tire

Abstract

To provide a tire with improved durability of the stress relaxation layer at around 50°C. [Solution] 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, and the tread rubber (38) contains rubber having voids (60).

Description

[Technical Field]

[0001] This invention relates to tires. [Background technology]

[0002] To suppress groove cracks that occur at the bottom of the main grooves, tires are known that have a stress-relieving layer at the bottom of the main grooves. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-147115 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, in the tire disclosed in Patent Document 1, when stress is applied to the bottom of the groove during large steering maneuvers (of the vehicle to which the tire is attached), the stress relaxation effect is insufficient, and consequently, peeling stress may occur between the stress relaxation layer and the tread rubber. In such cases, the stress relaxation layer may peel off from the bottom or wall of the groove, and consequently, the occurrence of groove cracks may not be suppressed.

[0005] Furthermore, in recent years, there has been a demand for tires to suppress the occurrence of groove cracks not only at temperatures around 50°C but also at temperatures around -10°C.

[0006] The present invention aims to provide a tire with improved durability of the stress relaxation layer at least around 50°C (more preferably around -10°C). [Means for solving the problem]

[0007] The tire of the present invention is characterized in that, on the tread surface of the tread rubber, the land area is demarcated by main grooves, a stress-relieving layer is formed on the surface of at least one groove bottom of the main grooves, the stress-relieving layer mainly consists of a diene-based rubber material and a non-diene-based rubber material and also contains carbon and a vulcanizing agent, and the tread rubber contains rubber having voids. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a tire in which the durability of the stress relaxation layer is improved at least around 50°C. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is an end view showing the meridional cross-sectional shape of the tire according to this embodiment. [Figure 2] Figure 2 is an enlarged view showing the region enclosed by the tire width direction range II and tire radial direction range II in Figure 1. [Figure 3] Figure 3 shows an example of the dotted line region X in Figure 2. [Figure 4] Figure 4 shows an example of the dotted line region X in Figure 2. [Figure 5] Figure 5 shows an example of the dotted line region X in Figure 2. [Figure 6] Figure 6 shows an example of the dotted line region X in Figure 2. [Figure 7] Figure 7 shows an example of the dotted line region X in Figure 2. [Figure 8] Figure 8 shows an example of the dotted line region X in Figure 2. [Figure 9] Figure 9 shows an example of the dotted line region X in Figure 2. [Modes for carrying out the invention]

[0010] <Mode of the present invention> The present invention encompasses the following embodiments. [Form 1] A tire in which, on the tread surface of the tread rubber, land portions are demarcated by main grooves, and a stress relaxation layer is formed on the surface of at least one groove bottom of the main grooves. The stress relaxation layer is mainly composed of a diene rubber material and a non-diene rubber material and contains carbon and a vulcanizing agent. The tire is characterized in that the tread rubber contains a rubber having voids. [Form 2] In a cross-sectional view of an interface forming a boundary between the tread rubber and the stress relaxation layer, at least a part of the interface protrudes toward the tread rubber side, or the voids are present near the boundary. The tire according to Form 1. [Form 3] When a linear reference length in the cross-sectional width direction is defined as Lc, and a length considering the protruding state of the interface toward the tread rubber side, or a length considering the presence state of the voids at the interface, is defined as Lp, the ratio Lp / Lc is 1.03 or more and 1.50 or less. The tire according to Form 2. [[ID=!]] [Form 4] The average diameter dave of the voids is 10 μm or more and 150 μm or less. The tire according to any one of Forms 1 to 3. [Form 5] In a cross-sectional view of an interface between the tread rubber and the stress relaxation layer, the minimum value hmin of the tire radial dimension of the stress relaxation layer is 5 μm or more and 100 μm or less. The tire according to any one of Forms 1 to 4. [[ID=2!]] [Form 6] The voids are formed by the tread rubber and a porous body, and the bulk specific gravity of the porous body is 0.05 g / cm 3 or more and 0.20 g / cm 3 or less. The tire according to any one of Claims 1 to 5. [Form 7] The embrittlement temperature Tgt of the tread rubber is -45°C or lower. The tire according to any one of Forms 1 to!. [Form 8] A tire according to any one of embodiments 1 to 7, 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 20°C or less. [Form 9] A tire according to any one of embodiments 1 to 8, wherein the difference (|Hsc-Hst|) between the hardness Hsc of the stress relaxation layer at -10°C and the hardness Hst of the tread rubber at -10°C is 10 or less.

