tire

Abstract

To provide a tire that can improve the durability of the stress relaxation layer at low temperatures. [Solution] The tire comprises a tread portion having a tread surface and provided with tread rubber. The tread surface has a plurality of circumferential main grooves and a plurality of land portions partitioned by the plurality of circumferential main grooves. At least one of the plurality of circumferential main grooves has a stress-relieving layer on the surface of the groove bottom. The stress-relieving layer mainly consists of diene-based rubber material and non-diene-based rubber material, and also contains carbon and a vulcanizing agent, and has a brittleness temperature Tgc of -20°C or lower.

Description

Technical Field

[0001] The present invention relates to a tire.

Background Art

[0002] In order to suppress groove cracks generated at the bottom of the main groove, a tire having a stress relaxation layer provided at the bottom of the main groove is known.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For tires, it is required to suppress the generation of groove cracks even at low temperatures.

[0005] An object of the present invention is to provide a tire capable of improving the durability of a stress relaxation layer at low temperatures.

Means for Solving the Problems

[0006] A tire according to one aspect of the present invention includes a tread portion having a tread surface provided with tread rubber, the tread surface having a plurality of circumferential main grooves and a plurality of land portions partitioned and formed by the plurality of circumferential main grooves, at least one of the plurality of circumferential main grooves having a stress relaxation layer on the surface of the groove bottom, the stress relaxation layer being mainly composed of a diene rubber material and a non-diene rubber material and containing carbon and a vulcanizing agent, and having a glass transition temperature Tgc of -20°C or lower.

Effects of the Invention

[0007] According to the present invention, the durability of the stress relaxation layer at low temperatures can be improved.

Brief Description of the Drawings

[0008] [Figure 1] This is an end view showing the meridional cross-sectional shape of the tire according to this embodiment. [Figure 2] This is a partially enlarged view along line II-II in Figure 1. [Modes for carrying out the invention]

[0009] Embodiments of the present invention relate to the following aspects.

[0010] [Aspect 1] It has a tread surface and a tread portion provided with tread rubber, The tread surface has a plurality of circumferential main grooves and a plurality of land areas partitioned by the plurality of circumferential main grooves, At least one of the plurality of circumferential main grooves has a stress-relieving layer on the surface of the groove bottom, The stress-relaxing layer is mainly composed of diene-based rubber material and non-diene-based rubber material, and also contains carbon and a vulcanizing agent, and has a brittleness temperature Tgc of -20°C or lower, in a tire. [Aspect 2] The tire according to embodiment 1, wherein the embrittlement temperature Tgt of the tread rubber is -20°C or lower. [Aspect 3] The tire according to embodiment 1 or 2, wherein the difference (|Tgc-Tgt|) between the embrittlement temperature Tgc of the stress relaxation layer and the embrittlement temperature Tgt of the tread rubber is 40°C or less. [Aspect 4] The tire according to any one of embodiments 1 to 3, wherein the hardness Hsc of the stress relaxation layer at -10°C is 45 degrees or more and 70 degrees or less. [Aspect 5] A tire according to any one of embodiments 1 to 4, wherein the ratio (Hsc / Hst) of the hardness Hsc of the stress relaxation layer at -10°C to the hardness Hst of the tread rubber at -10°C is 0.6 or more and 1.1 or less. [Aspect 6] The tire according to any one of Aspects 1 to 5, wherein the ratio (Dc / Dt) of the thermal conductivity Dc of the stress relaxation layer to the thermal conductivity Dt of the tread rubber is 0.5 or more and 1.5 or less. [Aspect 7] The tire according to any one of Aspects 1 to 6, wherein the ratio [EB(-10) / EB(20)] of the elongation at break EB(-10) of the stress relaxation layer at -10°C to the elongation at break EB(20) at 20°C is 0.5 or more. [Aspect 8] The tire according to any one of Aspects 1 to 7, wherein the thickness of the stress relaxation layer is 5 μm or more and 200 μm or less. [Aspect 9] Let R be the ratio Hg / Hmax of the minimum thickness Hg of the tread portion to the maximum thickness Hmax of the tread portion. The ratio R, the minimum distance Gu in the tire radial direction from the groove bottom of the circumferential main groove to the reinforcing layer, and the thickness Ga of the stress relaxation layer satisfy the following formula (1). The tire according to any one of Aspects 1 to 8. [Number] [Aspect 10] The tire according to any one of Aspects 1 to 9, wherein the stress relaxation layer is provided in the circumferential main groove provided within the maximum belt width region. [Aspect 11] The minimum distance Gu in the tire radial direction from the groove bottom of the circumferential main groove to the reinforcing layer, the thickness Ga of the stress relaxation layer, the thermal conductivity Dc of the stress relaxation layer, and the thermal conductivity Dt of the tread rubber satisfy the following formula (2). The tire according to any one of Aspects 1 to 9. [Number]

