material. Generally, the closer the tan δ (60°C) value is to 0, the higher the elasticity. The larger the value of tan δ (60°C), the higher the viscosity tends to be. Furthermore, the closer the value of tan δ (60°C) is to 0, the lower the heat build-up and the lower the rolling resistance coefficient tends to be.

[0032] In the present configuration, the total thickness TOGa of the protector tread rubber 11A and the base rubber 11B, and the thickness UTGa of the base rubber 11B, satisfy the relationship 0.20 ≤ UTGa / TOGa ≤ 0.40. The hardness UTHs of the base rubber 11B is in the range of 62 or more and 67 or less, and the hardness UTHs of the base rubber 11B and the hardness CapHs of the protector tread rubber 11A satisfy the relationship 0.90 ≤ CapHs / UTHs ≤ 1.20. The tan δ (60°C) of the base rubber 11B is 0.06 or less. Consequently, the thickness UTGa of the base rubber 11B can be relatively thick compared to the total thickness TOGa, and the hardness UTHs of the base rubber 11B can be of medium hardness. Furthermore, the base rubber 11B can exhibit low heat buildup. As a result, the stiffness of the tread section 1 can be ensured, good steering stability can be guaranteed, and the rolling resistance coefficient can be reduced.

[0033] Here, if UTGa / TOGa is less than 0.20, the amount of base rubber is small, and therefore the effect of reducing the rolling resistance coefficient is insufficient. If UTGa / TOGa is greater than 0.40, the amount of base rubber is also too large, and thus steering stability is reduced. Furthermore, the stiffness of tread section 1 is insufficient if the hardness UTHs of the base rubber 11B is less than 62, and thus steering stability is reduced. If the hardness UTHs is greater than 67, there is also a problem that a low heat build-up of the base rubber cannot be maintained, and thus the rolling resistance coefficient worsens. If CapHs / UTHs is less than 0.90, there is also a problem that the protective tread rubber 11A is too soft relative to the base rubber 11B, and thus steering stability cannot be maintained.If CapHs / UTHs is greater than 1.20, the base rubber 11B is too soft relative to the protective tread rubber 11A, and therefore the stiffness of tread section 1 is insufficient, which impairs steering stability. Furthermore, if the tan δ (60°C) of the base rubber 11B is greater than 0.06, there is a problem with high heat buildup in the base rubber 11B, which worsens the rolling resistance coefficient.

[0034] Furthermore, in the present configuration, since the tan δ (60°C) of the protector tread rubber 11A is set to 0.10 or more and 0.30 or less, rubber with a relatively higher viscosity than the protector tread rubber 11A can be used, thereby increasing the frictional force of the rubber. As a result, the gripping force of the tread section 1 can be improved, and steering stability can be enhanced.

[0035] Furthermore, the tread section 1 used in the pneumatic tire 50 described above deteriorates due to various factors such as oxygen, ozone, light, and dynamic fatigue during use. In the present configuration, the protector tread rubber 11A contains an amine-based anti-aging agent of 2.0 phr or more, and the base rubber 11B contains an amine-based anti-aging agent of 2.0 phr or more. In other words, the base rubber 11B contains a volume of the amine-based anti-aging agent equal to or greater than that of the protector tread rubber 11A. The amine-based anti-aging agent prevents the aging (deterioration) of the rubber to suppress groove cracking that occurs in the groove bottom of the main groove 10 or the like of the tread section 1. For example, N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine (brand name: NOCRAC (trade name) 6C) can be used.It should be noted that phr specifies the parts by weight of the amine-based antioxidant in relation to 100 parts by weight of the rubber component.

[0036] Here, the base rubber 11B is not exposed to the outside, and the main groove 10 is formed within the protective tread rubber 11A. Therefore, the amine-based oxidizing agent may only be contained within the protective tread rubber 11A to suppress groove cracking in the groove base of the main groove 10. However, in one case where the amine-based oxidizing agent is only contained within the protective tread rubber 11A, it was observed that the amine-based oxidizing agent flows from the protective tread rubber 11A to the base rubber 11B (also known as migration), thus reducing the amine-based oxidizing agent content of the protective tread rubber 11A and causing groove cracking.Accordingly, in the present configuration, by allowing the amine-based anti-aging agent to be contained in the base rubber 11B at a concentration of 2.0 phr or more, the migration of the amine-based anti-aging agent from the protector tread rubber 11A to the base rubber 11B can be suppressed, and groove cracking occurring in the groove bottom of the main groove 10 can be suppressed.