[0011] <Definition> The radial direction of a tire refers to the direction perpendicular to the tire's axis of rotation. The inner side of the tire's radial direction refers to the side facing the tire's axis of rotation, while the outer side of the tire's radial direction refers to the side moving away from the tire's axis of rotation. The circumferential direction of a tire refers to the direction around the tire's axis of rotation. The tire width direction refers to the direction parallel to the tire's axis of rotation. The inner side in the tire width direction refers to the side facing the tire's equator, while the outer side in the tire width direction refers to the side moving away from the tire's equator. The tire equatorial plane is a plane that is perpendicular to the tire's axis of rotation and passes through the center of the tire's width. The main groove is a groove with a wear indicator on its groove wall that shows the end of wear, such as a circumferential groove, and generally has a groove width of 3.0 mm or more and a groove depth of 5.0 mm or more. However, the groove width and groove depth of the circumferential main groove are not limited to the above ranges. The groove width is the maximum distance between opposing groove walls at the groove opening on the tread surface when the tire is mounted on a standard rim and filled to the standard internal pressure in an unloaded state (the distance measured in a direction perpendicular to the direction in which the groove extends). If there is a notch or chamfer at the groove opening, the groove width is the value measured with the endpoint being 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 groove depth direction. The groove depth is the maximum distance from the tread surface to the bottom of the groove (measured in the radial direction of the tire) when the tire is mounted on a standard rim, filled to the standard internal pressure, and under no load. If the groove in question has partial unevenness or sipes at the bottom of the groove, the groove depth shall be the value measured excluding the unevenness or sipes. The tread edge refers to the ends of the tread pattern on a tire, and is also called the design end. In this specification, unless otherwise specified, the shape, position, and length (distance) of each component are based on the shape, position, and length of the tire meridional section (in an unloaded state with the tire mounted on a standard rim and filled to standard internal pressure). A "regular rim" refers to an "applicable rim" as defined by JATMA, a "Design Rim" as defined by TRA, or a "Measuring Rim" as 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] <Basic tire configurations> 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 according to this embodiment. Note that the figure shows the tire portion in an unloaded state, mounted on a regular rim and subjected to regular internal pressure.

[0013] The tire 10 of this embodiment, when viewed in a meridional cross-section of the tire, comprises 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 an inner liner 19, a carcass 20, a belt 22 (consisting of two belt layers 22a and 22b), a belt cover 24, sidewall rubber 36, and tread rubber 38, just like a typical tire.

[0014] The tread rubber 38 has a tread surface 37 that is exposed at the outermost radial end of the tire, is made of a rubber material with excellent contact characteristics and weather resistance, and has multiple voids (not shown in Figure 1 but shown in Figure 2 later). Preferably, the tread rubber 38 contains silica, wax, and an anti-aging agent.

[0015] As for the wax, plant-derived waxes, paraffin wax, microcrystalline wax, polyethylene wax, and mixtures thereof can be selected. 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, and for example, a wax with a melting point of 40 to 65°C is preferably selected. The tread rubber 38 preferably contains 1.0 part by mass or more of wax when the rubber component is 100 parts by mass.

[0016] The anti-aging agent is preferably an amine-based anti-aging agent. The amine-based anti-aging agent is selected from, for example, "N-phenyl-N'-1,3-dimethylbutyl-p-phenylenediamine" and "2,2,4-trimethyl-1,2-dihydroquinoline polymer". The tread rubber 38 preferably contains 0.5 parts by mass or more of the anti-aging agent when the rubber component is 100 parts by mass.

[0017] The tread surface 37 is provided with multiple (four in Figure 1) circumferential main grooves 40 (forming a continuous annular shape in the circumferential direction of the tire). These circumferential main grooves 40 divide the tread surface 37 into multiple (five rows in Figure 1) land areas 42.

[0018] Figure 2 is an enlarged view showing the region enclosed by the tire width direction range II and the tire radial direction range II in Figure 1. As shown in Figure 2, the land area demarcated by the circumferential main groove 40 has a groove bottom 44 and a pair of groove walls 46. The groove bottom 44 is the bottom of the circumferential main groove 40 and serves as the reference for the groove depth of the circumferential main groove 40. The groove bottom 44 is composed of a surface that follows the tread surface 37, with the groove width direction (tire width direction in the example shown in Figure 2) as the short side and the direction perpendicular to both the groove width direction and the tire radial direction (tire circumferential direction in the example shown in Figure 2) as the long side. The pair of groove walls 46 are continuous with the outer edge of the groove bottom 44 in the tire width direction and serve as the reference for the groove width of the circumferential main groove 40. The pair of groove walls 46 are composed of a surface that intersects the tread surface 37, with the direction inclined in the groove width direction (tire width direction) relative to the tire radial direction as the short side and the tire circumferential direction as the long side.