[0011] (Definition) The tire radial direction refers to the direction perpendicular to the tire rotation axis. The tire radial direction inner side refers to the side facing the tire rotation axis in the tire radial direction, and the tire radial direction outer side refers to the side away from the tire rotation axis in the tire radial direction. The tire circumferential direction refers to the circumferential direction around the tire rotation axis as the central axis. The tire width direction refers to the direction parallel to the tire rotation axis. The inner side in the tire width direction means the side facing the tire equatorial plane (tire equatorial plane) in the tire width direction, and the outer side in the tire width direction means the side away from the tire equatorial plane in the tire width direction. The tire equatorial plane refers to a plane that is orthogonal to the tire rotation axis and passes through the center of the tire width. "Along" a certain reference includes following the direction within a range of less than ±20°, less than ±10°, or less than ±5° with respect to a certain reference. "Center" includes the midpoint where the distances from two certain points are equal and the range of ±10% of the distance between the two points from the midpoint. The circumferential main groove is a circumferential groove having a wear indicator indicating the end stage of wear, and generally has a groove width of 3.0 [mm] or more and a groove depth of 5.0 [mm] or more. Note that the groove width and groove depth of the circumferential main groove are not limited to the above ranges. The groove width is measured as the maximum value of the distance between the opposing groove walls at the groove opening on the tread surface in a no-load state where the tire is mounted on a standard rim and filled with a standard internal pressure. In the case of a configuration having a notch or chamfer at the groove opening, the groove width is taken as the value measured with the intersection of the extension line of the tread surface and the extension line of the groove wall in a cross-sectional view parallel to the groove width direction and the groove depth direction as the end point. The groove depth is measured as the maximum value of the distance from the tread surface to the groove bottom in a no-load state where the tire is mounted on a standard rim and filled with a standard internal pressure. When the target groove has partial irregularities or sipes at the groove bottom, the groove depth is taken as the value measured excluding the partial irregularities or sipes. The tread ends are the both ends of the tread pattern portion of the tire, and are also referred to as design ends. In this specification, unless otherwise specified, the shape, position, and length (distance) of each component are based on the shape, position, and length in the tire meridian cross-section. The standard rim refers to the "Applicable Rim" defined by JATMA, the "Design Rim" defined by TRA, or the "Measuring Rim" defined by ETRTO. Standard internal pressure refers to the "maximum air pressure" specified by JATMA, the maximum value listed in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" specified by TRA, or the "INFLATION PRESSURES" specified by ETRTO. Standard load refers to the "maximum load capacity" specified by JATMA, the maximum value listed in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" specified by TRA, or the "LOAD CAPACITY" specified by ETRTO.

[0012] (Tire configuration) One embodiment of the present invention will be described below with reference to the drawings. Figure 1 is an end view showing the meridional cross-sectional shape of the tire 10 according to this embodiment. In this figure, the tire portion is shown in a state where it is mounted on a regular rim and subjected to regular internal pressure, and in an unloaded state.

[0013] The tire 10 of this embodiment comprises, in a meridional cross-sectional view of the tire, a pair of bead portions 12, a pair of sidewall portions 14, a pair of shoulder portions 16, and a tread portion 18 arranged from the inside to the outside in the radial direction of the tire. The tire 10 comprises a carcass layer 20 and a reinforcing layer 22. The tire has an inner liner layer 23 arranged along the carcass layer 20 and exposed to the inner cavity of the tire.

[0014] Each pair of bead sections 12 has a bead core 24 and a bead filler 26. The bead core 24, although not shown, has a bead wire wound in an annular manner around the tire rotation axis. The bead wire is made of steel. The bead filler 26 is provided on the radially outer side of the bead core 24.

[0015] The carcass layer 20 has carcass plies (not shown) and, in a meridional cross-sectional view of the tire, extends from the tread portion 18 to the bead portions 12 on both outer sides in the tire width direction, and is wound back from the inside to the outside in the tire width direction so as to enclose a pair of bead cores 24 and bead fillers 26. The carcass plies are formed by covering multiple carcass cords made of steel or organic fiber material (e.g., aramid, nylon, polyester, rayon, etc.) with coating rubber and then rolling them.

[0016] The reinforcing layer 22 has a belt layer 28. The belt layer 28 has one or more belts 30. The belts 30 are formed by covering a belt cord made of steel or organic fiber material with a coating rubber and rolling it, and are positioned on the radially outer side of the carcass layer 20. The reinforcing layer 22 may further have a belt cover layer 32. The belt cover layer 32 has one or more belt covers 34a and edge covers 34b. The belt covers 34a and edge covers 34b are formed by covering a belt cover cord made of steel or organic fiber material with a coating rubber and rolling it, and are positioned on the radially outer side of the belt layer 28. The length between the outer edge of the belt layer 28 in the tire width direction and the tire equatorial plane CP is Dc. The belt cover 34a covers the entire width of the belt layer 28, and the edge cover 34b locally covers the end of the belt layer 28 in the tire width direction.

[0017] Each of the pair of sidewall sections 14 is provided with sidewall rubber 36. The sidewall rubber 36 is positioned on the outer side in the tire width direction of the carcass layer 20 and extends from the bead section 12 to the shoulder section 16.

