JP7725864B2 - tire - Google Patents

Description

The present invention relates to a tire.

Black ice (a frozen road surface with a thin layer of ice on a snow-free surface) can be difficult for drivers to see, making it prone to tire slippage. For this reason, it is necessary to be able to corner on black ice without slipping, and to achieve good responsiveness and handling stability.

Patent Document 1 describes a winter tire that offers excellent performance on ice and low fuel consumption.

Japanese Patent Application Laid-Open No. 2017-203111

Increasing the heat generation in the tread area could be considered to achieve good responsiveness or handling stability on frozen roads, but there is a concern that the heat generated by friction between the tread surface and the icy road surface could melt the ice, creating a water film between the tread surface and the road surface, which could actually lead to slipping.

The present invention aims to provide a tire with improved overall performance in terms of fuel economy, grip performance on ice, and handling stability on ice.

After extensive research, the inventors discovered that the above-mentioned problems could be solved by providing three or more rubber layers in the tread portion and blending two or more rubber components into the first rubber layer that makes up the tread surface, and thus completed the present invention.

That is, the present invention provides:
[1] A tire having a tread portion including at least a first layer constituting a tread surface, a second layer adjacent to the radially inner side of the first layer, and a third layer adjacent to the radially inner side of the second layer, wherein the tread portion has a plurality of circumferential grooves extending continuously in the tire circumferential direction, the first layer being constituted by a rubber composition containing a rubber component including at least two kinds selected from the group consisting of isoprene-based rubber, styrene-butadiene rubber, and butadiene rubber, the second layer being constituted by a rubber composition containing a rubber component including at least one kind selected from the group consisting of isoprene-based rubber, styrene-butadiene rubber, and butadiene rubber, and the third layer being constituted by a rubber composition containing a rubber component including at least one kind selected from the group consisting of isoprene-based rubber, styrene-butadiene rubber, and butadiene rubber,
[2] The tire according to the above [1], wherein the rubber component constituting the second layer contains 10 to 90% by mass of an isoprene-based rubber and 10 to 90% by mass of a butadiene rubber.
[3] The tire according to [1] or [2] above, wherein the rubber component constituting the third layer includes at least one rubber selected from the group consisting of an isoprene-based rubber and a butadiene rubber.
[4] The tire according to any one of [1] to [3] above, wherein the rubber composition constituting the first layer contains 60 parts by mass or more of a reinforcing filler per 100 parts by mass of the rubber component.
[5] The tire according to any one of [1] to [4] above, wherein the amount of sulfur in the rubber composition constituting the second layer is less than the amount of sulfur in the rubber composition constituting the third layer.
[6] The tire according to any one of [1] to [5] above, wherein the tan δ at 30 ° C of the rubber composition constituting the second layer and the tan δ at 30 ° C of the rubber composition constituting the third layer are smaller than the tan δ at 30 ° C of the rubber composition constituting the first layer.
[7] The tire according to any one of [1] to [6] above, wherein the total styrene content of the rubber component constituting the second layer is less than 25% by mass.
[8] The tire according to any one of [1] to [7] above, wherein the rubber composition constituting the first layer contains at least one selected from the group consisting of a terpene-based resin and a cyclopentadiene-based resin.
[9] The tire according to any one of [1] to [8] above, wherein the rubber composition constituting the first layer contains modified liquid butadiene rubber.
[10] The tire according to any one of [1] to [9] above, wherein the rubber composition constituting the first layer has a Shore hardness (Hs) of 40 to 60, measured in accordance with JIS K 6253-3:2012 using a durometer type A at a temperature of 23°C.
[11] The tire according to any one of [1] to [10] above, wherein the modulus of the first layer at 100% stretch is greater than the modulus of the second layer at 100% stretch.
[12] The tire according to any one of [1] to [11] above, wherein the deepest part of the groove bottom of any one of the circumferential grooves is formed to be located radially inward of the outermost part of the second layer in the land portion adjacent to that circumferential groove.

The present invention provides a tire with improved overall performance in terms of fuel economy, grip performance on ice, and handling stability on ice.

1 is an enlarged cross-sectional view showing a portion of a tread portion of a tire according to an embodiment of the present disclosure.

A tire according to one embodiment of the present disclosure has a tread portion including at least a first layer constituting a tread surface, a second layer adjacent to and radially inward of the first layer, and a third layer adjacent to and radially inward of the second layer, wherein the tread portion has a plurality of circumferential grooves extending continuously in the tire circumferential direction, the first layer being constituted by a rubber composition containing a rubber component including at least two rubbers selected from the group consisting of an isoprene-based rubber, a styrene-butadiene rubber, and a butadiene rubber, the second layer being constituted by a rubber composition containing a rubber component including at least one rubber selected from the group consisting of an isoprene-based rubber, a styrene-butadiene rubber, and a butadiene rubber, and the third layer being constituted by a rubber composition containing a rubber component including at least one rubber selected from the group consisting of an isoprene-based rubber, a styrene-butadiene rubber, and a butadiene rubber.

By providing three or more rubber layers in the tread and compounding two or more rubber components in the first rubber layer that makes up the tread surface, the resulting tire has improved overall performance in terms of fuel economy, grip performance on ice, and handling stability on ice. While not intending to be bound by theory, the reasons for this are thought to be as follows.

The tire of the present disclosure (1) has three or more rubber layers in the tread, forming multiple rubber layer interfaces in the tread. This is believed to enable energy loss to occur due to friction caused by micromolecular motion of the rubber phases at the interfaces when shear deformation occurs in the tread. Furthermore, (2) the first rubber layer contains at least two different rubber components, forming a sea-island structure, allowing the island phases to function as spikes that partially bite into the icy road surface. This suppresses the generation of a water film on the tread surface while adsorbing to the road surface, thereby enabling energy loss to occur at the tread interface, which is believed to improve grip performance and handling stability on ice. At the same time, shear deformation of the tread surface is small during rolling, which is believed to suppress heat generation during normal driving. These are believed to achieve the remarkable effect of improving overall fuel economy, grip performance on ice, and handling stability on ice.

The rubber component constituting the second layer preferably contains 10 to 90% by mass of isoprene-based rubber and 10 to 90% by mass of butadiene rubber. The total styrene content of the rubber component constituting the second layer is preferably less than 25% by mass. Furthermore, the rubber component constituting the third layer preferably contains at least one rubber selected from the group consisting of isoprene-based rubber and butadiene rubber.

By using the rubber component constituting the second layer as described above, the second layer also has an island-sea structure, which makes the interface between the first and second layers more likely to be discontinuous, making it easier for friction to occur at the interface and also making it easier to transmit forces caused by handling at the joints made by the common polymer. Furthermore, by preferably including either an isoprene-based rubber or a butadiene rubber, the interface between the second and third layers is more likely to be discontinuously bonded with the island-sea structure of the second layer, generating heat at the interface and also making it easier for forces caused by handling to be transmitted at the joints made by the common polymer, which is thought to result in good responsiveness on ice.

The rubber composition constituting the first layer preferably contains 60 parts by mass or more of a reinforcing filler per 100 parts by mass of the rubber component.

By including 60 parts by mass or more of reinforcing filler in the rubber composition constituting the first layer, it is believed that the reinforcing filler is more easily dispersed within the island phases in the rubber matrix of the first layer, making it easier to form hard domains.

It is preferable that the tan δ at 30°C of the rubber composition constituting the second layer and the tan δ at 30°C of the rubber composition constituting the third layer are smaller than the tan δ at 30°C of the rubber composition constituting the first layer.

By reducing the heat generation properties of the second and third layers, heat generation within the tread during rolling is suppressed, which is thought to help prevent ice on the road surface from liquefying due to an increase in the tread surface temperature.

The sulfur content of the rubber composition constituting the second layer is preferably less than the sulfur content of the rubber composition constituting the third layer.

By reducing the amount of sulfur in the rubber composition that makes up the second layer compared to the amount of sulfur in the rubber composition that makes up the third layer, it is believed that the pressure applied from inside the tire during vulcanization makes it easier for sulfur to migrate from the third layer to the second layer, resulting in good bonding between the rubber layers and making it easier to generate reaction force.

The rubber composition constituting the first layer preferably contains at least one resin selected from the group consisting of terpene-based resins and cyclopentadiene-based resins.

By blending a terpene resin and/or a cyclopentadiene resin into the rubber composition, the water repellency of the tread rubber can be improved, and it is believed that using this rubber composition in the tread portion of a tire can improve wet grip performance.

The rubber composition constituting the first layer preferably contains modified liquid butadiene rubber.

The modified liquid butadiene rubber interacts with the filler and selectively enters the butadiene portion of the styrene butadiene rubber or the butadiene rubber phase (BR phase). This forms fine, hard domains of filler with the liquid butadiene rubber on the surface within the system, which is thought to make it easier to achieve a gripping effect on the road surface.

It is preferable that the ratio of tan δ at -30°C (-30°C tan δ) to the complex modulus (MPa) at 0°C (0°C E*) of the rubber composition constituting the first layer (-30°C tan δ/0°C E*) is 0.070 or greater.