[0019] [Characteristics of the basic form of a tire] (Tread rubber) As described above, the tread rubber 38 is made of a rubber material with excellent contact characteristics and weather resistance, and has a plurality of voids 60 as shown in Figure 2. The voids 60 can be formed as simple cavities within the tread rubber 38, and can be formed by components that disappear as bubbles during vulcanization (i.e., by including a foaming agent in the tread rubber in a green tire, such as dinitrosopentamethylenetetramine (DPT) or azodicarbonamide (ADCA)).

[0020] With respect to such voids 60, a shell (e.g., an acrylonitrile copolymer) can be present at the interface between the solid portion and the cavity portion. In green tires, a shell can be formed by vulcanizing the tread rubber with, for example, heat-expandable hollow capsules. Here, the shell refers to a thin film that is harder than the tread rubber and forms the boundary between the solid portion and the cavity portion.

[0021] Alternatively, the void 60 can be formed by including a porous material other than rubber within the tread rubber 38, and as the porous material, for example, crystalline cellulose aggregates can be selected.

[0022] (Stress relaxation layer) Next, the tire 10 is provided with a stress-relieving layer 52 on the surface of at least the groove bottom 44 of the land portion demarcated by the circumferential main groove 40. The stress-relieving layer 52 includes a bottom portion 54 provided on the groove bottom 44. The bottom portion 54 is formed over the entire groove bottom 44. The bottom portion 54 forms a continuous annular shape in the circumferential direction of the tire.

[0023] The stress-relaxing layer 52 may include a wall portion 56 and a surface portion 58 in addition to the bottom portion 54 described above. The wall portion 56 is provided on each of the pair of groove walls 46. 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, starting from 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 at 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, on the opposite side in the groove width direction from the edge 48 from the one end 56U.

[0024] 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.

[0025] The stress relaxation layer 52 does not necessarily have to contain an anti-aging agent. This is because, since the stress relaxation layer 52 has a small thickness Ga, the anti-aging agent contained in the adjacent tread rubber 38 migrates to the stress relaxation layer 52 and compensates 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.

[0026] <Functions and Effects of the Basic Form of Tires> As described above, in the tire of this embodiment, a stress-relieving layer 52 is formed at least on the groove bottom 44 of the land portion demarcated by the main groove 40, and the tread rubber 38 has voids 60. With this configuration, a certain degree of crack resistance can be ensured by forming the stress-relieving layer 52 on the groove bottom 44. Based on this, because the tread rubber 38 contains voids 60, when a vehicle equipped with the tire of this embodiment is turned sharply and stress is applied to the groove bottom 44, the voids 60 relieve the stress applied to the tread rubber 38, so that the stress-relieving layer 52 follows the tread rubber 38 well, and as a result the occurrence of peeling stress between the stress-relieving layer 52 and the tread rubber 38 can be reduced to a high degree. As a result, peeling of the stress-relieving layer 52 from the groove bottom 44 and groove wall 46 is suppressed, and consequently the occurrence of groove cracks is suppressed, thereby improving the peel resistance of the stress-relieving layer 52.

[0027] <Preferred tire configuration> As shown in Figure 2, the stress relaxation layer 52 may extend to the tread surface 37, but from the viewpoint of ensuring grip with the road surface, it is preferable that the stress relaxation layer 52 is not exposed to the tread surface 37.

[0028] Typically, circumferential main grooves (grooves indicated by reference numeral 40 in Figure 1) are often formed on the tread surface 37 in a range of two to five grooves. Therefore, it is sufficient for the stress relaxation layer 52 to be formed in at least one of these grooves, but considering the occurrence of cracks at the groove bottom 44, it is preferable that it be formed in all the main grooves formed on the tread surface 37 (all four circumferential main grooves 40 as shown in Figure 1).

[0029] The preferred region for forming the stress relaxation layer 52 is the area between 0% and 30% from the inner side of the tire radially in the main groove depth. Therefore, in the case of the stress relaxation layer 52 shown in Figure 2, it is naturally preferable that the bottom portion 54 be formed, but it is also preferable that at least a part of the wall portion 56 be formed.

[0030] In the tires shown in Figures 1 and 2, it is preferable that, in a cross-sectional view of the interface forming the boundary between the tread rubber 38 and the stress relaxation layer 52 (Figures 3 to 9 below), at least a portion of the interface protrudes towards the tread rubber 38 side, or that a void exists near the boundary.

[0031] In other words, this preferred configuration is one in which, in a cross-sectional view of the interface, the interface between the tread rubber 38 and the stress relaxation layer 52 does not extend in a straight line. With this configuration, the length of the interface in the cross-sectional view can be made larger (than when the interface is straight), thereby suppressing the peeling of the stress relaxation layer 52 from the tread rubber 38.

