JP6922180B2 - Pneumatic tires - Google Patents

Description

The present invention relates to a pneumatic tire having a thin under-groove gauge for reducing rolling resistance. More specifically, even if the under-groove gauge is thin, it is possible to prevent the occurrence of groove cracks and to improve steering stability. Regarding pneumatic tires that can be maintained well.

In recent years, in order to reduce the rolling resistance of pneumatic tires and improve fuel efficiency, it has been possible to thin the gauge (under-groove gauge) of the rubber layer that constitutes the lower part of the main groove extending in the tire circumferential direction at the tread portion. It has been. However, when the under-groove gauge becomes extremely thin, the rigidity of the rubber layer constituting the lower part of the main groove becomes significantly smaller than that of the reinforcing layer including the reinforcing cord such as the belt layer and the belt reinforcing layer, and the bottom portion of the main groove becomes extremely thin. There is a problem that stress tends to be concentrated on the tire, and when the tire is used for a long period of time and deteriorates over time, groove cracks are likely to occur.

On the other hand, the rubber layer (tread rubber layer) constituting the tread portion is generally composed of a cap tread exposed on the tread surface and an under tread arranged on the inner peripheral side thereof. In this case, the cap tread needs to have high hardness physical characteristics in order to improve the steering stability of the tire, and on the other hand, it tends to have a characteristic that it is difficult to stretch. On the other hand, since the under tread is composed of a rubber composition that is more flexible than the cap tread for the purpose of reducing heat generation of the tire, for example, as shown in Patent Document 1, the thickness of the under tread is increased and the under tread is grooved. It is considered that if it is exposed to the bottom, it will be a countermeasure against the above-mentioned groove crack. However, there is a problem that when the amount of under tread used increases, the cap tread that contributes to steering stability inevitably decreases and steering stability deteriorates.

Japanese Unexamined Patent Publication No. 4-118305

An object of the present invention is air that can prevent the occurrence of groove cracks and maintain good steering stability even when the under-groove gauge is thinned to reduce rolling resistance. It is to provide tires with wheels.

The pneumatic tire of the present invention for achieving the above object has a tread portion extending in the tire circumferential direction to form an annular shape, a pair of sidewall portions arranged on both sides of the tread portion, and these sidewall portions. A pair of bead portions arranged inside in the tire radial direction, a carcass layer mounted between the pair of bead portions, a reinforcing layer arranged on the outer peripheral side of the carcass layer in the tread portion, and the tread. A tread rubber layer arranged on the outer peripheral side of the carcass layer and the reinforcing layer in the portion is provided, and a main groove extending in the tire circumferential direction and a land portion partitioned by the main groove are formed on the outer surface of the tread portion. In the pneumatic tire, the tread rubber layer is composed of a cap tread exposed on the surface of the tread portion and an under tread arranged on the inner peripheral side of the cap tread, and is under the groove of the main groove in the meridional cross section. The ratio of the under tread to the region is 95% or more, the height Ht from the outer peripheral surface of the reinforcing layer to the surface of the land portion is 30 mm or less, and the outer peripheral surface of the reinforcing layer to the bottom surface of the main groove. The height Hg up to is 5 mm or less, and the height h from the outer peripheral surface of the reinforcing layer at the center position in the width direction of the land portion to the boundary between the cap tread and the under tread is 0.5 mm or more and the height. The rubber composition that satisfies the relationship of h <Hg with respect to Hg and constitutes the under tread contains three types of natural rubber, polybutadiene rubber, and styrene butadiene rubber in the rubber component, and the natural rubber in the rubber component. The ratio of tread is 15% to 80% by weight, the ratio of polybutadiene rubber is 10% to 60% by weight, and 20 parts by weight to 80 parts by weight of the filler with respect to 100 parts by weight of the rubber component and 2 The filler contains 2.0 parts by weight to 6.0 parts by weight of sulfur , the filler is carbon black and silica, the elastic coefficient Eu of the rubber composition constituting the under tread is 11 MPa or less, and the cap. The rubber composition constituting the tread is characterized in that the elastic coefficient Ec satisfies the relationship of Ec / Eu ≦ 5.0 with respect to the elastic coefficient Eu.