The rubber composition constituting the first layer preferably has a Shore hardness (Hs) of 40 to 60, measured in accordance with JIS K 6253-3:2012 using a durometer type A at a temperature of 23°C.

By setting the Shore hardness (Hs) of the rubber composition constituting the first layer within the above range, it is believed that good macro-conformity can be achieved.

It is preferable that the modulus of the first layer at 100% stretch is greater than the modulus of the second layer at 100% stretch.

By making the modulus of the first layer at 100% stretch greater than the modulus of the second layer at 100% stretch, it is thought that deformation occurring in the first layer is more easily transmitted to the second layer, making it easier for internal energy loss to occur during shear deformation.

The tire of the present disclosure is preferably formed so that the deepest portion of the groove bottom of any one of the circumferential grooves is located radially inward of the outermost portion of the second layer in the land portion adjacent to that circumferential groove.

By forming the deepest part of the circumferential groove bottom to be located radially inward of the outermost part of the second layer within the land portion adjacent to that circumferential groove, it is believed that shear deformation is more likely to occur in the second layer, making it easier to reduce energy loss between layers.

<Definition>
A "genuine rim" is a rim that is determined for each tire by the standard system that includes the standard on which the tire is based, and is called a "standard rim" in the case of JATMA, a "design rim" in the case of TRA, and a "measuring rim" in the case of ETRTO.

"Normal internal pressure" is the air pressure specified for each tire by each standard in the standard system, including the standard on which the tire is based. For JATMA, it is the "maximum air pressure," for TRA, it is the maximum value listed in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES," and for ETRTO, it is the "INFLATION PRESSURE."

"Normal condition" refers to a condition in which a tire is mounted on a normal rim, inflated to the normal internal pressure, and unloaded. Unless otherwise specified, all tire dimensions (such as tire cross-sectional width Wt) in this specification are measured in this normal condition.

"Normal load" is the load specified for each tire by each standard in the standard system, including the standard on which the tire is based. For JATMA, it is "maximum load capacity," for TRA, it is the maximum value listed in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES," and for ETRTO, it is "LOAD CAPACITY."

"Tread edge" refers to the outermost contact point in the tire width direction when a tire in normal condition is loaded with a normal load and in contact with a flat surface with a camber angle of 0 degrees.

"Land portion" refers to the area in the tread portion that is bounded by the tread contact edge and multiple circumferential grooves that extend continuously around the tire. For example, if there are two circumferential grooves, the land portion is divided into a pair of shoulder land portions and a center land portion sandwiched between them. If there are three circumferential grooves, the center land portion is further divided into a land portion that faces the inside of the vehicle when mounted on the vehicle and a land portion that faces the outside of the vehicle.

The "groove depth of the circumferential groove" is determined by the distance between the tread surface and the extension of the deepest part of the groove bottom of the circumferential groove 1. Note that, for example, when there are multiple circumferential grooves 1, the groove depth H is the distance between the tread surface 3 and the extension of the deepest part of the groove bottom of the circumferential groove 1 that has the deepest groove depth among the multiple circumferential grooves 1.

The "total styrene amount in the rubber component" refers to the total content (mass%) of styrene moieties contained in 100% by mass of the rubber component, and is calculated by Σ(styrene content (mass%) of each styrene-containing rubber × content (mass%) of each styrene-containing rubber in the rubber component/100). For example, if the rubber component consists of 30% by mass of a first SBR (styrene content 25% by mass), 60% by mass of a second SBR (styrene content 27.5% by mass), and 10% by mass of BR, the total styrene amount (S) in 100% by mass of the rubber component is 24.0% by mass (= 25 × 30/100 + 27.5 × 60/100). The styrene amount of the styrene-containing rubber is calculated by ¹ H-NMR measurement.

"Oil content" includes the amount of oil contained in oil-extended rubber.

<Measurement method>
The "sulfur content" is the sulfur content (mass%) measured by an oxygen combustion flask method in accordance with JIS K 6233:2016. The sample for measuring the sulfur content is a vulcanized rubber composition having a length of 20 mm, a width of 4 mm, and a thickness of 1 mm. When preparing the sample by cutting it out from a tire, the sample is cut out from the tread portion of the tire so that the long side is in the tire circumferential direction and the thickness direction is in the tire radial direction.

"30°C tan δ" is the loss tangent measured under the following conditions: temperature 30°C, initial strain 5%, dynamic strain 1%, and frequency 10 Hz. The sample used for measuring the loss tangent is a vulcanized rubber composition measuring 20 mm in length, 4 mm in width, and 1 mm in thickness. When cutting out from a tire, the sample is cut out from the tread portion of the tire with the long side aligned circumferentially and the thickness aligned radially.

"-30°C tan δ" is the loss tangent measured under the following conditions: temperature -30°C, initial strain 10%, dynamic strain 0.25%, and frequency 10 Hz. The sample for this measurement is prepared in the same manner as for 30°C tan δ.

"0°C E*" is the complex modulus (MPa) measured under the following conditions: temperature 0°C, initial strain 10%, dynamic strain 0.25%, and frequency 10 Hz. Samples for measuring the complex modulus are prepared in the same manner as for 30°C tan δ.

"Shore hardness" refers to the Shore hardness (Hs) measured in accordance with JIS K 6253-3:2012 using a durometer type A at a temperature of 23°C. Samples for measuring Shore hardness are cut out from the tread portion so that the tire's radial direction is the thickness direction. Measurements are also performed by pressing a measuring tool against the sample from the contact surface side of the hardness measurement sample.

"Modulus at 100% stretch" is the tensile stress at 100% elongation in the grain direction (the rolling direction when forming a rubber sheet by extrusion or shearing) measured in accordance with JIS K 6251:2017 at a tensile speed of 3.3 mm/sec in an atmosphere of 23°C. The sample used to measure the modulus at 100% stretch is a 1 mm thick dumbbell-shaped No. 7 vulcanized rubber test piece. When cutting out from a tire, the sample is cut out from the tread portion of the tire so that the tensile direction is in the tire circumferential direction and the tire radial direction is in the thickness direction.

The "styrene content" is a value calculated by ^1H -NMR measurement, and is applied to, for example, rubber components having repeating units derived from styrene, such as SBR. The "vinyl content (amount of 1,2-bonded butadiene units)" is a value calculated by infrared absorption spectroscopy in accordance with JIS K 6239-2:2017, and is applied to, for example, rubber components having repeating units derived from butadiene, such as SBR and BR. The "cis content (amount of cis-1,4-bonded butadiene units)" is a value calculated by infrared absorption spectroscopy in accordance with JIS K 6239-2:2017, and is applied to, for example, rubber components having repeating units derived from butadiene, such as BR.

The "weight average molecular weight (Mw)" can be determined by converting the measured value into standard polystyrene using gel permeation chromatography (GPC) (e.g., a GPC-8000 series manufactured by Tosoh Corporation, a differential refractometer as the detector, and a TSKGEL SUPERMALTIPORE HZ-M column manufactured by Tosoh Corporation). This applies to, for example, SBR, BR, etc.

"N ₂ SA of carbon black" is measured in accordance with JIS K 6217-2 "Fundamental properties of carbon black for rubber - Part 2: Determination of specific surface area - Nitrogen adsorption method - Single point method.""N ₂ SA of silica" is measured by the BET method in accordance with ASTM D3037-93. "Average primary particle size of silica" can be determined by observing with a transmission or scanning electron microscope, measuring 400 or more primary silica particles observed within the field of view, and averaging the results. "Average particle size of rubber powder" is the average particle size by mass calculated from the particle size distribution measured in accordance with JIS Z 8815:1994.

The "softening point of the resin component" is the temperature at which the ball drops when the softening point specified in JIS K 6220-1:2001 is measured using a ring and ball softening point tester.

The manufacturing procedure for a tire, which is one embodiment of the present disclosure, is described in detail below. However, the following description is an example for explaining the present disclosure and is not intended to limit the technical scope of the present invention to the described range. Note that in this specification, when a numerical range is indicated using "to", it is intended to include both ends of the range.

<Tires>
1 is an enlarged cross-sectional view showing a portion of a tire tread portion , in which the vertical direction is the tire radial direction, the left-right direction is the tire width direction, and the direction perpendicular to the paper surface is the tire circumferential direction.

The tread portion of the present disclosure has multiple circumferential grooves 1 extending continuously in the tire circumferential direction. Having two or more circumferential grooves 1 divides the land portion 2 into at least one pair of shoulder land portions and a center land portion sandwiched between them. Having three or more circumferential grooves 1 further divides the center land portion into a land portion that faces the inside of the vehicle when mounted on the vehicle and a land portion that faces the outside of the vehicle. This allows the tread patterns of each land portion to be different, which is preferable as it increases the degree of freedom in designing the tread pattern. When there are three or more circumferential grooves, the pair of circumferential grooves located at the outermost sides in the tire width direction are referred to as the outermost grooves. The number of circumferential grooves may be four or more, or may be five or more. The circumferential grooves 1 may extend linearly in the circumferential direction, or may extend in a wavy, sinusoidal, or zigzag pattern.