[0032] Figures 3, 4, and 5 show examples of the dotted line region X in Figure 2. More specifically, Figure 3 shows an example where the void is not covered by a shell, Figure 3(A) shows an example where there is no void at the interface, and Figure 3(B) shows an example where there is a void at the interface. Figure 4 shows an example where the void is covered by a shell, Figure 4(A) shows an example where there is no shelled void at the interface, and Figure 4(B) shows an example where there is a shelled void at the interface. Figure 5 shows an example where the void is formed by a porous material, Figure 5(A) shows an example where there is no porous material at the interface, and Figure 5(B) shows an example where there is a porous material at the interface.

[0033] In the example shown in Figure 3(A), the stress-relieving layer 52 is coated onto the tread rubber 38 after tire vulcanization. This results in a linear interface between the tread rubber 38 and the stress-relieving layer 52 in cross-sectional view. In contrast, in the example shown in Figure 3(B), the stress-relieving layer material is coated onto the tread rubber material before vulcanization, and then vulcanization is performed.

[0034] Due to these differences in manufacturing methods, in the example shown in Figure 3(B), unlike the example shown in Figure 3(A), a portion of the stress relaxation layer 52 protrudes from the tread rubber 38 side at the interface, resulting in a non-linear interface profile line. As a result, in the example shown in Figure 3(B), the interface profile line is longer compared to the example shown in Figure 3(A), and consequently, the contact area between the tread rubber 38 and the stress relaxation layer 52 is increased compared to the linear case shown in Figure 3(A).

[0035] Similarly, in the example shown in Figure 4(A), the stress-relieving layer 52 is coated onto the tread rubber 38 after tire vulcanization. This results in a linear interface between the tread rubber 38 and the stress-relieving layer 52 in cross-sectional view. In contrast, in the example shown in Figure 4(B), the stress-relieving layer material is coated onto the tread rubber material (rubber material with a shell that encloses the voids mixed in beforehand) before vulcanization, and then vulcanization is performed.

[0036] Due to these differences in manufacturing methods, in the example shown in Figure 4(B), unlike the example shown in Figure 4(A), a void portion 60b with a shell remains near the interface between the tread rubber 38 and the stress relaxation layer 52, resulting in a non-linear interface profile line. As a result, in the example shown in Figure 4(B), the interface profile line is longer compared to the example shown in Figure 4(A), and consequently, the contact area between the tread rubber 38 and the stress relaxation layer 52 is increased compared to the linear case shown in Figure 4(A).

[0037] Similarly, in the example shown in Figure 5(A), the stress-relieving layer 52 is coated onto the tread rubber 38 after tire vulcanization. This results in a linear interface between the tread rubber 38 and the stress-relieving layer 52 in cross-sectional view. In contrast, in the example shown in Figure 5(B), the stress-relieving layer material is coated onto the tread rubber material (a rubber material with a porous body containing air voids mixed in beforehand) before vulcanization, and then vulcanization is performed.

[0038] Due to these differences in manufacturing methods, in the example shown in Figure 5(B), unlike the example shown in Figure 5(A), voids 60c contained in the porous material remain near the interface between the tread rubber 38 and the stress relaxation layer 52, resulting in a non-linear interface profile line. As a result, in the example shown in Figure 5(B), the interface profile line is longer compared to the example shown in Figure 5(A), and consequently, the contact area between the tread rubber 38 and the stress relaxation layer 52 is increased compared to the linear case shown in Figure 5(A).

[0039] As explained above, in the examples shown in Figures 3(A), 4(A), and 5(A), the interface forming the boundary between the tread rubber 38 and the stress relaxation layer 52 is linear, whereas in the examples shown in Figures 3(B), 4(B), and 5(B), the interface forming the boundary between the tread rubber 38 and the stress relaxation layer 52 is non-linear. More specifically, in the example shown in Figure 3(B), at least a part of the interface protrudes towards the tread rubber 38 side, in the example shown in Figure 4(B), a void 60b (having a shell) exists near the boundary, and in the example shown in Figure 5(B), a void 60c formed by a porous material exists near the boundary.

[0040] Therefore, in the examples shown in Figures 3(B), 4(B), and 5(B), compared to the examples shown in Figures 3(A), 4(A), and 5(A), the length of the interface can be increased in the cross-sectional view of the interface, that is, the contact area between the tread rubber 38 and the stress relaxation layer 52 can be increased. This further suppresses the peeling of the stress relaxation layer 52 from the tread rubber 38, and consequently further suppresses the occurrence of groove cracks, thereby further improving the peel resistance of the stress relaxation layer 52.

[0041] Figure 6 shows an example of the dotted line region X in Figure 2, and more specifically, it shows the reference length Lc and the actual interface length Lp in the example shown in Figure 4(B).