[0018] The tread portion 18 has a tread surface 37 and is provided with tread rubber 38. The tread rubber 38 is located on the radially outer side of the carcass layer 20, belt layer 28, and belt cover layer 32, and extends from one shoulder portion 16 in the tire width direction, through the tire equatorial plane CP, to the other shoulder portion 16 in the tire width direction.

[0019] The tread rubber 38 is formed from a rubber material with excellent contact characteristics and weather resistance, and is exposed on the tread surface 37. Preferably, the tread rubber 38 contains silica, wax, and an anti-aging agent.

[0020] The wax can be a plant-derived wax, paraffin wax, microcrystalline wax, polyethylene wax, or a mixture thereof. In particular, to ensure crack resistance at low temperatures, a low-melting-point wax that can easily precipitate and spread on the groove bottom surface even at low temperatures is preferred, for example, a wax with a melting point of 40 to 65°C is preferably selected. The tread rubber preferably contains 1.0 part by mass or more of wax when the rubber component is 100 parts by mass.

[0021] The anti-aging agent is preferably an amine-based anti-aging agent. Examples of amine-based anti-aging agents include "N-phenyl-N'-1,3-dimethylbutyl-p-phenylenediamine" and "2,2,4-trimethyl-1,2-dihydroquinoline polymer".

[0022] The embrittlement temperature Tgt of the tread rubber 38 is preferably -20°C or lower. By having an embrittlement temperature Tgt of -20°C or lower, the tread portion 18 can maintain its flexibility even at low temperatures.

[0023] The tread surface 37 has multiple circumferential main grooves 40, four in the case of Figure 1. The tread surface 37 is divided into multiple, five rows of land areas 42 in the case of Figure 1 by the circumferential main grooves 40. The circumferential main grooves 40 are annular and continuous in the circumferential direction of the tire.

[0024] As shown in Figure 2, the circumferential main groove 40 has a groove bottom 44, a pair of groove walls 46, and a pair of edges 48. The groove bottom 44 is the bottom of the circumferential main groove 40 and defines the groove depth of the circumferential main groove 40. The groove bottom 44 has a surface that follows the tread surface 37, with the tire width direction as the short direction and the tire circumferential direction as the long direction. The pair of groove walls 46 are continuous with the outer edge of the groove bottom 44 in the tire width direction and define the width of the circumferential main groove 40. The groove width of the circumferential main groove 40 gradually decreases from the tread surface 37 toward the inside in the tire radial direction. The pair of groove walls 46 has a surface that intersects the tread surface 37, with the tire radial direction as the short direction and the tire circumferential direction as the long direction. The pair of edges 48 are the boundary between the pair of groove walls 46 and the tread surface 37. Let Gu be the minimum distance in the radial direction of the tire from the groove bottom 44 to the belt layer 28 (or the belt cover layer 32 if one is present).

[0025] The maximum thickness of the tread portion 18, i.e., the length between the tread surface 37 and the inner surface 50 of the tire in the land portion 42, is defined as Hmax (Figure 1). The minimum thickness of the tread portion 18, i.e., the length between the groove bottom 44 of the circumferential main groove 40 and the inner surface 50 of the tire, is defined as Hg (Figure 1).

[0026] The groove area ratio on the tread surface 37 is preferably 15% or more and 45% or less. The groove area ratio is a value (in %) defined by groove area / (contact area + groove area) × 100. Groove area refers to the opening area of ​​the grooves on the contact surface. Grooves include the circumferential main grooves of the tread but do not include sipes. If circumferential fine grooves and lug grooves are formed on the tread surface 37, these are included in the grooves. Contact area refers to the contact area between the tire and the contact surface. The groove area and contact area are measured at the contact surface between the tire 10 and the flat plate when the tire 10 is mounted on a regular rim, subjected to regular internal pressure, and placed perpendicular to a flat plate in a stationary state, with a load corresponding to a regular load (80% of the maximum load capacity) applied.

[0027] (Stress relaxation layer) The tire 10 is provided with a stress-relieving layer 52 on the surface of the groove bottom 44 of the circumferential main groove 40. The stress-relieving layer 52 has a bottom portion 54 provided on the groove bottom 44 of the circumferential main groove 40. The bottom portion 54 is formed over the entire groove bottom 44. The bottom portion 54 is annular and continuous in the circumferential direction of the tire along the circumferential main groove 40.

[0028] The stress-relieving layer 52 may further have a wall portion 56 and a surface portion 58. The wall portion 56 is provided on each of the pair of groove walls 46 of the circumferential main groove 40. One end 56U of the wall portion 56 on the inner side in the tire radial direction is connected to the bottom portion 54, and the other end 56T on the outer side in the tire radial direction may be located within the groove wall 46 or may reach the edge 48. The surface portion 58 is provided on the tread surface 37 along the edge 48. One end 58G of the surface portion 58 in the tire width direction is connected to the other end 56T of the wall portion 56 on the outer side in the tire radial direction via the edge 48, and the other end 58L in the tire width direction is located at a predetermined length, for example, 0.5 mm to 5 mm, in the tire width direction from the edge 48 toward the land portion 42.