As shown in the figure, the tread portion of a tire of the present disclosure comprises a first rubber layer 6, a second rubber layer 7, and a third rubber layer 8 (hereinafter, sometimes simply referred to as the "first layer 6," "second layer 7," and "third layer 8"). The outer surface of the first layer 6 constitutes the tread surface 3, the second layer 7 is adjacent to the radially inner side of the first layer 6, and the third layer 8 is adjacent to the radially inner side of the second layer 7. The first layer 6 typically corresponds to a cap tread. The third layer 8 typically corresponds to a base tread or undertread. The second layer 7 does not have a specific shape, and may therefore be either a base tread or an undertread. Furthermore, one or more rubber layers may be present between the third layer 8 and the belt layer, as long as the objectives of the present disclosure are achieved.

In Figure 1, the double arrow t1 indicates the maximum thickness of the first layer 6, the double arrow t2 indicates the maximum thickness of the second layer 7, and the double arrow t3 indicates the maximum thickness of the third layer 8. In Figure 1, any point on the tread surface where no grooves are formed is indicated by the symbol P. The line indicated by the symbol N is a line (normal) that passes through point P and is perpendicular to the tangent plane at point P. In this specification, thicknesses t1, t2, and t3 are measured along the normal N drawn from point P on the tread surface at a location where no grooves are present in the cross section of Figure 1.

In the present disclosure, the maximum thickness t1 of the first layer 6 is not particularly limited, but from the viewpoint of wet grip performance, it is preferably 1.0 mm or more, more preferably 1.5 mm or more, and even more preferably 2.0 mm or more. On the other hand, from the viewpoint of heat buildup, the maximum thickness t1 of the first layer 6 is preferably 6.0 mm or less, more preferably 5.5 mm or less, and even more preferably 5.0 mm or less.

In the present disclosure, the maximum thickness t2 of the second layer 7 is not particularly limited, but is preferably 1.0 mm or more, more preferably 2.0 mm or more, and even more preferably 3.0 mm or more. Furthermore, the maximum thickness t2 of the second layer 7 is preferably 10.0 mm or less, more preferably 9.0 mm or less, and even more preferably 8.0 mm or less.

In the present disclosure, the maximum thickness t3 of the third layer 8 is not particularly limited, but is preferably 1.0 mm or more, more preferably 1.5 mm or more, and even more preferably 2.0 mm or more. Furthermore, the maximum thickness t3 of the third layer 8 is preferably 10.0 mm or less, more preferably 9.0 mm or less, and even more preferably 8.0 mm or less.

From the standpoint of the effects of the present disclosure, the ratio of the thickness of the second layer to the total thickness of the first layer 6, second layer 7, and third layer 8 (t2/(t1+t2+t3)) is preferably 0.30 or greater, more preferably 0.40 or greater, and even more preferably 0.45 or greater.

It is preferable that the thickness of the first layer 6 is thinner than the thickness of the second layer 7.

The tread portion of the present disclosure has a plurality of circumferential grooves 1 extending continuously in the tire circumferential direction. The circumferential grooves 1 extend linearly along the circumferential direction, but are not limited to this form, and may extend in a wave-like, sinusoidal, or zigzag shape along the circumferential direction, for example.

The tread portion of the present disclosure has land portions 2 separated by circumferential grooves 1 in the tire width direction.

The groove depth H1 of the circumferential groove 1 is determined by the distance between the extension line 4 of the land portion 2 and the extension line 5 of the deepest part of the groove bottom of the circumferential groove 1. Note that, for example, when there are multiple circumferential grooves 1, the groove depth H1 can be the distance between the extension line 4 of the land portion 2 and the extension line 5 of the deepest part of the groove bottom of the circumferential groove 1 with the deepest groove depth among the multiple circumferential grooves 1 (the circumferential groove 1 on the left side in Figure 1).

In the present disclosure, the deepest portion of the groove bottom of the circumferential groove 1 having the deepest groove depth among the multiple circumferential grooves 1 (the left circumferential groove 1 in Figure 1) is formed so as to be located radially inward of the outermost portion of the second layer 7 in the land portion 2 adjacent to that circumferential groove. In other words, the extension line 5 of the deepest portion of the groove bottom of the circumferential groove 1 having the deepest groove depth among the multiple circumferential grooves 1 (the left circumferential groove 1 in Figure 1) is located radially inward of the extension line 9 of the outermost portion of the second layer 7 in the land portion 2 adjacent to that circumferential groove.

In this disclosure, immediately below (radially inward in the tire direction) the circumferential groove 1 having the deepest groove depth among the multiple circumferential grooves 1 (the left circumferential groove 1 in Figure 1), there is a recess that is recessed radially inward in the tire direction relative to the outermost part of the second layer 7 within the land portion 2 adjacent to that circumferential groove, and a portion of the first layer 6 is formed to a predetermined thickness within this recess of the second layer 7. By forming the first layer 6 and second layer 7 in this manner, the complex modulus of the first layer 6 that remains around the circumferential groove 1 after tire wear is higher than the complex modulus of the second layer 7, which is thought to more easily function as an edge during cornering and improve snow traction, etc.

In the present disclosure, the deepest portion of the groove bottom of the circumferential groove 1 having the deepest groove depth among the multiple circumferential grooves 1 (the left circumferential groove 1 in Figure 1) is formed so as to be located radially outward of the outermost portion of the third layer 8 in the land portion 2 adjacent to that circumferential groove. In other words, the extension line 5 of the deepest portion of the groove bottom of the circumferential groove 1 having the deepest groove depth among the multiple circumferential grooves 1 (the left circumferential groove 1 in Figure 1) is located radially outward of the extension line of the outermost portion of the third layer 8 in the land portion 2 adjacent to that circumferential groove.

In the present disclosure, it is preferable that the widthwise length of at least one land portion 2 has a portion where it increases from the outer side toward the inner side in the tire radial direction, and it is even more preferable that the widthwise length of the land portion 2 gradually increases from the outer side toward the inner side in the tire radial direction. This configuration allows the contact area to be increased as the tire wears during driving, thereby maintaining wet handling stability and ice grip performance until the end of driving. Note that, although the groove walls of the circumferential grooves 1 in the present disclosure extend linearly from the outer side toward the inner side in the tire radial direction, this configuration is not limited to this and may extend, for example, in a curved or stepped manner.

From the viewpoint of handling response on ice, the sulfur content of the rubber composition constituting the first layer 6 is preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, and particularly preferably 0.8 or more. The sulfur content of the rubber composition constituting the second layer is preferably 0.6 or more, more preferably 0.7 or more, even more preferably 0.8 or more, and particularly preferably 0.9 or more. The sulfur content of the rubber composition constituting the third layer 8 is preferably 1.0 or more, more preferably 1.3 or more, even more preferably 1.6 or more, and particularly preferably 1.8 or more. On the other hand, from the viewpoint of abrasion resistance, the sulfur content of the rubber compositions constituting the first layer 6, second layer 7, and third layer 8 is preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.0 or less. In the present disclosure, the sulfur content of the rubber composition constituting the second layer 7 is preferably lower than the sulfur content of the rubber composition constituting the third layer 8. By reducing the amount of sulfur in the rubber composition that makes up the second layer 7 to a level lower than the amount of sulfur in the rubber composition that makes up the third layer 8, it is believed that good vulcanization adhesion between the second layer 7 and the third layer 8 can be ensured.

From the viewpoint of grip performance on ice, the 30°C tan δ of the rubber composition constituting the first layer 6 (hereinafter referred to as the 30°C tan δ of the first layer 6) is preferably 0.11 or greater, more preferably 0.15 or greater, even more preferably 0.18 or greater, and particularly preferably 0.20 or greater. The 30°C tan δ of the second layer 7 is preferably 0.10 or greater, more preferably 0.12 or greater, and even more preferably 0.14 or greater. The 30°C tan δ of the third layer 8 is preferably 0.10 or greater, more preferably 0.12 or greater, and even more preferably 0.14 or greater. On the other hand, from the viewpoint of fuel economy, the 30°C tan δ of the first layer 6, second layer 7, and third layer 8 is preferably 0.40 or less, more preferably 0.35 or less, and even more preferably 0.30 or less. In the present disclosure, the 30°C tan δ of the first layer 6 is preferably greater than the 30°C tan δ of the second layer 7 and the 30°C tan δ of the third layer 8. By making the 30°C tan δ of the first layer 6 greater than the 30°C tan δ of the second layer 7 and the 30°C tan δ of the third layer 8, it is believed that heat generation in the first layer during driving is increased, resulting in good wet grip performance. The ratio of the 30°C tan δ of the first layer 6 to the 30°C tan δ of the second layer 7 is preferably 1.0 or greater, more preferably 1.1 or greater, even more preferably 1.2 or greater, and particularly preferably 1.3 or greater. There is no upper limit to the ratio of the 30°C tan δ of the first layer 6 to the 30°C tan δ of the second layer 7, but it is preferably 2.5 or less, more preferably 2.2 or less, even more preferably 2.0 or less, and particularly preferably 1.8 or less.