[0042] In the tires shown in Figures 1 and 2 (particularly the tire shown in Figure 4(B)), when Lc is defined as the linear reference length in the cross-sectional width direction, and Lp is defined as the length considering the protrusion of the interface toward the tread rubber side, or the length considering the existence of the void at the interface (actual interface length), it is preferable that the ratio Lp / Lc is 1.03 or more and 1.50 or less.

[0043] Here, in Figure 6, the actual interface length Lp is set to the tread rubber side 38 (lower side of the paper) rather than the stress relaxation layer 52 side (upper side of the paper) around the void portion 60b (Figure 4(B)) containing the shell. However, the present invention is not limited to this, and the stress relaxation layer 52 side (upper side of the paper) may also be used. Furthermore, around the void portion 60c (Figure 5(B)) containing the porous body, the tread rubber side 38 (lower side of the paper) is used rather than the stress relaxation layer 52 side (upper side of the paper).

[0044] Here, the reference length Lc and the actual interface length Lp are both values ​​measured using an optical microscope. Furthermore, the region of the tread rubber 38 and stress relaxation layer 52 measured using an optical microscope is defined as the region where the groove width center of the circumferential main groove 40 shown in Figure 2 is at the center position in the longitudinal direction (direction of the reference length Lc) shown in Figure 6, and there are no particular restrictions on its longitudinal range (lateral dimension in Figure 6).

[0045] By setting the ratio Lp / Lc to 1.03 or higher, the contact area between the tread rubber 38 and the stress relaxation layer 52 can be further increased, thereby further improving the peel resistance of the stress relaxation layer 52 to the tread rubber 38.

[0046] In contrast, by setting the ratio Lp / Lc to 1.50 or less, the thickness of the stress relaxation layer 52 does not become locally smaller in many areas as shown in the cross-sectional view in Figure 6, and the peel resistance of the stress relaxation layer 52 to the tread rubber 38 can be further improved.

[0047] Furthermore, the ratio Lp / Lc is more preferably 1.04 or more and 1.45 or less, and most preferably 1.05 or more and 1.40 or less.

[0048] Furthermore, while the above preferred configurations regarding the ratio Lp / Lc have mainly been described for cases where the void portion 60b has a shell, as shown in Figure 4(B), such preferred configurations can also be applied to cases where the void portion 60a does not have a shell, as shown in Figure 3(B), or when a porous body is included, as shown in Figure 5(B).

[0049] Figure 7 shows an example of the dotted line region X in Figure 2, and more specifically, it shows the radius dn (where n is a natural number greater than or equal to 1) of each void in the example shown in Figure 4(B).

[0050] In the tires shown in Figures 1 and 2 (particularly the tire shown in Figure 4(B)), it is preferable that the average diameter dave of the void portion 60b is 10 μm or more and 150 μm or less.

[0051] Here, the average diameter dave is calculated as follows. First, the area of ​​each void 60b shown in Figure 7 is measured using an optical microscope (for example, with a measurement magnification of 500x; the same applies hereafter). Then, the radius of a circle equivalent to the area of ​​each void is calculated, and the provisional radii of each void are set as d1, d2, d3, ..., dn. The average value of these radii is then calculated to obtain the average diameter dave.

[0052] Furthermore, the regions of the tread rubber 38 and stress relaxation layer 52 measured by optical microscope are defined as the region where the groove width center of the circumferential main groove 40 shown in Figure 2 is located at the longitudinal center position shown in Figure 7, and there are no particular restrictions on the longitudinal range.

[0053] By setting the average diameter dave to 10 μm or more, when stress is applied to the groove bottom, the void portion 60b further relieves the stress applied to the tread rubber 38, allowing the stress relief layer 52 to follow the tread rubber 38 more favorably. In addition, setting the average diameter dave to 10 μm or more allows for an even larger contact area between the tread rubber 38 and the stress relief layer 52. Therefore, these effects combined can further improve the peel resistance of the stress relief layer 52 to the tread rubber 38.

[0054] In contrast, by setting the average diameter dave to 150 μm or less, the thickness of the stress relaxation layer 52 does not become locally smaller in the cross-sectional view shown in Figure 7, and the peel resistance of the stress relaxation layer 52 to the tread rubber 38 can be further improved.

[0055] Furthermore, the average diameter dave is more preferably 15 μm to 125 μm, and more preferably 30 μm to 100 μm.

[0056] Furthermore, although the above preferred example regarding the average diameter dave was described for the case where the void portion 60b has a shell, as shown in Figure 4(B), such preferred examples can also be applied to cases where the void portion 60a does not have a shell, as shown in Figure 3(B).

[0057] Figure 8 shows an example of the dotted line region X in Figure 2, and more specifically, it shows the minimum value hmin of the tire radial dimension of the stress relaxation layer 52 in the example shown in Figure 4(B).