[0029] The thickness Ga of the stress relaxation layer 52 is preferably 5 μm to 200 μm, and more preferably 10 μm to 100 μm. The thickness Ga of the stress relaxation layer 52 is the value measured at the center of the groove width of the circumferential main groove in the meridional cross-section of the tire. The thickness Ga of the stress relaxation layer 52 being within the above range allows it to easily follow the deformation of the tread portion 18.

[0030] The stress relaxation layer 52 mainly consists of diene-based rubber material and non-diene-based rubber material, and also contains carbon and a vulcanizing agent. The diene-based rubber is selected from the group consisting of diene polymers including natural rubber and synthetic diene-based rubber (isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butadiene-isoprene rubber (BIR), styrene-isoprene rubber (SIR), styrene-isoprene-butadiene rubber (SBIR), chloroprene rubber (CR), etc.). The non-diene-based rubber is selected from the group consisting of non-diene polymers including synthetic non-diene-based rubber (butyl rubber (IIR), ethylene-propylene rubber (EPDM, EPM), urethane rubber, silicone rubber, etc.). Furthermore, it is preferable that the stress relaxation layer 52 does not contain resin components in order to ensure weather resistance.

[0031] The stress relaxation layer 52 does not necessarily have to contain an anti-aging agent. Because the stress relaxation layer 52 has a thin thickness Ga, the anti-aging agent contained in the tread rubber is migrated to the stress relaxation layer 52 to compensate for this. The stress relaxation layer 52 can further suppress the occurrence of cracks by containing an anti-aging agent. If the stress relaxation layer 52 contains an anti-aging agent, it is preferable to use other anti-aging agents (e.g., phenolic, phosphite, organic thioacid, benzimidazole, etc.) in an amount of 0.1 parts by mass to 5 parts by mass per 100 parts by mass of the rubber component, rather than using amine-based anti-aging agents.

[0032] The embrittlement temperature Tgc of the stress relaxation layer 52 is -20°C or lower, preferably -30°C or lower. By keeping the embrittlement temperature Tgc below the above upper limit, the stress relaxation layer 52 can follow the deformation of the tread portion 18 even at low temperatures.

[0033] The difference between the embrittlement temperature Tgc of the stress relaxation layer 52 and the embrittlement temperature Tgt of the tread rubber is preferably 40°C or less, and more preferably 30°C or less. The difference between Tgc and Tgt (|Tgc-Tgt|) is an absolute value. By keeping the difference between Tgc and Tgt below the above upper limit, the stress difference between the stress relaxation layer 52 and the tread portion 18 can be reduced. The embrittlement temperature is a value measured in accordance with JIS K 6261-2.

[0034] The hardness Hsc of the stress relaxation layer 52 at -10°C is preferably 45 degrees or more and 70 degrees or less, and more preferably 50 degrees or more and 65 degrees or less. By having a low hardness of the stress relaxation layer 52 at low temperatures, the ability of the stress relaxation layer 52 to follow the deformation of the tread portion 18 can be improved.

[0035] The ratio (Hsc / Hst) of the hardness Hsc of the stress relaxation layer 52 at -10°C to the hardness Hst of the tread rubber 38 at -10°C is preferably 0.6 or more and 1.1 or less, and more preferably 0.7 or more and 1.0 or less. By reducing the difference in hardness between the stress relaxation layer 52 and the tread portion 18 at low temperatures, the ability of the stress relaxation layer 52 to follow the deformation of the tread portion 18 can be improved more reliably. The hardness Hsc and Hst are measured in accordance with JIS K6253 using a durometer type A under temperature conditions of -10°C ± 2°C. The rubber hardness Hst of the tread rubber 38 is measured as the rubber hardness of the rubber material that is in surface contact with the stress relaxation layer 52 among the rubber materials constituting the tread rubber 38.

[0036] The ratio (Dc / Dt) of the thermal conductivity Dc of the stress relaxation layer 52 to the thermal conductivity Dt of the tread rubber 38 is preferably 0.5 or more and 1.5 or less, and more preferably 0.7 or more and 1.3 or less. By having similar thermal conductivity values ​​for the stress relaxation layer 52 and the tread rubber 38, the temperatures of the stress relaxation layer 52 and the tread rubber 38 can change uniformly. The thermal conductivity values ​​were measured in accordance with JIS A 1412-2.

[0037] The ratio [EB(-10) / EB(20)] of the fracture elongation EB(-10) of the stress relaxation layer 52 at -10°C to the fracture elongation EB(20) at 20°C is preferably 0.5 or higher, and more preferably 0.7 or higher. When the ratio [EB(-10) / EB(20)] is above the above lower limit, the change in fracture elongation of the stress relaxation layer 52 between low temperature and room temperature is small. The fracture elongation is a value (unit: %) measured at room temperature (20°C) and low temperature (-10°C) in accordance with JIS K6251.