[0037] Here, if the content of the amine-based aging inhibitor in the base rubber 11B is less than 2.0 phr, a problem can arise in that groove cracking is likely to occur due to the lack of aging inhibitor in the groove bottom of the main groove 10. Consequently, the content of the amine-based aging inhibitor in the base rubber 11B is preferably 2.0 phr or more. Furthermore, the CPM content of the amine-based aging inhibitor in the protector tread rubber 11A and the UTM content of the amine-based aging inhibitor in the base rubber 11B preferably satisfy the relationship 0.5 ≤ (UTM / CPM) ≤ 1.5.

[0038] Furthermore, in the pneumatic tire 50 described above, the total thickness TOGa of the protector tread rubber 11A and the base rubber 11B, the thickness UTGa of the base rubber 11B, and a tread width TW of the tread section 1 satisfy the relationship 0.0025 ≤ (UTGa / TOGa) / TW ≤ 0.0040 according to the invention. As in Fig. As illustrated in Figure 1, the tread width TW is the distance between the ground contact edges T, T of the tread section 1 in the tire width direction and is measured in a state in which the pneumatic tire 50 is mounted on a specified rim, inflated to a specified internal pressure and loaded with 70% of a specified load.

[0039] As described above, the pneumatic tire 50 according to the present embodiment is suitable for mounting on a small car or a compact car (an A-segment vehicle). A pneumatic tire for a small car or the like has a tread width TW that is narrower than that of a pneumatic tire for a regular passenger car, which is likely to impair steering stability. Furthermore, a pneumatic tire for a small car or the like requires a rolling resistance coefficient that is lower than that of a pneumatic tire for a regular passenger car. If (UTGa / TOGa) / TW is less than 0.0012, the effect of reducing the rolling resistance coefficient cannot be sufficiently achieved here, since the amount of base rubber is small relative to the tread width TW (in other words, the tire size).Meanwhile, if (UTGa / TOGa) / TW is greater than 0.0040, the amount of base rubber in relation to the tread width TW is large, which can lead to a decrease in steering stability.

[0040] In the present configuration, by adjusting the total thickness TOGa of the protector tread rubber 11A and the base rubber 11B, the thickness UTGa of the base rubber 11B, and the tread width TW of tread section 1 within the range to achieve 0.0025 ≤ (UTGa / TOGa) / TW ≤ 0.0040, the value of (UTGa / TOGa) with respect to the tread width TW can be relatively large. Consequently, even a pneumatic tire size 50, when fitted to a small or compact car, can provide good steering stability and a reduction in the rolling resistance coefficient in a compatible manner.

[0041] Furthermore, in the pneumatic tire 50 described above, the thickness CPGa of the protective tread rubber 11A and the average groove depth GD of the main grooves 10 preferably meet 1.0 ≤ (GD / CPGa) ≤ 1.3, and the average groove depth GD of the main grooves 10 is preferably in the range of 5.0 mm or more and 9.0 mm or less. Accordingly, the thickness CPGa of the protective tread rubber 11A can be optimized with respect to the average groove depth GD of the main grooves 10, and steering stability can be improved.

[0042] Furthermore, in the pneumatic tire 50 described above, a rubber component of a rubber composition for a tire, which is used in the base rubber 11B, is a diene rubber, which certainly includes a natural rubber and a terminally modified butadiene rubber. The natural rubber used can be any rubber that is typically used in rubber compositions for tires. By incorporating a natural rubber, sufficient rubber strength can be achieved in a rubber composition for tires. If the total diene rubber is 100 wt%, the amount of natural rubber in the mixture is 50 wt% or more, preferably 50 wt% or more and 70 wt% or less, and more preferably 60 wt% or more and 65 wt% or less. If the amount of natural rubber in the mixture is less than 50 wt%, the rubber strength is reduced.