[0058] In the tires shown in Figures 1 and 2 (particularly the tire shown in Figure 4(B)), it is preferable that the minimum value hmin of the radial dimension of the stress relaxation layer 52 in the cross-sectional view of the interface between the tread rubber 38 and the stress relaxation layer 52 is 5 μm or more and 100 μm or less.

[0059] Here, the minimum value hmin is the value measured using an optical microscope. Furthermore, the region of the tread rubber 38 and stress relaxation layer 52 measured using an optical microscope is the region where the groove width center of the circumferential main groove 40 shown in Figure 2 is at the longitudinal center position shown in Figure 8, and there are no particular restrictions on the longitudinal range.

[0060] By setting the minimum value hmin to 5 μm or more, the peel resistance of the stress relaxation layer 52 to the tread rubber 38 can be further improved in areas where the thickness of the stress relaxation layer 52 is locally reduced.

[0061] In contrast, by setting the minimum hmin value to 100 μm or less, it is possible to avoid excessively large tire weight, thereby suppressing tire rolling resistance and ultimately improving fuel efficiency.

[0062] Furthermore, the minimum hmin value is more preferably 10 μm or more and 95 μm or less, and most preferably 15 μm or more and 90 μm or less.

[0063] Furthermore, although the above preferred embodiment for the minimum value hmin was described when the void portion 60b has a shell, as shown in Figure 4(B), such preferred embodiments can also be applied when the void portion 60c is formed of a porous material, as shown in Figure 5(B).

[0064] Figure 9 is a diagram showing an example of the dotted line region X in Figure 2, and more specifically, it is a diagram illustrating the effects of two types of voids (i.e., voids 60c1 formed by the porous material present in the tread rubber 38, and voids 60c2 formed by the porous material near the boundary between the tread rubber 38 and the stress relaxation layer 52) in the example shown in Figure 5(B) (an example using a porous material).

[0065] In the tire shown in Figure 5(B), the voids are formed as gaps between the tread rubber and the porous material, and the bulk density of the porous material is 0.05 g / cm³. 3 More than 0.20g / cm 3 The following is preferable:

[0066] Here, the porous body includes a solid portion and openings communicating with the outer surface of the porous body, and in some cases also includes closed pores as internal voids. In this embodiment, the bulk density of the porous body refers to the mass per unit volume including all of the solid portion, openings, and closed pores. In other words, the bulk density of the porous body refers to the density obtained by dividing the mass by the volume determined from the external dimensions.

[0067] The bulk density of the porous material is 0.05 g / cm³. 3By setting it as above, for both the void portion 60c1 and the void portion 60c2, the rigidity of the tread rubber 38 can be increased, and thus the handling stability can be improved.

[0068] On the other hand, by setting the bulk specific gravity of the porous body to 0.20 g / cm 3 or less, the volume of the void portion contained in the porous body can be increased. Therefore, when paying attention to the porous body that constitutes the void portion 60C2 (in particular, a part of the void portion is not filled with rubber) among the void portion 60C1 and the void portion 60C2 shown in FIG. 9, the contact area between this porous body and the rubber that constitutes the stress relaxation layer 52 can be made larger, and thus the peeling of the stress relaxation layer 52 from the tread rubber 38 can be further suppressed, and the durability of the stress relaxation layer 52 can be further enhanced.

[0069] Incidentally, the bulk specific gravity of the porous body is preferably 0.06 g / cm 3 or more and 0.18 g / cm 3 or less, and more preferably 0.07 g / cm 3 or more and 0.15 g / cm 3 or less, and extremely preferably so.

[0070] [[ID=2)] In the tire shown in FIGS. 1 and 2, it is preferable that the embrittlement temperature Tgt of the tread rubber 38 is -45°C or lower. Here, the embrittlement temperature is a value measured in accordance with JIS K 6261-2.

[0071] Since the embrittlement temperature Tgt of the tread rubber 38 is -45°C or lower, the tread portion 18 can maintain flexibility even at low temperatures. Incidentally, the embrittlement temperature Tgt of the tread rubber 38 is more preferably -50°C or lower, and extremely preferably -55°C or lower.

[0072] In the tires shown in Figures 1 and 2, it is preferable that the difference between the embrittlement temperature Tgc of the stress relaxation layer 52 and the embrittlement temperature Tgt of the tread rubber 38 (|Tgc-Tgt|) is 20°C or less. By keeping the difference in embrittlement temperatures (|Tgc-Tgt|) at 20°C or less, the difference in elastic modulus (specifically, the modulus at 100% elongation) between the stress relaxation layer 52 and the tread rubber 38 at low temperatures can be reduced. This further improves the peel resistance of the stress relaxation layer 52 relative to the tread rubber 38.