From the viewpoint of the effects of the present disclosure, the ratio of -30°C tan δ to 0°C E* (MPa) of the rubber composition constituting the first layer 6 (-30°C tan δ/0°C E*) is preferably 0.060 or greater, more preferably 0.063 or greater, even more preferably 0.066 or greater, and particularly preferably 0.070 or greater. Furthermore, -30°C tan δ/0°C E* is preferably 0.200 or less, more preferably 0.150 or less, even more preferably 0.120 or less, and particularly preferably 0.090 or less.

The 30°C tan δ, -30°C tan δ, and 0°C E* of the rubber compositions constituting the first layer 6, second layer 7, and third layer 8 are measured using the above-mentioned measurement method. The 30°C tan δ, -30°C tan δ, and 0°C E* of the rubber compositions constituting the first layer 6, second layer 7, and third layer 8 can be adjusted as appropriate by adjusting the types and amounts of rubber components, fillers, plasticizers, etc. (especially plasticizers) described below.

The Shore hardness (Hs) of the rubber composition constituting the first layer 6 (hereinafter referred to as the Shore hardness (Hs) of the first layer 6) is preferably 70 or less, more preferably 65 or less, even more preferably 60 or less, and particularly preferably 58 or less. The Shore hardness (Hs) of the second layer 7 is preferably 75 or less, more preferably 70 or less, even more preferably 65 or less, and particularly preferably 60 or less. By maintaining the Shore hardness (Hs) of the first layer 6 and the second layer 7 within the above range, it is believed that the road-following ability is not impaired and reaction force is more easily obtained, thereby improving grip performance on ice. On the other hand, from the viewpoint of maintaining the block rigidity of the tire, the Shore hardness (Hs) of the first layer 6 and the second layer 7 is preferably 30 or more, more preferably 35 or more, even more preferably 40 or more, even more preferably 45 or more, and particularly preferably 50 or more. The rubber hardness of each rubber layer can be adjusted appropriately by changing the type and amount of the rubber components, fillers, plasticizers, etc., described below.

The modulus of the first layer 6 at 100% stretch is preferably 1.0 MPa or more, more preferably 1.1 MPa or more, even more preferably 1.2 MPa or more, and particularly preferably 1.3 MPa or more. The modulus of the second layer 7 at 100% stretch is preferably 1.0 MPa or more, more preferably 1.2 MPa or more, even more preferably 1.4 MPa or more, and particularly preferably 1.6 MPa or more. While there are no particular upper limits for the moduli of the first layer 6 and the second layer 7 at 100% stretch, they are typically 4.0 MPa or less, and preferably 3.5 MPa or less. In the present disclosure, the modulus of the first layer 6 at 100% stretch is preferably greater than the modulus of the second layer 7 at 100% stretch. By making the modulus of the first layer 6 at 100% stretch greater than the modulus of the second layer 7 at 100% stretch, it is believed that deformation occurring in the first layer 6 is more easily transmitted to the second layer 7, making it easier for internal energy loss to occur during shear deformation. The difference between the modulus of the second layer 7 at 100% stretch and the modulus of the first layer 6 at 100% stretch is preferably 0.1 MPa or more, more preferably 0.2 MPa or more, and even more preferably 0.3 MPa or more. The modulus of each rubber layer at 100% stretch can be adjusted appropriately by changing the types and amounts of rubber components, fillers, plasticizers, etc., as described below.

[Rubber composition]
The tire of the present disclosure can improve the overall performance of fuel economy, grip performance on ice, and steering stability on ice by combining the above-mentioned shape of the tread portion with the above-mentioned physical properties of the rubber composition constituting each layer of the tread portion.

<Rubber component>
The rubber composition constituting each layer of the tread portion of the present disclosure (hereinafter, unless otherwise specified, referred to as the rubber composition according to the present disclosure) preferably uses isoprene-based rubber, styrene-butadiene rubber (SBR), and butadiene rubber (BR) as the rubber component. The rubber component constituting the first layer 6 includes at least two rubbers selected from the group consisting of isoprene-based rubber, styrene-butadiene rubber, and butadiene rubber. The rubber components constituting the second layer 7 and the third layer 8 include at least one rubber selected from the group consisting of isoprene-based rubber, styrene-butadiene rubber, and butadiene rubber. The rubber components constituting the first layer 6, the second layer 7, and the third layer 8 preferably include isoprene-based rubber, more preferably include isoprene-based rubber and BR, and may be rubber components consisting only of isoprene-based rubber and BR.

(Isoprene rubber)
Examples of isoprene-based rubbers that can be used include those commonly used in the tire industry, such as isoprene rubber (IR) and natural rubber. Natural rubber includes unmodified natural rubber (NR), as well as modified natural rubbers such as epoxidized natural rubber (ENR), hydrogenated natural rubber (HNR), deproteinized natural rubber (DPNR), high-purity natural rubber, and grafted natural rubber. These isoprene-based rubbers may be used alone or in combination of two or more.

NR is not particularly limited, and any commonly used NR in the tire industry can be used, such as SIR20, RSS#3, TSR20, etc.

When an isoprene-based rubber is contained, its content in the rubber component is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, and particularly preferably 25% by mass or more, from the viewpoint of complex modulus. Meanwhile, the upper limit of the isoprene-based rubber content in the rubber component is not particularly limited and may be 100% by mass, but from the viewpoint of ensuring damping performance in the tread portion, it is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, and particularly preferably 60% by mass or less.

(BR)
The BR is not particularly limited, and examples thereof include BR having a cis content of less than 50 mol% (low cis BR), BR having a cis content of 90 mol% or more (high cis BR), rare earth butadiene rubber (rare earth BR) synthesized using a rare earth catalyst, BR containing syndiotactic polybutadiene crystals (SPB-containing BR), and modified BR (high cis modified BR, low cis modified BR), which are commonly used in the tire industry. These BRs may be used alone or in combination of two or more.

High-cis BR can be commercially available from, for example, Zeon Corporation, Ube Industries, Ltd., JSR Corporation, etc. The inclusion of high-cis BR can improve low-temperature properties and wear resistance. The cis content is preferably 95 mol% or more, more preferably 96 mol% or more, even more preferably 97 mol% or more, and particularly preferably 98 mol% or more. The cis content of BR is measured by the above-mentioned measurement method.

Rare earth BR is synthesized using a rare earth catalyst and has a vinyl content of preferably 1.8 mol% or less, more preferably 1.0 mol% or less, and even more preferably 0.8 mol% or less, and a cis content of preferably 95 mol% or more, more preferably 96 mol% or more, even more preferably 97 mol% or more, and particularly preferably 98 mol% or more. Such rare earth BR can be commercially available from, for example, LANXESS K.K.

SPB-containing BR is one in which 1,2-syndiotactic polybutadiene crystals are not simply dispersed in BR, but are dispersed after being chemically bonded to the BR. Examples of such SPB-containing BR that can be used include those commercially available from Ube Industries, Ltd., etc.

The preferred modified BR is modified butadiene rubber (modified BR) whose terminals and/or main chain have been modified with functional groups containing at least one element selected from the group consisting of silicon, nitrogen, and oxygen.

Other modified BRs include those obtained by polymerizing 1,3-butadiene with a lithium initiator and then adding a tin compound, in which the terminals of the modified BR molecules are further bonded with tin-carbon bonds (tin-modified BR). Furthermore, modified BR may be either non-hydrogenated or hydrogenated.

The BRs listed above may be used alone or in combination of two or more.

From the viewpoint of abrasion resistance, the weight average molecular weight (Mw) of BR is preferably 300,000 or more, more preferably 350,000 or more, and even more preferably 400,000 or more. Furthermore, from the viewpoint of crosslinking uniformity, etc., it is preferably 2,000,000 or less, and more preferably 1,000,000 or less. The Mw of BR is measured by the above-mentioned measurement method.

When BR is contained, the content of BR in 100% by mass of the rubber component is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and particularly preferably 35% by mass or more. Furthermore, the content is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, and particularly preferably 65% by mass or less.

(SBR)
The SBR is not particularly limited, and examples thereof include solution-polymerized SBR (S-SBR), emulsion-polymerized SBR (E-SBR), and modified SBRs thereof (modified S-SBR, modified E-SBR). Examples of modified SBR include SBR whose ends and/or main chains are modified, and modified SBRs (condensates, those having a branched structure, etc.) coupled with tin, silicon compounds, etc. Among these, S-SBR and modified SBR are preferred. Furthermore, hydrogenated products of these SBRs (hydrogenated SBR), etc. can also be used. These SBRs may be used alone or in combination of two or more.

S-SBR that can be used in this disclosure includes S-SBR manufactured and sold by JSR Corporation, Sumitomo Chemical Co., Ltd., Ube Industries, Ltd., Asahi Kasei Corporation, ZS Elastomers Co., Ltd., and others.