[0038] The ratio R of the maximum thickness Hmax of the tread portion 18 to the minimum thickness Hg of the tread portion 18 is defined as Hg / Hmax. Preferably, the above ratio R, the minimum distance Gu in the tire radial direction from the groove bottom 44 to the belt layer 28 (or the belt cover layer 32 if a belt cover layer 32 is present), and the thickness Ga of the stress relaxation layer 52 satisfy the following formula (1). By satisfying the following formula (1), the tire 10 can achieve both durability of the stress relaxation layer 52 and suppression of tire rolling resistance.

[0039]

number

[0040] When the maximum thickness Hmax is large and the minimum thickness Hg is small, i.e., when R is small, the amount of deformation of the tread rubber 38 at the bottom of the groove 44 is large, making it difficult to obtain durability of the stress relaxation layer 52, and the rolling resistance of the tire 10 tends to increase. On the other hand, when R is large, the amount of deformation at the bottom of the groove 44 is kept small, making it easier to obtain durability of the stress relaxation layer 52, and the rolling resistance of the tire 10 tends to decrease.

[0041] Furthermore, when the minimum distance Gu is large and the thickness Ga is small (Ga / Gu is small), the thickness Ga of the stress relaxation layer 52 becomes small relative to the thickness of the tread rubber 38 at the minimum distance Gu, i.e., the groove bottom 44, making it difficult to obtain durability for the stress relaxation layer 52. On the other hand, when Ga / Gu is large, the thickness Ga of the stress relaxation layer 52 becomes thicker relative to the thickness of the tread rubber 38 at the groove bottom 44, making it easier to obtain durability for the stress relaxation layer 52.

[0042] By being above the lower limit of equation (1) above, the stress relaxation layer 52 has sufficient thickness relative to the thickness of the tread rubber 38 at the groove bottom 44. Therefore, the tire 10 can achieve the effect of suppressing the occurrence of groove cracks. In addition, because the tread rubber 38 at the groove bottom 44 has sufficient thickness, the amount of deformation at the groove bottom 44 can be suppressed. Therefore, the tire 10 can suppress the increase in rolling resistance.

[0043] By keeping the value below the upper limit of equation (1) above, the stress relaxation layer 52 does not become too thick relative to the thickness of the tread rubber 38 at the groove bottom 44, and can follow the deformation of the tread portion 18. Therefore, the tire 10 can obtain the effect of suppressing the occurrence of groove cracks. In addition, by maintaining the appropriate thickness of the tread rubber 38 at the groove bottom 44, an unnecessary increase in tire mass can be suppressed. Therefore, the tire 10 can suppress an increase in rolling resistance.

[0044] It is preferable that the minimum distance Gu, the thickness Ga of the stress relaxation layer 52, the thermal conductivity Dc of the stress relaxation layer 52, and the thermal conductivity Dt of the tread rubber 38 satisfy the following formula (2).

[0045]

number

[0046] By staying within the above numerical range, the tire 10 can more reliably improve the ability of the stress-relieving layer 52 to follow the deformation of the tread portion 18.

[0047] It is preferable to adjust the thermal conductivity Dc of the stress relaxation layer 52 relative to the thermal conductivity Dt of the tread rubber 38, in accordance with the thickness Ga of the stress relaxation layer 52 relative to the thickness of the tread rubber 38 at the bottom of the groove 44. For example, when Ga / Gu is large, that is, when the thickness Ga of the stress relaxation layer 52 is large relative to the thickness of the tread rubber 38 (minimum distance Gu) at the bottom of the groove 44, it is preferable to increase the thermal conductivity Dc of the stress relaxation layer 52 relative to the thermal conductivity Dt of the tread rubber 38. As a result, while it takes time for the temperature of the stress relaxation layer 52 to reach the ambient temperature due to its larger thickness, by increasing the thermal conductivity Dc of the stress relaxation layer 52 relative to the thermal conductivity Dt of the tread rubber 38, the temperature of the tread rubber 38 and the stress relaxation layer 52 at the bottom of the groove 44 can change uniformly. This makes it less likely for the hardness difference between the stress relaxation layer 52 and the tread rubber 38 to become large. Therefore, the tire 10 can more reliably improve the ability of the stress relaxation layer 52 to follow the deformation of the tread portion 18.

[0048] Furthermore, when Ga / Gu is small, that is, when the thickness Ga of the stress relaxation layer 52 is small relative to the thickness of the tread rubber 38 (minimum distance Gu) at the bottom of the groove 44, the tread rubber 38 and the stress relaxation layer 52 can undergo a uniform temperature change at the bottom of the groove 44 by reducing the thermal conductivity Dc of the stress relaxation layer 52 relative to the thermal conductivity Dt of the tread rubber 38. In this case, the difference in hardness between the stress relaxation layer 52 and the tread rubber 38 is less likely to become large. Therefore, the tire 10 can more reliably improve the ability of the stress relaxation layer 52 to follow the deformation of the tread portion 18.