[0073] Furthermore, the difference in embrittlement temperatures (|Tgc-Tgt|) is more preferably 15°C or less, and most preferably 10°C or less.

[0074] In the tires shown in Figures 1 and 2, it is preferable that the difference (|Hsc-Hst|) between the hardness Hsc of the stress relaxation layer 52 at -10°C and the hardness Hst of the tread rubber at -10°C is 10 or less.

[0075] Here, the hardness values ​​Hsc and Hst are both measured in accordance with JIS K6253 using a Type A durometer at a temperature of -10°C ± 2°C. The hardness Hsc 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 that make up the tread rubber 38.

[0076] By keeping the hardness difference (|Hsc-Hst|) at low temperatures to 10 or less, the stress relaxation layer 52 can conform and deform appropriately when the tread rubber 38 deforms, further improving the peel resistance of the stress relaxation layer 52 relative to the tread rubber 38.

[0077] Furthermore, the hardness difference (|Hsc-Hst|) at low temperatures is more preferably 8 or less, and most preferably 6 or less.

[0078] Furthermore, by setting the hardness Hst of the tread rubber at -10°C to between 50 and 62, the flexibility of the tread rubber 38 is further enhanced at low temperatures, resulting in an even higher level of crack resistance at low temperatures.

[0079] <Other suitable examples of tires> In the tires shown in Figures 1 and 2, 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 radial direction of the tire from the groove bottom 44 to the belt cover 24 (or to the belt 22 if there is no belt cover 24), 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.

[0080]

number

[0081] 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.

[0082] 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.

[0083] 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 obtain 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.

[0084] 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.

[0085] In the tires shown in Figures 1 and 2, 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 layer 22b or belt cover 24, which is formed as a reinforcing layer and located on the outermost side in the tire radial direction. The maximum belt width region WB has high rigidity and low strain during driving because the belt 22 or belt cover 24 is provided therein. 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 where the belt cover 24 is provided within the maximum belt width region WB.

[0086] In the tires shown in Figures 1 and 2, among the circumferential main grooves 40 provided with a 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 denoted as Dg. The distance between the outer edge of the belt layer 22b or belt cover 24 located on the outermost side in the tire radial direction and the tire equatorial plane CP is denoted as Df. The ratio of Dg to Df (Dg / Df) is preferably 0.3 or more and 0.7 or less.

[0087] 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.

[0088] In the tires shown in Figures 1 and 2, 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 specified load (80% of the maximum load capacity) applied.

[0089] <Tire manufacturing method> 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 the green tire, the vulcanization process, and the inspection process after vulcanization. When manufacturing the tire 10 of this embodiment, a foaming agent containing, for example, dinitrosopentamethylenetetramine (DPT) or azodicarbonamide (ADCA) is added to the tread rubber of the green tire before vulcanization so that bubbles are formed and voids are created during vulcanization. In addition, a coating agent for the stress relaxation layer 52 is applied to the green tire before vulcanization in a predetermined location, namely the region including the location 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 at least carbon and a vulcanizing agent. Subsequently, by going through the vulcanization process, a tire 10 can be obtained in which a stress relaxation layer 52 is formed at least at the groove bottom 44 of the circumferential main groove 40 and in which voids are present in the tread rubber. In the vulcanization process, a vulcanization mold is used, in which protrusions and recesses corresponding to a predetermined tread pattern are formed on the inner wall.

[0090] In particular, as shown in Figures 3(B), 4(B), and 5(B), when the interface between the tread rubber 38 and the stress relaxation layer 52 is non-linear, in the molding process of the green tire, the stress relaxation layer material is coated onto the tread rubber material, which includes an uncovered void portion 60a, a void portion 60b covered by a void portion 60c formed by a porous material, and then vulcanization is performed. [Examples]

[0091] The following describes the results of evaluating the durability (GC resistance) of the stress relaxation layer of each test tire, which was prepared as shown below, at test temperatures of 50°C and -10°C.

[0092] (Preparation of test tires) Each test tire (the conventional example and Invention Examples 1 to 9 described later) was prepared with a tire size of 225 / 65R17, having the tread pattern shown in Figure 1, with a stress relaxation layer formed on the surface of the groove bottom of all circumferential main grooves, these stress relaxation layers mainly composed of butadiene rubber and containing carbon and a vulcanizing agent, and the tread rubber containing rubber with voids. Each test tire was then mounted on a rim with a rim size of 17 × 6.5J, and an internal pressure of 230 [kPa] and a specified load of 6.0 [kN] were applied to evaluate the crack resistance at 50°C and at 10°C, respectively.