From the viewpoints of wet grip performance and abrasion resistance, the styrene content of SBR is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more. Furthermore, from the viewpoints of the temperature dependence of grip performance and blow resistance, the styrene content is preferably 60% by mass or less, more preferably 55% by mass or less, and even more preferably 50% by mass or less. The styrene content of SBR is measured using the above-mentioned measurement method.

From the viewpoints of ensuring reactivity with silica, wet grip performance, rubber strength, and abrasion resistance, the vinyl content of SBR is preferably 10 mol% or more, more preferably 15 mol% or more, and even more preferably 20 mol% or more. Furthermore, from the viewpoints of preventing an increase in temperature dependency, elongation at break, and abrasion resistance, the vinyl content of SBR is preferably 70 mol% or less, more preferably 65 mol% or less, and even more preferably 60 mol% or less. The vinyl content of SBR is measured using the above-mentioned measurement method.

From the viewpoint of wet grip performance, the weight average molecular weight (Mw) of SBR is preferably 200,000 or more, more preferably 250,000 or more, and even more preferably 300,000 or more. Furthermore, from the viewpoint of crosslinking uniformity, the weight average molecular weight is preferably 2,000,000 or less, more preferably 1,800,000 or less, and even more preferably 1,500,000 or less. The Mw of SBR is measured by the above-mentioned measurement method.

When SBR is contained, its content in the rubber component is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 30% by mass or more, from the viewpoint of grip performance on ice. On the other hand, there is no particular upper limit for the SBR content in the rubber component, and it may be set to 100% by mass.

The total styrene content of the rubber component constituting the second layer 7 is preferably less than 25% by mass, more preferably less than 20% by mass, even more preferably less than 15% by mass, even more preferably less than 10% by mass, particularly preferably less than 5% by mass, and may even be 0% by mass.

(Other rubber components)
The rubber component according to the present disclosure may contain rubber components other than the isoprene-based rubber, SBR, and BR. Examples of other rubber components that can be used include crosslinkable rubber components commonly used in the tire industry, such as styrene-isoprene-butadiene copolymer rubber (SIBR), styrene-isobutylene-styrene block copolymer (SIBS), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), hydrogenated nitrile rubber (HNBR), butyl rubber (IIR), ethylene propylene rubber, polynorbornene rubber, silicone rubber, chlorinated polyethylene rubber, fluororubber (FKM), acrylic rubber (ACM), and hydrin rubber. These other rubber components may be used alone or in combination of two or more.

<Reinforcing filler>
The rubber composition according to the present disclosure preferably uses a reinforcing filler containing carbon black and/or silica. The rubber compositions constituting the first layer 6 and the second layer 7 preferably contain silica as a filler, more preferably carbon black and silica, or may contain a filler consisting solely of carbon black and silica. The rubber composition constituting the third layer 8 preferably contains carbon black as a filler, or may contain a filler consisting solely of carbon black.

(silica)
The silica is not particularly limited, and can be, for example, silica prepared by a dry method (anhydrous silica) or silica prepared by a wet method (hydrated silica), which are commonly used in the tire industry. Among them, hydrated silica prepared by a wet method is preferred because it contains a large number of silanol groups. These silicas can be used alone or in combination of two or more.

From the viewpoint of ensuring reinforcement and damping properties in the tread portion, the nitrogen adsorption specific surface area ( _N2SA ) of silica is preferably 140 ^m2 /g or more, more preferably 150 ^m2 /g or more, even more preferably 160 ^m2 /g or more, and particularly preferably 170 ^m2 /g or more. From the viewpoint of heat buildup and processability, it is preferably 350 ^m2 /g or less, more preferably 300 ^m2 /g or less, and even more preferably 250 ^m2 /g or less. The _N2SA of silica is measured by the above-mentioned measurement method.

The average primary particle size of silica is preferably 20 nm or less, more preferably 18 nm or less. There is no particular lower limit to the average primary particle size, but it is preferably 1 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more. By having the average primary particle size of silica within the above range, the dispersibility of the silica can be further improved, and the reinforcement properties, fracture properties, and abrasion resistance can be further improved. The average primary particle size of silica is measured using the above-mentioned measurement method.

When the rubber composition constituting the first layer 6 and the second layer 7 contains carbon black, the content per 100 parts by mass of the rubber component is preferably 30 parts by mass or more, more preferably 40 parts by mass or more, even more preferably 50 parts by mass or more, and particularly preferably 55 parts by mass or more, from the viewpoint of ensuring damping properties in the tread portion and grip performance on ice. Furthermore, from the viewpoint of reducing the specific gravity of the rubber and achieving weight reduction, the content is preferably 130 parts by mass or less, more preferably 120 parts by mass or less, even more preferably 110 parts by mass or less, and particularly preferably 105 parts by mass or less.

When the rubber composition constituting the third layer 8 contains silica, the content per 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more. The content is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less.

(carbon black)
The carbon black is not particularly limited, and may be, for example, one commonly used in the tire industry, such as GPF, FEF, HAF, ISAF, SAF, etc. These carbon blacks may be used alone or in combination of two or more.

From the viewpoints of weather resistance and reinforcing properties, the nitrogen adsorption specific surface area ( _N2SA ) of carbon black is preferably 50 ^m2 /g or more, more preferably 80 ^m2 /g or more, and even more preferably 100 ^m2 /g or more. From the viewpoints of dispersibility, fuel efficiency, fracture properties, and durability, it is preferably 250 ^m2 /g or less, more preferably 220 ^m2 /g or less. The _N2SA of carbon black is measured by the above-mentioned measurement method.

When the rubber composition constituting the first layer 6 and the second layer 7 contains carbon black, the content per 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, from the viewpoints of weather resistance and reinforcement. Furthermore, from the viewpoint of fuel economy, the content is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, even more preferably 20 parts by mass or less, and particularly preferably 15 parts by mass or less.

When the rubber composition constituting the third layer 8 contains carbon black, the amount of carbon black per 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more. From the standpoint of fuel economy, the amount is preferably 70 parts by mass or less, more preferably 60 parts by mass or less, and even more preferably 55 parts by mass or less.

(Other reinforcing fillers)
As reinforcing fillers other than silica and carbon black, those conventionally used in the tire industry, such as aluminum hydroxide, calcium carbonate, alumina, clay, and talc, can be blended.

In the rubber composition constituting the first layer 6 and the second layer 7, the ratio of the carbon black content to the silica content is preferably 0.40 or less, more preferably 0.30 or less, even more preferably 0.21 or less, even more preferably 0.17 or less, even more preferably 0.13 or less, and particularly preferably 0.10 or less. By keeping the ratio of the carbon black content to the silica content within the above range, grip performance on ice can be further improved. On the other hand, the lower limit of the ratio of the carbon black content to the silica content is not particularly limited and can be, for example, 0.01 or more, 0.02 or more, or 0.05 or more. The carbon black may also be a reinforcing filler that does not contain carbon black.

In the rubber composition constituting the first layer 6 and the second layer 7, the total content of the reinforcing filler per 100 parts by mass of the rubber component is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, even more preferably 70 parts by mass or more, and particularly preferably 75 parts by mass or more, from the viewpoint of grip performance on ice. Furthermore, from the viewpoint of the effects of the present disclosure, this content is preferably 140 parts by mass or less, more preferably 130 parts by mass or less, even more preferably 120 parts by mass or less, and particularly preferably 110 parts by mass or less.

In the rubber composition constituting the third layer 8, the total content of reinforcing fillers per 100 parts by mass of the rubber component is preferably 30 parts by mass or more, more preferably 35 parts by mass or more, even more preferably 40 parts by mass or more, and particularly preferably 45 parts by mass or more, from the viewpoint of ensuring reinforcement and damping properties in the tread portion. Furthermore, from the viewpoint of the effects of the present disclosure, this content is preferably 120 parts by mass or less, more preferably 110 parts by mass or less, and even more preferably 105 parts by mass or less.

(Silane coupling agent)
Silica is preferably used in combination with a silane coupling agent. The silane coupling agent is not particularly limited, and any silane coupling agent conventionally used in combination with silica in the tire industry can be used, including, for example, mercapto-based silane coupling agents such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyltrimethoxysilane, and 2-mercaptoethyltriethoxysilane; sulfide-based silane coupling agents such as bis(3-triethoxysilylpropyl)disulfide and bis(3-triethoxysilylpropyl)tetrasulfide; 3-octanoylthio-1-propyltriethoxysilane, 3-hexanoylthio-1-propyltriethoxysilane, and 3-octanoylthio-1-propyltrimethoxysilane; Examples of suitable silane coupling agents include thioester-based silane coupling agents such as silane; vinyl-based silane coupling agents such as vinyltriethoxysilane and vinyltrimethoxysilane; amino-based silane coupling agents such as 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, and 3-(2-aminoethyl)aminopropyltriethoxysilane; glycidoxy-based silane coupling agents such as γ-glycidoxypropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; nitro-based silane coupling agents such as 3-nitropropyltrimethoxysilane and 3-nitropropyltriethoxysilane; and chloro-based silane coupling agents such as 3-chloropropyltrimethoxysilane and 3-chloropropyltriethoxysilane. Among these, sulfide-based silane coupling agents and/or mercapto-based silane coupling agents are preferred. Examples of suitable silane coupling agents include those commercially available from Momentive, Inc. These silane coupling agents may be used alone or in combination of two or more.