[0049] The stress relaxation layer 52 is preferably provided in the circumferential main groove 40 located within the maximum belt width region WB. The maximum belt width region WB is the region from both outer ends in the tire width direction to the inner end in the tire width direction of the belt 30 or belt cover 34, which is located on the outermost side in the tire radial direction within the reinforcing layer 22. The maximum belt width region WB has high rigidity and low strain during driving because the belt 30 or belt cover 34 is provided there. Therefore, by providing the stress relaxation layer 52 in the circumferential main groove 40 located within the maximum belt width region WB, strain can be reduced and durability can be improved. The stress relaxation layer 52 is preferably provided in the circumferential main groove 40 located in the region within the maximum belt width region WB where two or more belt covers 34 are provided.

[0050] Of the circumferential main grooves 40 provided with the stress relaxation layer 52, the circumferential main groove 40 located on the outermost side in the tire width direction is specifically called the outermost main groove 40S. The distance between the center of the groove width of the outermost main groove 40S and the tire equatorial plane CP is defined as Dg. The distance between the outer edge of the belt 30 or belt cover 34 located on the outermost side in the tire width direction of the reinforcing layer 22 and the tire equatorial plane CP is defined as Df. The ratio of Dg to Df (Dg / Df) is preferably 0.3 or more and 0.7 or less.

[0051] Generally, during the manufacturing process, when the green tire is inflated, when the tire 10 is mounted on the rim, and when the tire 10 is brought to the ground, the tread portion 18 experiences greater stress closer to the tread edge. As a result, the strain generated at the groove bottom 44 of the outermost main groove 40S, which is located near the tread edge, also increases. Therefore, by keeping the above ratio (Dg / Df) above the lower limit, the strain generated at the groove bottom 44 can be suppressed, thereby improving the durability of the stress relaxation layer 52. By keeping the above ratio (Dg / Df) below the upper limit, appropriate rigidity of the tread portion 18 can be obtained, resulting in excellent handling stability.

[0052] The tire 10 of this embodiment, as described above, is obtained through the usual manufacturing processes, namely the mixing process of tire materials, the processing process of tire materials, the molding process of green tires, the vulcanization process, and the inspection process after vulcanization. When manufacturing the tire 10 of this embodiment, a coating agent that forms a stress-relieving layer 52 is applied to the green tire before vulcanization, to a region including a predetermined position, i.e., the position where the circumferential main groove 40 is formed. The coating agent mainly consists of the above-mentioned diene-based rubber material and non-diene-based rubber material, and also contains carbon, a vulcanizing agent, and a vulcanization accelerator. Subsequently, by going through the vulcanization process, a tire 10 can be obtained in which a stress-relieving layer 52 is formed at least at the groove bottom 44 of the circumferential main groove 40. In the vulcanization process, a vulcanization mold is used, which has convex and concave portions corresponding to a predetermined tread pattern formed on its inner wall.

[0053] (Mechanism of Action and Effects) Generally, during the extrusion process in tire manufacturing, residual stress tends to occur at the groove bottom 44 of the circumferential main groove 40 when the tread portion 18 is extruded. This residual stress can cause groove cracks to occur depending on the usage conditions.

[0054] The tire 10 according to this embodiment can alleviate residual stress and suppress the occurrence of groove cracks by providing a stress-relieving layer 52 at the bottom of the groove 44. In particular, since the embrittlement temperature of the stress-relieving layer 52 according to this embodiment is -20°C or lower, the stress-relieving layer 52 can follow the deformation of the tread portion 18 even at low temperatures. Therefore, the tire 10 can improve the durability of the stress-relieving layer 52 at low temperatures and suppress the occurrence of groove cracks at low temperatures.

[0055] Since the embrittlement temperature Tgt of the tread rubber 38 is -20°C or lower, the flexibility of the tread rubber 38 is maintained at low temperatures, and the tire 10 can suppress the occurrence of groove cracks at low temperatures.

[0056] By keeping the difference between the embrittlement temperature Tgc of the stress relaxation layer 52 and the embrittlement temperature Tgt of the tread rubber 38 to 40°C or less, the stress difference between the stress relaxation layer 52 and the tread portion 18 is reduced. Therefore, the tire 10 can reduce the stress generated between the stress relaxation layer 52 and the tread portion 18, thereby suppressing the occurrence of groove cracks at low temperatures.

[0057] By having a hardness Hsc of the stress relaxation layer 52 at -10°C of 45 degrees or more and 70 degrees or less, the ability of the stress relaxation layer 52 to follow the deformation of the tread portion 18 can be improved. Furthermore, by having a ratio (Hsc / Hst) of the hardness Hsc of the stress relaxation layer 52 at -10°C and the hardness Hst of the tread rubber 38 at -10°C of 0.6 or more and 1.1 or less, the ability of the stress relaxation layer 52 to follow the deformation of the tread portion 18 can be improved more reliably.