In the pneumatic tire of the present invention, since most of the under-groove region is composed of the under tread having the above-mentioned composition, the groove bottom can be made flexible and can flexibly follow the strain, and groove cracks occur. Can be suppressed. At this time, after setting the height Hg and the height Ht in the above range, the height h is limited to the above range to suppress the proportion of the under tread under the land portion, so that the rigidity of the entire tread portion ( In particular, the rigidity of the land portion) is not lowered, and the steering stability can be maintained satisfactorily. Further, since the height Hg (that is, the rubber gauge under the groove) is suppressed within the above range, the rolling resistance can also be reduced. In the present invention, the "groove subregion" is a region below the bottom surface of the main groove in the meridian cross section, and specifically, it is lowered from the connecting point between the wall surface and the bottom surface of the main groove toward the reinforcing layer. The area between each perpendicular. When the connecting portion between the wall surface and the bottom surface of the main groove forms a smooth arc, the starting point of the arc on the wall surface side is regarded as the above-mentioned connecting point. The ratio of the under tread to the sub-groove region is the ratio of the cross-sectional area of the under tread contained in the sub-groove region to the cross-sectional area of the entire under-groove region in the meridian cross section. Further, in the present invention, the elastic modulus refers to the conditions of a frequency of 20 Hz, an initial strain of 10%, a dynamic strain of ± 2%, and a temperature of 20 ° C. using a viscoelastic spectrometer (manufactured by Toyo Seiki Seisakusho) in accordance with JIS K6394. It means the storage elastic modulus (E') measured in.

In the present invention, the rubber composition constituting the under tread, as described above, natural rubber 15 wt% to 80 wt% in the rubber component comprises a polybutadiene rubber 10 wt% to 60 wt%, filler as comprises carbon black with shea silica, including 2.0 parts by weight to 6.0 parts by weight of sulfur with respect to 100 parts by weight of the rubber component. By limiting the composition of the rubber composition constituting the under tread in this way, the heat generation and fatigue resistance of the under tread are improved, rolling resistance is reduced, groove cracks are prevented, and steering stability is maintained. It is advantageous to balance the above.

In the present invention, it is preferable that the under tread is exposed on at least a part of the bottom surface of the inner surface of the main groove. By exposing the under tread in this way, it becomes possible to prevent groove cracks more effectively.

In the present invention, the center position in the width direction of the land portion for measuring the "height h" is the tire width between the ends of the treads of each land portion in the case of the land portion partitioned between the pair of main grooves. It is the center position in the direction, and the distance from the end of the land portion to the center position in the width direction is 1/2 of the width W of each land portion. On the other hand, in the case of a land portion (so-called shoulder land portion) partitioned outside the tire width direction of the outermost main groove in the tire width direction, between the end on the main groove side and the ground contact end on the tread surface of the land portion. The distance from the end on the main groove side to the center position in the width direction or the distance from the ground contact end to the center position in the width direction is 1/2 of the width W of the land portion. The ground contact end is an end portion in the tire axial direction when the tire is rim-assembled on a regular rim, placed vertically on a flat surface with a regular internal pressure applied, and a regular load is applied. A "regular rim" is a rim defined for each tire in a standard system including a standard on which a tire is based. For example, a standard rim for JATTA, "DesignRim" for TRA, or ETRTO. If so, it is set to "Measuring Rim". "Regular internal pressure" is the air pressure defined for each tire in the standard system including the standard on which the tire is based. JATTA is the maximum air pressure, and TRA is the table "TIRE ROAD LIMITED AT VARIOUS". The maximum value described in "COLD INFLATION PRESSURES" is "INFLATION PRESSURE" for ETRTO, but 180 kPa when the tire is a passenger car. "Regular load" is the load defined for each tire in the standard system including the standard on which the tire is based. JATMA has the maximum load capacity, and TRA has the table "TIRE ROAD LIMITED AT VARIOUS". The maximum value described in "COLD INFLATION PRESSURES", and in the case of ETRTO, it is "LOAD CAPACITY".

It is a meridian cross-sectional view of the pneumatic tire which comprises embodiment of this invention. It is a cross-sectional view of the meridian which shows the tread part of FIG. 1 enlarged. FIG. 5 is an enlarged meridian cross-sectional view showing a tread portion of a pneumatic tire according to another embodiment of the present invention. FIG. 5 is an enlarged meridian cross-sectional view showing a tread portion of a pneumatic tire according to another embodiment of the present invention.

Hereinafter, the configuration of the present invention will be described in detail with reference to the accompanying drawings.

As shown in FIG. 1, the pneumatic tire of the present invention includes a tread portion 1 extending in the tire circumferential direction to form an annular shape, a pair of sidewall portions 2 arranged on both sides of the tread portion 1, and side surfaces. It includes a pair of bead portions 3 arranged inside the wall portion 2 in the tire radial direction. In FIG. 1, CL indicates the tire equator.