[0043] End-modified butadiene rubber is a butadiene rubber in which one or both ends of the molecular chain are modified with an organic compound containing a functional group. Incorporating such an end-modified butadiene rubber increases the affinity for carbon black described below and improves dispersibility. Consequently, the effect of the carbon black is further enhanced, while heat build-up is kept to a minimum, thus increasing the rubber's hardness. From, for example, a hydroxyl group, an amino group, an amide group, an alkoxyl group, an epoxy group, and a siloxane linkage group, at least one is selected to be used as the functional group enabling end-modification of the molecular chain. It should be noted that the siloxane linkage group is a functional group with an -O-Si-O structure.

[0044] If the total diene rubber is 100% by mass, the amount of terminally modified butadiene rubber mixed in is 35% by mass or more and 50% by mass or less, and preferably 40% by mass or more and 50% by mass or less. If the amount of terminally modified butadiene rubber mixed in is less than 35% by mass, fuel efficiency deteriorates. If the amount of terminally modified butadiene rubber mixed in is greater than 50% by mass, rubber strength is reduced.

[0045] The molecular weight distribution (Mw / Mn) of the terminally modified butadiene rubber is preferably 2.0 or less, and more preferably 1.1 or more and 1.6 or less. As described above, using a terminally modified butadiene rubber with a narrow molecular weight distribution results in even better rubber properties, thus effectively improving steering stability and durability in tires while reducing rolling resistance. If the molecular weight distribution (Mw / Mn) of the terminally modified butadiene rubber is greater than 2.0, the hysteresis loss increases, the heat buildup of the rubber increases, and the compression set resistance is reduced.

[0046] The glass transition temperature Tg of the terminally modified butadiene rubber used in the present configuration is preferably -85°C or lower, and more preferably -100°C or higher and -90°C or lower. By adjusting the glass transition temperature Tg as described above, heat buildup can be effectively reduced. If the glass transition temperature Tg is higher than -80°C, the heat buildup reduction effect cannot be sufficiently maintained. It should be noted that the glass transition temperature Tg of the natural rubber is not particularly limited, but can, for example, be set to -80°C or higher and -70°C or lower.

[0047] Additionally, the terminally modified butadiene rubber used in the present configuration preferably has a vinyl content of 0.1 wt% or more and 20 wt% or less, and more preferably a vinyl content of 0.1 wt% or more and 15 wt% or less. If the vinyl content of the terminally modified butadiene rubber is less than 0.1 wt%, its affinity for carbon black becomes insufficient, making it difficult to adequately reduce heat buildup. If the vinyl content of the terminally modified butadiene rubber is greater than 20 wt%, the glass transition temperature Tg of the rubber composition is increased, and the rolling resistance and wear resistance cannot be adequately improved. It should be noted that the vinyl content of the terminally modified butadiene rubber is measured by infrared spectroscopy (Hampton method).The increase or decrease in the vinyl unit content in the terminally modified butadiene rubber can be appropriately adjusted using a conventional method, such as the use of a catalyst.

[0048] Furthermore, in the pneumatic tire 50 described above, carbon black is necessarily mixed as a filler into a rubber compound for the tire, which is used in the base rubber 11B. The addition of carbon black increases the strength of the rubber compound. In particular, the carbon black mixed into the rubber compound for a tire according to the present configuration has a nitrogen adsorption-specific surface area N2SA of 70 m². 2 / g or less, preferably 35 m 2 / g or more and 60 m 2 / g or less and preferably more than 35 m 2 / g or more and 50 m 2 / g or less. By mixing a combination of carbon black with such a large particle diameter and the modified butadiene rubber described above, while keeping the heat build-up low, the rubber hardness can be effectively improved. If the nitrogen adsorption-specific surface area N2SA of carbon black is greater than 70 m² 2 The heat build-up deteriorates when the nitrogen adsorption capacity (N2SA) is lower. It should be noted that the nitrogen adsorption-specific surface area N2SA of the carbon black is measured according to JIS K 6217-2.