[0058] The ratio (Dc / Dt) of the thermal conductivity Dc of the stress relaxation layer 52 to the thermal conductivity Dt of the tread rubber 38 is between 0.5 and 1.5. This allows the stress relaxation layer 52 and the tread rubber 38 to undergo a uniform temperature change, thus preventing a large difference in hardness between the stress relaxation layer 52 and the tread rubber 38. As a result, the ability of the stress relaxation layer 52 to follow the deformation of the tread portion 18 can be more reliably improved. Therefore, the tire 10 can suppress the occurrence of groove cracks at low temperatures.

[0059] Incidentally, when the ratio of thermal conductivity Dc to Dt (Dc / Dt) is large, when a tire stored in a normal temperature environment is exposed to a low temperature environment, the greater the difference in thermal conductivity, the greater the time difference between the stress relaxation layer 52 and the tread rubber 38 until they reach the ambient temperature. During this time, a difference in hardness occurs, making it difficult for the stress relaxation layer 52 to follow the deformation of the tread portion 18.

[0060] The ratio [EB(-10) / EB(20)] of the fracture elongation EB(-10) of the stress relaxation layer 52 at -10°C to the fracture elongation EB(20) at 20°C is 0.5 or greater. As a result, the change in fracture elongation of the stress relaxation layer 52 between low temperature and room temperature is small, thus reducing the stress generated between it and the tread portion 18 when a temperature difference occurs.

[0061] A thickness Ga of 5 μm or more in the stress relaxation layer 52 more reliably suppresses the occurrence of groove cracks at low temperatures. A thickness Ga of 200 μm or less in the stress relaxation layer 52 allows it to more reliably follow the deformation of the tread portion 18.

[0062] The tread rubber 38, by containing wax, can form a protective layer (not shown) on the groove bottom 44 surface via a stress-relieving layer 52. Since the protective layer protects the stress-relieving layer from ozone and ultraviolet rays, the tire 10 can more reliably suppress the occurrence of groove cracks caused by ozone and ultraviolet rays.

[0063] The tread rubber 38, by containing an amine-based antioxidant, can form a protective layer (not shown) on the groove bottom 44 surface via the stress relaxation layer 52. Since the protective layer protects the stress relaxation layer from ozone and ultraviolet rays, the tire 10 can more reliably suppress the occurrence of groove cracks caused by ozone and ultraviolet rays.

[0064] Excellent wet performance can be obtained by having a groove area ratio of 15% or more on the tread surface 37. By having a groove area ratio of 45% or less on the tread surface 37, groove cracks are less likely to occur at the groove bottom 44 of the circumferential main grooves 40, thereby improving the durability of the tire 10.

[0065] (modified version) Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the invention.

[0066] In the above embodiments, the stress relaxation layer was described as having a bottom portion, a wall portion, and a surface portion, but the present invention is not limited thereto. For example, the stress relaxation layer does not need to have a wall portion and a surface portion, as long as it has at least a bottom portion. If it has a wall portion, it may have only one of a pair of wall portions. One end of the wall portion that is on the radial side of the tire may reach the edge or may be located within the groove wall. If one end of the wall portion that is on the radial side of the tire may reach the edge, it may also have a surface portion.

[0067] The present invention has described the case where the embrittlement temperature Tgt of the tread rubber is -20°C or lower, but the present invention is not limited to this, and may be greater than -20°C.

[0068] The present invention has described the case where the difference between the embrittlement temperature Tgc of the stress relaxation layer and the embrittlement temperature Tgt of the tread rubber (|Tgc-Tgt|) is 40°C or less, but the present invention is not limited to this case, and may exceed 40°C.

[0069] The present invention has described the case where the hardness Hsc of the stress relaxation layer at -10°C is 45 degrees or more and 70 degrees or less, but the present invention is not limited to this, and may also be less than 45 degrees or greater than 70 degrees.

[0070] The present invention has described a case where the ratio (Hsc / Hst) of the hardness of the stress relaxation layer at -10°C to the hardness of the tread rubber at -10°C is 0.6 or more and 1.1 or less. However, the present invention is not limited to this case, and the ratio may be less than 0.6 or greater than 1.1.

[0071] The present invention has described the case where the ratio (Dc / Dt) of the thermal conductivity Dc of the stress relaxation layer to the thermal conductivity Dt of the tread rubber is 0.5 or more and 1.5 or less, but the present invention is not limited to this, and may also be less than 0.5 or greater than 1.5.

[0072] The present invention has described a case where the ratio [EB(-10) / EB(20)] between the fracture elongation EB(-10) at -10°C and the fracture elongation EB(20) at 20°C is 0.5 or greater, but the present invention is not limited to this case, and may be less than 0.5.

[0073] Although the case where the thickness of the stress relaxation layer is 5 μm or more and 200 μm or less has been described, the present invention is not limited to this, and may also be less than 5 μm or greater than 200 μm.

[0074] In the above embodiment, the case in which a stress-relieving layer is provided in all circumferential main grooves has been described, but the present invention is not limited to this, and includes the case in which a stress-relieving layer is provided in at least one circumferential main groove. [Examples]

[0075] This paper describes the results of evaluating the durability of the stress relaxation layer at low temperatures in a tire manufactured in accordance with the invention defined in the claims of this application.