A carcass layer 4 is mounted between the pair of left and right bead portions 3. The carcass layer 4 includes a plurality of reinforcing cords extending in the tire radial direction, and is folded back from the inside to the outside of the vehicle around the bead cores 5 arranged in each bead portion 3. Further, a bead filler 6 is arranged on the outer periphery of the bead core 5, and the bead filler 6 is wrapped by a main body portion and a folded portion of the carcass layer 4.

A belt layer 7a (two belt layers 7a in the example of FIG. 1) is embedded on the outer peripheral side of the carcass layer 4 in the tread portion 1. Each belt layer 7a includes a plurality of reinforcing cords that are inclined with respect to the tire circumferential direction, and the reinforcing cords are arranged so as to intersect each other between the layers. In these belt layers 7a, the inclination angle of the reinforcing cord with respect to the tire circumferential direction is set in the range of, for example, 10 ° to 40 °. Further, a belt reinforcing layer 7b (two layers of belt reinforcing layers 7b in the example of FIG. 1) is provided on the outer peripheral side of the belt layer 7a. The belt reinforcing layer 7b contains an organic fiber cord oriented in the tire circumferential direction. In the belt reinforcing layer 7b, the angle of the organic fiber cord with respect to the tire circumferential direction is set to, for example, 0 ° to 5 °. In the present invention, the belt layer 7a and the belt reinforcing layer 7b are collectively referred to as the "reinforcing layer 7". In the present invention, the belt layer 7a is an essential element, but the belt reinforcing layer 7b is an arbitrary element. Therefore, in addition to the case where the belt layer 7a and the belt reinforcing layer 7b are provided as shown in the figure, the reinforcing layer 7 It is possible that only the belt layer 7a is provided.

On the outer surface of the tread portion 1, main grooves 8 extending in the tire circumferential direction (four main grooves 8 in FIG. 1) are provided, and the main grooves 8 provide land portions 9 (5 rows of land portions 9 in FIG. 1). ) Is partitioned. The land portion 9 may be either a rib extending continuously over the entire circumference of the tire or a block defined by a lug groove (not shown) extending in the tire width direction. At this time, the height Hg from the outer peripheral surface of the reinforcing layer 7 to the bottom surface of the main groove 8 (hereinafter, may be referred to as the under-groove gauge Hg) is set to 5 mm or less. Further, the height Ht from the outer peripheral surface of the reinforcing layer 7 to the surface (tread surface) of the land portion 9 is set to 30 mm or less. These heights Hg and Ht are heights based on the outer peripheral surface of the belt layer 7a when only the belt layer 7a is provided as the reinforcing layer 7, and the belt layer 7a and the belt reinforcing layer 7b are provided. In the case, the lower height is based on the outer peripheral surface of the belt reinforcing layer 7b. The outer peripheral surface of the reinforcing layer 7 (belt layer 7a, belt reinforcing layer 7b) is covered with a reinforcing cord (belt cord in the case of the belt layer 7a, organic fiber cord in the case of the belt reinforcing layer 7b) constituting the reinforcing layer 7. This is the outer surface of the coated rubber in the tire radial direction.

A tread rubber layer 10 is arranged on the outer peripheral side of the carcass layer 4 in the tread portion 1, and a side rubber layer 20 is arranged on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the sidewall portion 2. A rim cushion rubber layer 30 is arranged on the outer peripheral side (outside in the tire width direction) of the carcass layer 4. The tread rubber layer 10 has a structure in which two types of rubber layers (cap tread 11 and under tread 12) having different physical properties are laminated in the tire radial direction.

The present invention mainly defines the structure and physical properties of the undertread rubber layer 12 in such a general pneumatic tire as described later. Therefore, the basic cross-sectional structure of the pneumatic tire except for the under tread 12 (and the cap tread 11 whose structure may change depending on the under tread 12) is not limited to the above-mentioned structure.

In the present invention, as shown in FIG. 2, the ratio of the undertread 12 to the subgroove region of the main groove 8 in the meridian cross section is 95% or more, and most of the undergroove region is composed of the undertread 12. On the other hand, the height h from the outer peripheral surface of the reinforcing layer 7 at the center position in the width direction of the land portion 9 to the boundary between the cap tread 11 and the under tread 12 is 0.5 mm or more, and this height h. Satisfies the relationship of h <Hg, preferably h ≦ 0.95 Hg, with respect to the height Hg (groove gauge Hg). As described above, the under tread 12 is composed of a rubber composition different from that of the cap tread 11, but the rubber composition constituting the under tread 12 always contains a butadiene structure in the rubber component, and the rubber component 100 Always contains 20 to 80 parts by weight of filler with respect to parts by weight.