The amount of silane coupling agent per 100 parts by mass of silica is preferably 1.0 part by mass or more, more preferably 3.0 parts by mass or more, and even more preferably 5.0 parts by mass or more, from the viewpoint of improving the dispersibility of the silica. Furthermore, from the viewpoint of preventing a decrease in abrasion resistance, the amount is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 15 parts by mass or less.

<Plasticizer>
The rubber composition according to the present disclosure preferably contains a plasticizer, such as a resin component, oil, liquid rubber, or an ester-based plasticizer.

(Resin component)
The rubber composition according to the present disclosure preferably contains, as a resin component, at least one resin selected from the group consisting of terpene-based resins and cyclopentadiene-based resins.

Terpene resins and cyclopentadiene resins are characterized by their lower SP values than other adhesive resins used in tire rubber compositions, such as coumarone resins, petroleum resins (aliphatic petroleum resins, aromatic petroleum resins, alicyclic petroleum resins, etc.), phenolic resins, and rosin derivatives. Here, SP value refers to the solubility parameter calculated using the Hoy method based on the compound structure; the greater the difference in the SP values of two compounds, the lower their compatibility. Here, the SP value of water is approximately 23, while the SP values of the other adhesive resins are approximately 9 to 12. Therefore, terpene resins, which have lower SP values than other adhesive resins, are adhesive resins with lower compatibility with water. Containing these resins in rubber compositions can improve the water repellency of the rubber composition.

Terpene resins include polyterpene resins made from at least one terpene raw material selected from α-pinene, β-pinene, limonene, dipentene, and other terpene raw materials (non-hydrogenated terpene resins), such as aromatic-modified terpene resins made from terpene compounds and aromatic compounds, and terpene phenolic resins made from terpene compounds and phenolic compounds, as well as terpene resins obtained by hydrogenating these terpene resins (hydrogenated terpene resins). Examples of aromatic compounds used to make aromatic-modified terpene resins include styrene, α-methylstyrene, vinyltoluene, and divinyltoluene. Examples of phenolic compounds used to make terpene phenolic resins include phenol, bisphenol A, cresol, and xylenol.

Among terpene resins, hydrogenated terpene resins are preferred because they have a lower SP value, are more compatible with rubber components, and can further improve water repellency. Hydrogenated polyterpene resins are even more preferred because they can be hydrogenated to nearly 100% and also have excellent durability. Hydrogenation of terpene resins can be carried out using known methods, and commercially available hydrogenated terpene resins can also be used in the present disclosure.

The SP value of the terpene resin is preferably 8.6 or less, and more preferably 8.5 or less, because this can further improve the water repellency of the rubber composition. The lower limit of the SP value of the terpene resin is preferably 7.5 or more, from the viewpoint of compatibility with the rubber component.

The SP value of the cyclopentadiene resin is preferably 8.5 or less, and more preferably 8.4 or less, because this can further improve the water repellency of the rubber composition. The lower limit of the SP value of the cyclopentadiene resin is preferably 7.9 or more, from the viewpoint of compatibility with the rubber component.

From the viewpoint of achieving the effects of the present disclosure, the total content of the terpene resin and cyclopentadiene resin per 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 3 parts by mass or more, even more preferably 5 parts by mass or more, and particularly preferably 10 parts by mass or more. Furthermore, from the viewpoint of ensuring appropriate hardness, moldability, and viscosity of the rubber composition, the total content of the terpene resin and cyclopentadiene resin is preferably 40 parts by mass or less, more preferably 30 parts by mass or less.

Other resin components include, but are not limited to, petroleum resins, rosin-based resins, phenol-based resins, and the like commonly used in the tire industry. These resin components may be used alone or in combination of two or more.

From the viewpoint of wet grip performance, the softening point of the resin component (preferably a terpene resin or a cyclopentadiene resin) is preferably 60°C or higher, and more preferably 65°C or higher. Furthermore, from the viewpoint of processability and improving the dispersibility of the rubber component and filler, it is preferably 150°C or lower, more preferably 140°C or lower, and even more preferably 130°C or lower. The softening point of the resin component is measured using the above-mentioned measurement method.

When resin components are included, the total content per 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, from the viewpoint of achieving the effects of the present disclosure. Furthermore, from the viewpoint of ensuring appropriate hardness, moldability, and viscosity of the rubber composition, the content is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, even more preferably 25 parts by mass or less, and particularly preferably 20 parts by mass or less.

Examples of oils include process oil, vegetable oils, and animal fats. Examples of process oils include paraffinic process oil, naphthenic process oil, and aromatic process oil. Additionally, as an environmental measure, process oils with a low content of polycyclic aromatic compound (PCA) compounds can also be used. Examples of low-PCA process oils include mild extract solvates (MES), treated distillate aromatic extracts (TDAE), and heavy naphthenic oil.

When oil is contained, the content per 100 parts by mass of the rubber component is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, from the viewpoint of processability. Furthermore, from the viewpoint of abrasion resistance, the content is preferably 90 parts by mass or less, more preferably 70 parts by mass or less, and even more preferably 50 parts by mass or less.

(liquid rubber)
The liquid rubber is not particularly limited as long as it is a polymer that is in a liquid state at room temperature (25°C), and examples thereof include liquid butadiene rubber (liquid BR), liquid styrene butadiene rubber (liquid SBR), liquid isoprene rubber (liquid IR), liquid styrene isoprene rubber (liquid SIR), and liquid farnesene rubber. Of these, liquid BR is preferred, and modified liquid BR is more preferred. These liquid rubbers may be used alone or in combination of two or more.

The modified liquid BR is not particularly limited, and examples include modified liquid butadiene polymers whose ends and/or main chains have been modified with functional groups containing at least one element selected from the group consisting of silicon, nitrogen, and oxygen. The modified liquid BR of the present disclosure may be either non-hydrogenated or hydrogenated. In this disclosure, "liquid" means that the BR is in a liquid state at room temperature (25°C).

The functional group is not particularly limited and examples thereof include silyl group, ^R1 ( ^R2O ) ₂ silyl group, ( ^R1 ) _2R2O ^silyl group, ( ^R2O ) ₃ silyl group, amino group, amido group, isocyanate group, imino group, imidazole group, urea group, ether group, carbonyl group, oxycarbonyl group, sulfonyl group, sulfinyl group, thiocarbonyl group, ammonium group, imido group, hydrazo group, azo group, diazo group, nitrile group, pyridyl group, alkoxy group, hydroxyl group, oxy group, carboxyl group, epoxy group, (meth)acrylic group, etc. Here, ^R1 and ^R2 constituting the substituent of the silyl group each independently represent an alkyl group having 1 to 10 carbon atoms or an aryl group having 6 to 10 carbon atoms. The alkyl group having 1 to 10 carbon atoms may be linear, cyclic, or branched, and specific examples thereof include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an s-butyl group, a t-butyl group, an n-pentyl group, an n-hexyl group, an n-heptyl group, an n-octyl group, an n-nonyl group, an n-decyl group, a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, and a cyclooctyl group. Of these, from the viewpoint of the effects of the present disclosure, a linear alkyl group is preferred, and a methyl group or an ethyl group is more preferred. Specific examples of an aryl group having 6 to 10 carbon atoms include a phenyl group, an α-naphthyl group, and a β-naphthyl group.

From the viewpoint of the effects of the present disclosure, the functional group is preferably a functional group containing at least one element selected from the group consisting of silicon and oxygen, such as an ^R1 ( ^R2O ) ₂ silyl group, _a ( ^R1 ) ^2R2O silyl group, or a ( ^R2O ) ₃ silyl group, and among these, a trialkoxysilyl group is more preferred.Furthermore, examples of the trialkoxysilyl group include a trimethoxysilyl group and a triethoxysilyl group.

The modified liquid BR may be a commercially available product or one obtained by synthesis. Examples of commercially available products include those manufactured by Nippon Soda Co., Ltd., Cray Valley, and Noveon. There are no particular restrictions on the synthesis method, and any known method can be used. For example, a method may be used in which an unmodified liquid butadiene polymer is reacted with a compound containing at least one element selected from the group consisting of silicon, nitrogen, and oxygen in the presence of a metal catalyst to introduce a functional group.

Specific examples of modified liquid BR include those with a structure having a triethoxysilyl group in the main chain and those with hydroxyl groups at both ends.

One or more types of modified liquid BR can be used.