[0076] (sample) A test tire with tire size 225 / 65R17 was mounted on a rim with rim size 17×6.5J, and this test tire was subjected to an internal pressure of 230 [kPa] and a normal load of 6.0 [kN]. The tire conditions for Examples 1 to 28 and Reference Example 1 are as shown in Table 1 below.

[0077] (Evaluation of crack resistance performance) After leaving tires in a room maintained at an ozone concentration of 100±5 pphm, a temperature of -10±2°C, and an internal pressure of 230±2 kPa for 24 hours, the number of cracks formed in the circumferential main grooves of the tires was measured. Based on these measurement results, an index was expressed with Reference Example 1 as the baseline (100). In this evaluation, a higher numerical value is preferable.

[0078] [Table 1]

[0079] Examples 1 to 28 showed improved crack resistance when "Tgc" was -20°C or lower compared to Reference Example 1. In contrast, Reference Example 1 did not show any improvement in crack resistance because "Tgc" was -15°C. Example 6 showed improved crack resistance when "Tgt" was -20°C or lower compared to Examples 1 to 5. Examples 7 to 9 showed improved crack resistance when "|Tgt-Tgc|" was 40°C or lower compared to Example 6. Examples 8 and 9 showed further improved crack resistance when "|Tgt-Tgc|" was 30°C. Examples 10 to 14 showed improved crack resistance when "Hsc" (hardness) was between 45°C and 70°C compared to Example 9. Furthermore, it was found that the crack resistance performance of Examples 12 to 14 was further improved when "Hsc" was between 50 and 65 degrees. Compared to Example 14, Examples 15 to 19 were found to have improved crack resistance when "Hsc / Hst" was between 0.6 and 1.1. Furthermore, it was found that the crack resistance performance of Examples 17 to 19 was further improved when "Hsc / Hst" was between 0.7 and 1.0. Compared to Example 19, Examples 20 to 24 were found to have improved crack resistance when "Dc / Dt" was between 0.5 and 1.5. Furthermore, it was found that the crack resistance performance of Examples 22 to 24 was further improved when "Dc / Dt" was between 0.7 and 1.3. Furthermore, it was found that in Examples 26 and 27, crack resistance was further improved by having "EB(-10) / EB(20)" of 0.7 or higher. In Example 28, it was found that crack resistance was improved by having "Ga" of 50 μm. [Explanation of Symbols]

[0080] 10 tires 12 Bead section 14 Sidewall section 16 Shoulder section 18 Tread section 20 Carcass Layers 22 Reinforcement layer 23 Inner liner layer 24 Bead Core 26 Bead Filler 28 Belt Layer 30 belts 32 Belt cover layer 34a Belt cover 34b Edge Cover 36 Sidewall rubber 37 Tread surface 38 Tread Rubber 40 Circumferential main groove 40S outermost main groove 42 Land 44 Groove bottom 46 Ditch wall 48 Edge 50 Tire inner surface 52 Stress relaxation layer 54 Bottom 56 Wall 56U one end 56T other end 58 Surface part 58G one end 58L other end CP Tire Equatorial Plane WB Maximum Belt Width Area

Claims

1. It has a tread surface and a tread portion provided with tread rubber, The tread surface has a plurality of circumferential main grooves and a plurality of land areas partitioned by the plurality of circumferential main grooves, At least one of the plurality of circumferential main grooves has a stress-relieving layer on the surface of the groove bottom, The stress-relieving layer mainly consists of diene-based rubber material and non-diene-based rubber material, and also contains carbon and a vulcanizing agent. A tire with a brittleness temperature Tgc of -20°C or lower.

2. The tire according to claim 1, wherein the embrittlement temperature Tgt of the tread rubber is -20°C or lower.

3. The tire according to claim 1 or 2, wherein the difference between the embrittlement temperature Tgc of the stress relaxation layer and the embrittlement temperature Tgt of the tread rubber (|Tgc - Tgt|) is 40°C or less.

4. The tire according to claim 1, wherein the hardness Hsc of the stress relaxation layer at -10°C is 45 degrees or more and 70 degrees or less.

5. The tire according to claim 1, wherein the ratio (Hsc / Hst) of the hardness Hsc of the stress relaxation layer at -10°C to the hardness Hst of the tread rubber at -10°C is 0.6 or more and 1.1 or less.

6. The tire according to claim 1, wherein the ratio (Dc / Dt) of the thermal conductivity Dc of the stress relaxation layer to the thermal conductivity Dt of the tread rubber is 0.5 or more and 1.5 or less.

7. The tire according to claim 1, wherein the ratio [EB(-10) / EB(20)] of the stress relaxation layer's elongation at break EB(-10) at -10°C to the elongation at break EB(20) at 20°C is 0.5 or more.

8. The tire according to claim 1, wherein the thickness of the stress relaxation layer is 5 μm or more and 200 μm or less.

Images