Since the under tread 12 made of the rubber composition having the above-mentioned composition is more flexible than the cap tread 11 and has a property of being able to flexibly follow the strain, the under tread 12 occupies most of the subgroove region as described above. By configuring the tread portion 1 as described above, the occurrence of groove cracks can be suppressed. At this time, since the height h is limited to the above range to suppress the proportion occupied by the under tread 11 below the land portion 9, the rigidity of the entire tread portion 1 (particularly the rigidity of the land portion 9) is reduced. There is no such thing, and the steering stability can be maintained well. Further, since the height Hg and the height Ht are set in the above ranges as the structure of the entire tread portion 1, the rolling resistance can also be reduced.

If the ratio of the undertread 12 to the subgroove region of the main groove 8 in the meridian cross section is smaller than 95%, the effect of softening the bottom surface of the main groove 8 by the undertread 12 cannot be sufficiently obtained, and groove cracks occur. The effect of suppressing is not obtained. Ideally, the ratio of the undertread 12 to the subgroove region of the main groove 8 is close to 100%, and the undertread 12 is exposed on the bottom surface of the main groove 8, but it is shown in FIG. 3 in manufacturing. As described above, the cap tread 11 may be present on the bottom surface of the main groove 8. Even if the cap tread 11 is exposed to the groove bottom of the main groove 8 in this way, if the under tread 12 satisfies the above ratio (that is, the ratio of the cap tread 11 exposed to the groove bottom to the groove bottom region). (If is less than 5%), the effect of the present invention that suppresses the occurrence of groove cracks can be sufficiently exhibited. In FIG. 3, the cap tread 11 is exposed on the entire surface of the groove bottom of the main groove 8. In this case, the cap tread 11 is present on the groove bottom side of the under tread 12 in the groove bottom region. However, if the under tread 12 is exposed to a part of the groove bottom, the under tread 12 exposed to a part of the groove bottom can obtain an effective groove crack suppressing effect. That is, the under tread 12 that occupies 95% or more of the groove bottom region can suppress the groove crack even if it is not exposed to the groove bottom, but in order to enhance the effect of suppressing the groove crack, the main groove 8 is used. It is preferable that at least a part of the bottom surface of the inner surface is exposed, more preferably a large amount of under tread 12 is exposed on the inner surface of the main groove 8, and most preferably the under tread covers the entire bottom surface of the main groove 8. It is good that 12 is exposed.

When the cap tread 11 covers the entire bottom surface of the main groove 8 as shown in FIG. 3, the thickness of the under tread 12 under the bottom surface of the main groove 8 (reinforcing layer at the position where the height Hg is measured). Assuming that the height from the outer peripheral surface of 7 to the boundary between the cap tread 11 and the under tread 12) is h', it is better that the height h and the height h'satisfy the relationship of h <h'. It is preferable that the relationship of h ≦ 0.95 h'is satisfied. When the ratio of the under tread 12 to the subgroove region of the main groove 8 is 100%, the height h'consists with the height Hg, so that the above-mentioned relationship between the height h and the height Hg (h). <Hg, preferably h ≦ 0.95 Hg).

When the height Ht from the outer peripheral surface of the reinforcing layer 7 to the surface of the land portion 9 exceeds 30 mm, or the height Hg from the outer peripheral surface of the reinforcing layer 7 to the bottom surface of the main groove 8 exceeds 5 mm, the entire tread portion 1 Since the thickness of the tire is increased, the rolling resistance cannot be sufficiently reduced in addition to the deterioration of the steering stability. When the height h from the outer peripheral surface of the reinforcing layer 7 at the center position in the width direction of the land portion 9 to the boundary between the cap tread 11 and the under tread 12 is smaller than 0.5 mm, the under tread 12 is placed below the land portion 9. Since it virtually disappears, the original performance of the tire is impaired, and the heat generation property is significantly deteriorated. When the height h is Hg or more, the amount of the under tread 12 increases below the land portion 9, so that it becomes difficult to maintain the rigidity of the entire tread portion 1 (rigidity of the land portion 9), and the steering stability. Decreases. In the present invention, if the height h at the center position in the width direction of the land portion 9 is smaller than the height Hg, the rigidity of the land portion 9 can be sufficiently maintained. That is, for example, as shown in FIG. 4, while the undertread 12 is arranged so as to cover the bottom surface and the wall surface of the main groove 8, the thickness of the undertread decreases toward the center in the width direction of the land portion 9. In the meridian cross section, the boundary between the cap tread 11 and the under tread 12 is substantially V-shaped, and the height h satisfies the relationship of h <Hg only in the vicinity of the center position in the width direction of the land portion 9. There may be.