[0049] The amount of carbon black mixed in is preferably 50 parts by mass or more per 100 parts by mass of the aforementioned rubber component, preferably 55 parts by mass or more and 65 parts by mass or less, and more preferably 57 parts by mass or more and 60 parts by mass or less. If the amount of carbon black mixed in is less than 50 parts by mass, the hardness of the base rubber 11B is reduced.

[0050] Furthermore, in the pneumatic tire 50 described above, the hardness UTHs of the rubber composition used in the base rubber 11B is set in the range of 62 or more and 67 or less, as described above, and is preferably 65 or more and 67 or less. Additionally, in the pneumatic tire 50 described above, the rubber composition used in the base rubber 11B has a rebound elasticity modulus of 80% or more at 40°C, preferably a rebound elasticity modulus of 80% or more and 85% or less, and more preferably a rebound elasticity modulus of 82% or more and 85% or less. Since the base rubber 11B in the present configuration has the physical properties described above, steering stability can be improved while the rolling resistance coefficient is reduced.If the recoil modulus is less than 80%, heat buildup deteriorates and the rolling resistance coefficient cannot be reduced. It should be noted that this hardness and recoil modulus are not solely determined by the aforementioned compound and are physical properties that can also be influenced, for example, by kneading conditions or processes.

[0051] As described above, the pneumatic tire 50 according to the present embodiment includes the tread section 1, which extends in the circumferential direction of the tire and has an annular shape, and wherein the plurality of main grooves 10 are formed in the tread section 1 and extend in the circumferential direction of the tire. The tread section 1 includes at least the protector tread rubber 11A, which is arranged on the outer side in the tire radial direction, and the base rubber 11B, which is arranged on the inner side 11A of the protector tread rubber in the tire radial direction. The total thickness TOGa of the protector tread rubber 11A and the base rubber 11B and the thickness UTGa of the base rubber 11B satisfy the relationship 0.20 ≤ UTGa / TOGa ≤ 0.40 in the ground contact area defined by the pair of shoulder main grooves 10B located on the two outermost sides in the tire width direction in tread section 1.The hardness UTHs of the base rubber 11B is in the range of 62 or more and 67 or less. The hardness UTHs of the base rubber 11B and the hardness CapHs of the protector tread rubber 11A satisfy the relationship 0.90 ≤ CapHs / UTHs ≤ 1.20. The tan δ (60°C) of the base rubber 11B is less than 0.06. Accordingly, the thickness UTGa of the base rubber 11B can be relatively thick compared to the total thickness TOGa, and the hardness UTHs of the base rubber 11B can be of medium hardness. Furthermore, the base rubber 11B can exhibit low heat buildup. As a result, the stiffness of the tread section 1 is ensured, and good steering stability can be guaranteed. In addition, the rolling resistance coefficient can be reduced.

[0052] Furthermore, according to the present embodiment, the base rubber 11B contains an amine-based aging inhibitor of 2.0 phr or more, and the protector tread rubber 11A contains an amine-based aging inhibitor of 2.0 phr or more. Accordingly, the migration of the amine-based aging inhibitor from the protector tread rubber 11A to the base rubber 11B can be suppressed, and the groove cracking that occurs in the groove bottom of the main groove 10 can be suppressed.

[0053] Furthermore, according to the present embodiment, the CPM content of the amine-based aging inhibitor of the protector tread rubber 11A and the UTM content of the amine-based aging inhibitor of the base rubber 11B satisfy the relationship 0.5 ≤ (UTM / CPM) ≤ 1.5. Accordingly, the migration of the amine-based aging inhibitor from the protector tread rubber 11A to the base rubber 11B can be suppressed, and the groove cracking that occurs in the groove bottom of the main groove 10 can be suppressed.

[0054] Furthermore, according to the present embodiment, the total thickness TOGa of the protector tread rubber 11A and the base rubber 11B, the thickness UTGa of the base rubber 11B, and a tread width TW of the tread section 1 satisfy the relationship 0.0025 ≤ (UTGa / TOGa) / TW ≤ 0.0040 according to the invention. Accordingly, the pneumatic tire, even when mounted on, for example, a small car or a compact car, can provide good steering stability and a reduction in the rolling resistance coefficient in a compatible manner.