When liquid rubber (preferably liquid BR, more preferably modified liquid BR) is contained, the content per 100 parts by mass of the rubber component is preferably 1 part by mass or more, more preferably 2 parts by mass or more, even more preferably 3 parts by mass or more, and particularly preferably 5 parts by mass or more. Furthermore, the content of the liquid rubber is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 20 parts by mass or less. By keeping the liquid rubber content within the above range, the effects of the present disclosure can be more effectively exhibited.

(ester plasticizer)
Examples of ester-based plasticizers include dibutyl adipate (DBA), diisobutyl adipate (DIBA), dioctyl adipate (DOA), di-2-ethylhexyl azelate (DOZ), dibutyl sebacate (DBS), diisononyl adipate (DINA), diethyl phthalate (DEP), dioctyl phthalate (DOP), diundecyl phthalate (DUP), dibutyl phthalate (DBP), dioctyl sebacate (DOS), tributyl phosphate (TBP), trioctyl phosphate (TOP), triethyl phosphate (TEP), trimethyl phosphate (TMP), thymidine triphosphate (TTP), tricresyl phosphate (TCP), trixylenyl phosphate (TXP), etc. These ester-based plasticizers may be used alone or in combination of two or more.

From the viewpoint of grip performance on ice, the amount of plasticizer per 100 parts by mass of the rubber component (the total amount when multiple plasticizers are used) is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more. From the viewpoint of processability, the amount is preferably 120 parts by mass or less, more preferably 100 parts by mass or less, even more preferably 90 parts by mass or less, and particularly preferably 80 parts by mass or less.

<Other compounding agents>
In addition to the above components, the rubber composition according to the present disclosure may appropriately contain compounding agents that are generally used in the tire industry, such as wax, processing aids, antioxidants, vulcanizing agents such as stearic acid, zinc oxide, and sulfur, and vulcanization accelerators.

When wax is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, from the viewpoint of weather resistance of the rubber. Furthermore, from the viewpoint of preventing whitening of the tire due to bloom, the content is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less.

Processing aids that can be used include fatty acid metal salts, which are used to lower the viscosity of unvulcanized rubber and ensure releasability, and commercially available compatibilizers that suppress micro-layer separation of rubber components.

When a processing aid is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, from the viewpoint of improving processability. Furthermore, from the viewpoints of abrasion resistance and breaking strength, the content is preferably 10 parts by mass or less, and more preferably 8 parts by mass or less.

The anti-aging agent is not particularly limited, but examples include amine-based, quinoline-based, quinone-based, phenol-based, and imidazole-based compounds, as well as metal carbamates.

When an antioxidant is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, from the viewpoint of the ozone crack resistance of the rubber. Furthermore, from the viewpoint of abrasion resistance and wet grip performance, the content is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less.

When stearic acid is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, from the viewpoint of processability. Furthermore, from the viewpoint of vulcanization speed, the content is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less.

When zinc oxide is contained, the content per 100 parts by mass of the rubber component is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, from the viewpoint of processability. Furthermore, from the viewpoint of abrasion resistance, the content is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less.

Sulfur is preferably used as a vulcanizing agent. Examples of sulfur that can be used include powdered sulfur, oil-treated sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, and highly dispersible sulfur.

When sulfur is used as a vulcanizing agent, the content per 100 parts by mass of the rubber component is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, and even more preferably 0.5 parts by mass or more, from the viewpoint of ensuring a sufficient vulcanization reaction. Furthermore, from the viewpoint of preventing deterioration, the content is preferably 5.0 parts by mass or less, more preferably 4.0 parts by mass or less, and even more preferably 3.0 parts by mass or less. When oil-containing sulfur is used as the vulcanizing agent, the content of the vulcanizing agent is the total content of pure sulfur contained in the oil-containing sulfur.

Examples of vulcanizing agents other than sulfur include alkylphenol-sulfur chloride condensate, sodium 1,6-hexamethylene-dithiosulfate dihydrate, and 1,6-bis(N,N'-dibenzylthiocarbamoyldithio)hexane. These non-sulfur vulcanizing agents can be commercially available from Taoka Chemical Co., Ltd., Lanxess K.K., Flexis, and other companies.

Examples of vulcanization accelerators include sulfenamide-based, thiazole-based, thiuram-based, thiourea-based, guanidine-based, dithiocarbamic acid-based, aldehyde-amine-based or aldehyde-ammonia-based, imidazoline-based, and xanthate-based vulcanization accelerators. These vulcanization accelerators may be used alone or in combination of two or more. Among these, one or more vulcanization accelerators selected from the group consisting of sulfenamide-based, guanidine-based, and thiazole-based vulcanization accelerators are preferred, as they more suitably achieve the desired effects, and a combination of a sulfenamide-based vulcanization accelerator and a guanidine-based vulcanization accelerator is even more preferred.

Examples of sulfenamide vulcanization accelerators include N-tert-butyl-2-benzothiazolylsulfenamide (TBBS), N-cyclohexyl-2-benzothiazolylsulfenamide (CBS), and N,N-dicyclohexyl-2-benzothiazolylsulfenamide (DCBS). Of these, N-tert-butyl-2-benzothiazolylsulfenamide (TBBS) and N-cyclohexyl-2-benzothiazolylsulfenamide (CBS) are preferred.

Examples of guanidine vulcanization accelerators include 1,3-diphenylguanidine (DPG), 1,3-di-o-tolylguanidine, 1-o-tolylbiguanide, di-o-tolylguanidine salt of dicatechol borate, 1,3-di-o-cumenylguanidine, 1,3-di-o-biphenylguanidine, and 1,3-di-o-cumenyl-2-propionylguanidine. Of these, 1,3-diphenylguanidine (DPG) is preferred.

Examples of thiazole-based vulcanization accelerators include 2-mercaptobenzothiazole, cyclohexylamine salt of 2-mercaptobenzothiazole, and di-2-benzothiazolyl disulfide. Of these, 2-mercaptobenzothiazole is preferred.

When a vulcanization accelerator is contained, the content per 100 parts by mass of the rubber component is preferably 1.0 part by mass or more, more preferably 1.5 parts by mass or more, and even more preferably 2.0 parts by mass or more. Furthermore, the content of the vulcanization accelerator per 100 parts by mass of the rubber component is preferably 8.0 parts by mass or less, more preferably 7.0 parts by mass or less, even more preferably 6.0 parts by mass or less, and particularly preferably 5.0 parts by mass or less. By keeping the content of the vulcanization accelerator within the above range, breaking strength and elongation tend to be ensured.

<Manufacturing>
The rubber composition according to the present disclosure can be produced by a known method, for example, by kneading the above-described components using a rubber kneading device such as an open roll or an internal kneader (such as a Banbury mixer or kneader).

The kneading process may include, for example, a base kneading process in which compounding ingredients and additives other than the vulcanizing agent and vulcanization accelerator are kneaded, and a final kneading (F kneading) process in which the vulcanizing agent and vulcanization accelerator are added to the mixture obtained in the base kneading process and kneaded. Furthermore, the base kneading process can be divided into multiple processes as desired.

Kneading conditions are not particularly limited, but examples include a base kneading process in which the mixture is kneaded for 3 to 10 minutes at a discharge temperature of 150 to 170°C, and a final kneading process in which the mixture is kneaded for 1 to 5 minutes at 70 to 110°C. Vulcanization conditions are not particularly limited, but examples include a vulcanization process in which the mixture is vulcanized for 10 to 30 minutes at 150 to 200°C.

A tire having a tread portion including the first layer 6, the second layer 7, and the third layer 8 can be manufactured by a conventional method using the rubber composition. That is, an unvulcanized rubber composition obtained by blending the above-mentioned components with the rubber component as needed is extruded into the shapes of the first layer 6, the second layer 7, and the third layer 8 using an extruder equipped with a die of a predetermined shape, and the extruded rubber composition is then bonded together with other tire components on a tire building machine and molded by a conventional method to form an unvulcanized tire. The unvulcanized tire can then be heated and pressurized in a vulcanizer to manufacture the tire. The vulcanization conditions are not particularly limited, and examples include a method of vulcanizing at 150 to 200°C for 10 to 30 minutes.

<Application>
The tire of the present disclosure can be suitably used as a passenger car tire, a truck/bus tire, a motorcycle tire, or a racing tire, and is preferably used as a passenger car tire. Passenger car tires are tires designed to be mounted on four-wheeled vehicles and have a maximum load capacity of 1,000 kg or less. The tire of the present disclosure can be used as an all-season tire, a summer tire, or a winter tire such as a studless tire, and is preferably used as a winter tire.

This disclosure will be described based on examples, but is not limited to these examples.