[0093] The conditions for the tires in the conventional example and Invention Examples 1 to 9 are as shown in Table 1 below. The terms in Table 1 are all the same as the terms explained in this embodiment, and their descriptions have been partially simplified. In Table 1, regarding "whether the interface is linear or non-linear in cross-sectional view," if the elements constituting the void are shell-less voids, a non-linear interface means the example shown in Figure 3(B). Also, if the elements constituting the void are porous materials, a non-linear interface means the example shown in Figure 5(B).

[0094] (Evaluation of crack resistance at 50°C) Each test tire was left for 24 hours in a room maintained at an ozone concentration of 100±5 pphm, a temperature of 50±2°C, and an internal pressure of 230±2 kPa. The number of cracks that formed in the circumferential main grooves was then measured. Based on this measurement, an index evaluation was performed using a conventional example as the baseline (100). In this evaluation (see Table 1), a higher numerical value indicates higher crack resistance at 50°C.

[0095] (Evaluation of crack resistance at -10°C) Each test tire was left for 24 hours 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. The number of cracks that formed in the circumferential main grooves was then measured. Based on this measurement, an index evaluation was performed using a conventional example as the baseline (100). In this evaluation (see Table 1), a higher value indicates higher crack resistance at around -10°C.

[0096] [Table 1]

[0097] As shown in Table 1, the tires of Invention Examples 1 to 9, which fall within the technical scope of the present invention (a stress relaxation layer mainly composed of a diene-based rubber material and containing carbon and a vulcanizing agent, and a tread rubber containing rubber with voids), exhibit superior GC resistance compared to conventional tires that do not fall within the technical scope of the present invention.

[0098] Furthermore, as shown in the table, the tires of Invention Examples 7 to 8, which fall within the preferred technical scope of the present invention (where the tread rubber embrittlement temperature Tgt is -45°C or lower), exhibit superior low-temperature GC resistance compared to the tires of Invention Examples 1 to 6, which do not fall within the preferred technical scope of the present invention. [Explanation of Symbols]

[0099] 10 tires 12 Bead section 14 Sidewall section 16 Shoulder section 18 Tread section 19 Inner liner 20 Carcass 22 belts 22a, 22b Belt Layer 24 Belt 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 52 Stress relaxation layer 54 Bottom 56 Wall 56U one end 56T other end 58 Surface part 58G one end 58L other end 60 Void CP Tire Equatorial Plane Df is the distance between the outer edge of the belt layer 22b or belt cover 24 located on the outermost side in the tire's width direction and the tire's equatorial plane CP. Dg: The distance between the center of the groove width of the outermost main groove 40S and the tire's equatorial plane CP. Ga stress relaxation layer thickness Minimum tire radial distance from groove bottom 44 to belt cover 24 (or belt 22 if there is no belt cover) Hg tread section 18 minimum thickness Hmax tread section maximum thickness 18 WB Maximum Belt Width Area Area indicated by the dotted line X

Claims

1. A tire in which the tread surface of the tread rubber is divided into land areas by main grooves, and a stress-relieving layer is formed on the surface of at least one groove bottom of the main groove, The stress relaxation layer mainly consists of a diene-based rubber material and a non-diene-based rubber material, and also contains carbon and a vulcanizing agent. A tire characterized in that the tread rubber includes rubber having voids.

2. In a cross-sectional view of the interface forming the boundary between the tread rubber and the stress relaxation layer, At least a portion of the interface protrudes toward the tread rubber side, or The void exists near the boundary. The tire according to claim 1.

3. The tire according to claim 2, wherein Lc is a linear reference length in the cross-sectional width direction, and Lp is a length considering the protrusion state of the interface toward the tread rubber side, or a length considering the presence of the void portion at the interface, the ratio Lp / Lc is 1.03 or more and 1.50 or less.

4. The tire according to any one of claims 1 to 3, wherein the average diameter dave of the void portion is 10 μm or more and 150 μm or less.

5. In a cross-sectional view of the interface between the tread rubber and the stress relaxation layer, The tire according to claim 1 or 2, wherein the minimum value hmin of the radial dimension of the stress relaxation layer is 5 μm or more and 100 μm or less.

6. The aforementioned void is formed by tread rubber and a porous material, and the bulk density of the porous material is 0.05 g / cm³. ３ 0.20g / cm or more ３ The tire according to claim 1 or 2, which is as follows:

7. The tire according to claim 1 or 2, wherein the tread rubber has a brittleness temperature Tgt of -45°C or lower.

8. 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 20°C or less.

9. The tire according to claim 1 or 2, wherein the difference (|Hsc - Hst|) between the hardness Hsc of the stress relaxation layer at -10°C and the hardness Hst of the tread rubber at -10°C is 10 or less.

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