[0055] Furthermore, according to the present embodiment, since the tan δ (60°C) of the protector tread rubber 11A is 0.10 or more and 0.30 or less, rubber with a relatively high viscosity than the protector tread rubber 11A can be used, thereby increasing the frictional force of the rubber. As a result, the gripping force of the tread section 1 can be improved, and steering stability can be enhanced.

[0056] Furthermore, according to the present embodiment, since the average groove depth GD of the main groove 10 and the thickness CPGa of the protector tread rubber 11A satisfy the relationship 1.0 ≤ (GD / CPGa) ≤ 1.3, the thickness CPGa of the protector tread rubber 11A can be optimized with respect to the average groove depth GD of the main groove 10, and the steering stability can be improved.

[0057] Furthermore, according to the present embodiment, 50 parts by mass or more of carbon black with a nitrogen adsorption-specific surface area N2SA of 70 m² are used. 2 / g or less per 100 parts by mass of a rubber component comprising 50% or more by mass of natural rubber and 35% or more by mass of terminally modified butadiene rubber and 50% or less by mass of base rubber 11B, and having a modulus of rebound elasticity of the base rubber 11B at 40°C of 80% or more. As a result, the stiffness of the tread section 1 is ensured, and good steering stability can be guaranteed. In addition, the rolling resistance coefficient can be reduced. Examples

[0058] Fig. Figure 3 is a table showing the results of performance tests of pneumatic tires according to the present embodiment. In the performance tests, steering stability, rolling resistance coefficient, and groove cracking were evaluated for a variety of test tire types. In each of the test tires, a tread rubber layer 11, arranged in a tread section 1, includes: a protector tread rubber 11A, located on the outermost side in the tire radial direction, and a base rubber 11B, located adjacent to the protector tread rubber on the inner side 11A in the tire radial direction.The tires according to Examples 1 to 6 and according to Comparative Examples 1 to 6 were manufactured, exhibiting a UTGa / TOGa relationship between a total thickness TOGa of the protector tread rubber 11A and the base rubber 11B and a thickness UTGa of the base rubber 11B, a CapHs / UTHs relationship between a hardness CapHs of the protector tread rubber 11A and a hardness UTHs of the base rubber 11B, a hardness UTHs of the base rubber 11B, the content of an amine-based anti-aging agent of the base rubber 11B, a relationship between the aforementioned UTGa / TOGa and a tread width TW, and a GD / CPGa relationship between an average groove depth GC of a main groove and a thickness CPGa of the protector tread rubber 11A, as in . Fig. Figure 3 illustrates the following. The pneumatic tire according to Example 1 is a non-inventive embodiment. For comparison, prior art examples 1 and 2 were produced, which were provided with a base rubber of low hardness.

[0059] The test tires have a tire size of 155 / 65R 14 75S. Rolling resistance coefficient, steering stability, and groove cracking of the test tires are evaluated using the following test procedures, and the results are presented in Fig. 3. In evaluating rolling resistance coefficients, each of the test tires was mounted on a wheel with a rim size of 14 x 4.5J and mounted on a drum testing machine, and rolling resistance coefficients were measured under an air pressure of 240 kPa according to ISO 25280. The evaluation results are expressed as index values ​​using inverses of the measured values, with the prior art example 1 being assigned an index value of 100. Higher index values ​​indicate lower rolling resistance coefficients and superior results.

[0060] To assess steering stability, each test tire was mounted on a 14 x 4.5J rim, inflated to 240 kPa, mounted on a passenger car, and driven on a test track with a dry road surface. A test driver then performed the sensory evaluation. The results are also expressed as index values, with 100 representing the state of the art. Higher index values ​​indicate better steering stability.

[0061] In the evaluation of groove cracking, each of the test tires was mounted on a wheel with a rim size of 14 x 4.5J, inflated to a pressure of 240 kPa, and left for 24 hours in an ozone-treated test chamber. Groove cracks formed in the main groove were measured. The evaluation results are expressed as index values ​​using inverses of the measured values, with the prior art example 1 being assigned an index value of 100. Higher index values ​​indicate fewer occurrences of groove cracking and superior results.