The various chemicals used in the examples and comparative examples are listed below.
NR: TSR20
SBR: Tufuden 4850 manufactured by Asahi Kasei Corporation (unmodified S-SBR, styrene content: 40% by mass, vinyl content: 46 mol%, Mw: 350,000, contains 50 parts by mass of oil per 100 parts by mass of rubber solids)
BR: UBEPOL BR (registered trademark) 150B manufactured by Ube Industries, Ltd. (vinyl content: 1.5 mol%, cis content: 97 mol%, Mw: 440,000)
Carbon black: Showblack N220 (N ₂ SA: 111 m ² /g) manufactured by Cabot Japan Co., Ltd.
Silica: ULTRASIL (registered trademark) VN3 manufactured by Evonik Degussa (N ₂ SA: 175 m ² /g, average primary particle size: 17 nm)
Silane coupling agent: Si266 (bis(3-triethoxysilylpropyl)disulfide) manufactured by Evonik Degussa
Oil: H&R VivaTec 500 (TDAE oil)
Resin component 1: P125 (hydrogenated polyterpene resin, softening point: 125°C) manufactured by Yasuhara Chemical Co., Ltd.
Resin component 2: Oppera PR-120 manufactured by ExxonMobil (hydrogenated dicyclopentadiene resin, softening point: 120°C)
Liquid rubber: NISSO-PB GI-3000 (hydrogenated modified liquid BR having hydroxyl groups at both ends) manufactured by Nippon Soda Co., Ltd.
Zinc oxide: Zinc oxide type 2 manufactured by Mitsui Mining & Smelting Co., Ltd. Stearic acid: Beaded stearic acid camellia manufactured by NOF Corporation Sulfur: Powder manufactured by Karuizawa Sulfur Co., Ltd. Sulfur vulcanization accelerator 1: Noccela CZ (N-cyclohexyl-2-benzothiazolyl sulfenamide) manufactured by Ouchi Shinko Chemical Industry Co., Ltd.
Vulcanization accelerator 2: Noccelaer D (1,3-diphenylguanidine (DPG)) manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

Examples and Comparative Examples
According to the formulation shown in Table 1, chemicals other than sulfur and vulcanization accelerator were mixed using a 1.7 L closed-type Banbury mixer for 1 to 10 minutes until the discharge temperature reached 150 to 160°C, yielding a kneaded mixture. Next, sulfur and vulcanization accelerator were added to the resulting mixture using a two-screw open roll mill, and the mixture was mixed for 4 minutes until the temperature reached 105°C, yielding an unvulcanized rubber composition. The resulting unvulcanized rubber composition was molded to the shapes of the first layer (thickness: 5.0 mm), second layer (thickness: 5.0 mm), and third layer (thickness: 1.0 mm) of the tread portion , and then bonded together with other tire components to produce an unvulcanized tire. The tire was then vulcanized at 170°C to obtain the test tires (size: 195/65R15, rim: 15x6.0J, internal pressure: 230 kPa) listed in Table 2. The deepest circumferential groove depth was 8.0 mm.

<Measurement of loss tangent tanδ and complex modulus of elasticity E*>
Each rubber test piece was cut from the rubber layer of the tread portion of each test tire to a length of 20 mm, width of 4 mm, and thickness of 1 mm, with the long side aligned in the tire circumferential direction. Using a GABO Iplexer series tester, 30°C tan δ was measured under conditions of a temperature of 30°C, initial strain of 5%, dynamic strain of 1%, and frequency of 10 Hz. Furthermore, -30°C tan δ was measured under conditions of a temperature of -30°C, initial strain of 10%, dynamic strain of 0.25%, and frequency of 10 Hz. Furthermore, 0°C E* was measured under conditions of a temperature of 0°C, initial strain of 10%, dynamic strain of 0.25%, and frequency of 10 Hz.

<Measurement of rubber hardness (Hs)>
The Shore hardness (Hs) of each rubber test piece was measured at a temperature of 23° C. using a durometer type A in accordance with JIS K6253-3:2012. Each rubber test piece was cut out from inside the rubber layer of the tread portion of each test tire.

<Tensile test>
A 1 mm thick dumbbell-shaped No. 7 test piece was prepared by cutting out from inside the rubber layer of the tread portion of each test tire so that the tensile direction was the tire circumferential direction, and a tensile test was carried out in accordance with JIS K 6251:2017 under conditions of an atmosphere of 23°C and a tensile speed of 3.3 mm/sec, and the modulus (MPa) at 100% elongation was measured. The thickness direction of the sample was the tire radial direction.

<Low fuel consumption performance>
Using a rolling resistance tester, each new test tire was measured for rolling resistance when run under the conditions of a rim of 15x6.0J, an internal pressure of 230 kPa, a load of 4.24 kN, and a speed of 80 km/h, and the reciprocal of the measured rolling resistance was expressed as an index, with the reference comparative example (Comparative Example 1 in Table 3, Comparative Example 3 in Table 5, the same applies hereinafter) being set at 100. A larger value indicates lower rolling resistance and better fuel economy.

<Grip performance on ice>
Each test tire was mounted on each of the four wheels of a 2000cc front-wheel drive passenger car, and the braking distance from the point where the brakes were applied on an icy road at a speed of 15 km/h was measured. The reciprocal value of the braking distance was expressed as an index, with the braking distance of the control tire (Comparative Example 1) set at 100, and the evaluation results of each test tire were displayed.

<Steering stability on ice>
Each test tire was mounted on each of the four wheels of a 2000cc front-wheel drive passenger vehicle, and the vehicle was driven on an icy test course. The test drivers evaluated the handling characteristics based on the feeling they had when driving straight, changing lanes, and accelerating and decelerating while driving at 50 km/h. The evaluation was performed using an integer value from 1 to 10, with a higher score indicating better handling characteristics. The total score of the 10 test drivers was calculated based on this evaluation standard. The total score of the control tire (Comparative Example 1) when new was converted to a reference value (100), and the evaluation results of each test tire were displayed as an index proportional to the total score.

The target performance value for the overall performance of fuel economy, ice grip, and ice handling stability (the sum of the fuel economy index, ice grip index, and ice handling stability index) is over 300.

The results in Tables 1 and 2 show that tires of the present disclosure, which have three or more rubber layers in the tread and contain two or more rubber components in the first rubber layer that makes up the tread surface, have improved overall performance in terms of fuel economy, grip performance on ice, and steering stability on ice.

1 Circumferential groove 2 Land portion 3 Tread surface 4 Extension line of land portion 5 Extension line of deepest part of bottom of circumferential groove 6 First layer 7 Second layer 8 Third layer 9 Outermost extension line of second layer

Claims (12)

A tire having a tread portion including at least a first layer constituting a tread surface, a second layer adjacent to the radially inner side of the first layer, and a third layer adjacent to the radially inner side of the second layer,
The tread portion has a plurality of circumferential grooves extending continuously in the tire circumferential direction,
the first layer is composed of a rubber composition containing a rubber component including at least two rubbers selected from the group consisting of isoprene-based rubber, styrene-butadiene rubber, and butadiene rubber,
the second layer is made of a rubber composition containing a rubber component including at least one rubber selected from the group consisting of an isoprene-based rubber, a styrene-butadiene rubber, and a butadiene rubber,
the third layer is made of a rubber composition containing a rubber component including at least one rubber selected from the group consisting of an isoprene-based rubber, a styrene-butadiene rubber, and a butadiene rubber,
The tire has a ratio of -30°C tan δ to 0°C E* (MPa) of the rubber composition constituting the first layer (-30°C tan δ/0°C E*) of 0.060 to 0.150 . The tire of claim 1, wherein the rubber component constituting the second layer contains 10 to 90% by mass of isoprene-based rubber and 10 to 90% by mass of butadiene rubber. The tire according to claim 1 or 2, wherein the rubber component constituting the third layer includes at least one rubber selected from the group consisting of isoprene-based rubber and butadiene rubber. The tire described in any one of claims 1 to 3, wherein the rubber composition constituting the first layer contains 60 parts by mass or more of a reinforcing filler per 100 parts by mass of the rubber component. The tire described in any one of claims 1 to 4, wherein the sulfur content of the rubber composition constituting the second layer is less than the sulfur content of the rubber composition constituting the third layer. The tire described in any one of claims 1 to 5, wherein the tan δ at 30°C of the rubber composition constituting the second layer and the tan δ at 30°C of the rubber composition constituting the third layer are smaller than the tan δ at 30°C of the rubber composition constituting the first layer. The tire described in any one of claims 1 to 6, wherein the total styrene content of the rubber component constituting the second layer is less than 25% by mass. The tire described in any one of claims 1 to 7, wherein the rubber composition constituting the first layer contains at least one resin selected from the group consisting of terpene-based resins and cyclopentadiene-based resins. The tire described in any one of claims 1 to 8, wherein the rubber composition constituting the first layer contains modified liquid butadiene rubber. The tire described in any one of claims 1 to 9, wherein the rubber composition constituting the first layer has a Shore hardness (Hs) of 40 to 60, measured in accordance with JIS K 6253-3:2012 using a durometer type A at a temperature of 23°C. A tire according to any one of claims 1 to 10, wherein the modulus of the first layer at 100% elongation is greater than the modulus of the second layer at 100% elongation. A tire according to any one of claims 1 to 11, wherein the deepest portion of the groove bottom of any one of the circumferential grooves is formed so as to be located radially inward of the outermost portion of the second layer in the land portion adjacent to that circumferential groove.