[0062] As from Fig.As can be seen in Figure 3, the tires according to Examples 1 to 6 can achieve a reduction in the rolling resistance coefficient and the occurrence of groove cracking, while ensuring good steering stability, unlike the prior art example 1. However, since the tires according to comparative examples 1 to 6 do not meet the predetermined conditions, the desired effect of providing steering stability, reducing the rolling resistance coefficient, and minimizing groove cracking is not sufficiently achieved in a compatible manner. Furthermore, the tire according to prior art example 2 is a so-called studless tire, which incorporates a base rubber with lower hardness and relatively high thickness compared to prior art example 1, and in this case, steering stability is ultimately impaired. List of reference symbols 1 tread section 10 Main groove 10A Middle Main Groove 10B Shoulder groove 11 Tread rubber layer 11A Protector tread rubber 11B Base rubber 20 Bridge section 50 pneumatic tires CPGa thickness of the protector running surface rubber CapH's hardness of the protector running surface rubber TOGa Total thickness TW tread width UTGA thickness of the base rubber UTH's hardness of the base rubber

Claims

[1] Pneumatic tires (50), comprising: a tread section (1) extending in a circumferential direction of the tire and having a ring shape; and a plurality of main grooves (10) formed in the tread section (1) which run in a circumferential direction of the tire, wherein the tread section (1) comprises a protector tread rubber (11A) arranged at least on an outer side in the tire radial direction and a base rubber (11B) arranged on an inner side of the protector tread rubber (11A) in the tire radial direction, wherein a total thickness (TOGa) of the protector tread rubber (11A) and the base rubber (11B) and a thickness (UTGa) of the base rubber (11B) satisfy a relationship 0.20 ≤ UTGa / TOGa ≤ 0.40 in a ground contact area defined by a pair of the main grooves (10) located on both outermost sides in the tire width direction in the tread section (1), where the hardness UTHs of the base rubber (11B) is in the range of 62 or more and 67 or less, the hardness UTHs of the base rubber (11B) and the hardness CapHs of the outermost side satisfy the relationship 0.90 ≤ CapHs / UTHs ≤ 1.20 and a tan δ (60°C) of the base rubber (11B) is less than 0.06, where the total thickness (TOGa) of the protector tread rubber (11A) and the base rubber (11B), the thickness (UTGa) of the base rubber (11B) and a tread width (TW) of the tread section (1) satisfy a relationship 0.0025 ≤ (UTGa / TOGa) / TW ≤ 0.0040. [2] Pneumatic tire (50) according to claim 1, wherein the base rubber (11B) contains an amine-based anti-aging agent of 2.0 phr or more. [3] Pneumatic tire (50) according to claim 1 or 2, wherein the protector tread rubber (11A) contains an amine-based anti-aging agent of 2.0 phr or more. [4] Pneumatic tire (50) according to any one of claims 1 to 3, wherein a content of CPM of the amine-based anti-aging agent of the protector tread rubber (11A) and a content of UTM of the amine-based anti-aging agent of the base rubber (11B) satisfy a relationship 0.5 ≤ (UTM / CPM) ≤ 1.

5. [5] Pneumatic tire (50) according to any one of claims 1 to 4, wherein a tan δ (60°C) of the protector tread rubber (11A) is 0.10 or more and 0.30 or less. [6] Pneumatic tire (50) according to any one of claims 1 to 5, wherein an average groove depth (GD) of the main groove (10) and a thickness (CPGa) of the protector tread rubber (11A) satisfy a relationship 1.0 ≤ (GD / CPGa) ≤1.

3. [7] Pneumatic tires (50) according to any one of claims 1 to 6, wherein 50 parts by mass or more of carbon black with a nitrogen adsorption-specific surface area N2SA of 70 m² 2 / g or less per 100 parts by mass of a rubber component comprising 50 wt% or more natural rubber and 35 wt% or more and 50 wt% or less terminally modified butadiene rubber, blended into the base rubber (11B) and having a recoil elasticity modulus of the base rubber (11B) at 40°C of 80% or more. [8] Pneumatic tire according to any one of claims 1 to 7, wherein the pneumatic tire (50) is a summer tire or an all-season tire.

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