As described above, the cap tread 11 and the under tread 12 are composed of rubber compositions having different physical properties. Specifically, the elastic modulus Eu of the rubber composition constituting the under tread 12 is 12 MPa or less. Moreover, it is preferable that the elastic modulus Ec of the rubber composition constituting the cap tread 11 satisfies the relationship of Ec / Eu ≦ 5.0 with respect to the elastic modulus Eu of the rubber composition constituting the undertread 12. By setting the elastic modulus Eu in this way, the under tread 12 can be made sufficiently flexible, which is advantageous in suppressing the occurrence of groove cracks. Further, when the elastic modulus Eu and the elastic modulus Ec satisfy the above relationship, the physical properties of both are well balanced, and the rolling resistance is reduced, the groove crack is prevented, and the steering stability is maintained in a well-balanced manner. Will be advantageous. The elastic modulus Ec of the rubber composition constituting the cap tread 11 is not particularly limited as long as the above relationship is satisfied, but can be set to, for example, 4 MPa to 20 MPa.

In the rubber composition constituting the under tread 12, the rubber component is a diene-based rubber and always contains a butadiene structure as described above. As such a diene rubber, rubbers usually used in rubber compositions for tires such as natural rubber, butadiene rubber, styrene butadiene rubber, isoprene rubber, modified butadiene rubber having modified molecular ends, and modified styrene butadiene rubber are used. can do. In particular, it is preferable that the rubber component contains 15% by weight to 80% by weight of natural rubber or polyisoprene rubber and 10% by weight to 60% by weight of polybutadiene rubber. The butadiene rubber may be modified with a modifier, and the catalyst used at the time of polymerization is not limited, and a butadiene rubber produced by a catalyst such as nickel, titanium, chromium, neogeim, or lithium may be used. .. These diene rubbers can be used alone or as any blend. By using such a rubber component, the fatigue and heat generation of the subgroove rubber layer 40 are improved, and the reduction of rolling resistance, the prevention of groove cracks, and the maintenance of steering stability can be achieved in a well-balanced manner. Will be advantageous.

The rubber composition constituting the under tread always contains 20 parts by weight to 80 parts by weight of the filler with respect to 100 parts by weight of the rubber component described above, but the blending amount of the filler is 20 with respect to 100 parts by weight of the rubber component. If it is less than the weight part, the hardness of the rubber cannot be maintained and the steering stability deteriorates. If the blending amount of the filler is more than 80 parts by weight with respect to 100 parts by weight of the rubber component, the rolling resistance cannot be sufficiently reduced. As the filler, carbon black may be contained alone or together with silica.

The rubber composition constituting the under tread 12 preferably contains 2.0 parts by weight to 6.0 parts by weight of sulfur with respect to 100 parts by weight of the rubber component. By blending a predetermined amount of sulfur in this way, the toughness of the rubber composition constituting the undertread 12 is improved, which is advantageous in preventing groove cracks, and further, the hardness and heat generation that affect the steering stability. A good balance can be achieved. If the amount of sulfur blended is less than 2.0 parts by weight with respect to 100 parts by weight of the rubber component, the crosslink density is lowered, the elastic modulus is remarkably lowered, and the steering stability is deteriorated. If the amount of sulfur blended is more than 6.0 parts by weight with respect to 100 parts by weight of the rubber component, the vulcanization density increases, the elastic modulus becomes excessive, and the effect of preventing groove cracks cannot be sufficiently obtained.

Other compounding agents other than the above can be added to the rubber composition constituting the under tread 12. Examples of other compounding agents include various compounding agents generally used for pneumatic tires, such as vulcanization accelerators, liquid polymers, thermosetting resins, and thermoplastic resins. The blending amount of these blending agents can be a conventional general blending amount as long as it does not contradict the object of the present invention.

In the present invention, the physical properties of the under tread 12 that directly contributes to the suppression of groove cracks are important, and the composition of the under tread 12 is limited as described above, but the rubber constituting the cap tread 11 is formed. The composition is not particularly limited. For example, the rubber component of the rubber composition constituting the cap tread 11 includes natural rubber, butadiene rubber, styrene butadiene rubber, isoprene rubber, or modified butadiene rubber having a modified molecular end, modified styrene butadiene rubber, and the like for tires. Rubber usually used for the composition can be used, and as the filler, carbon black usually used for the rubber composition for tires, and as the silica, those produced by a wet method or a dry method such as precipitated silica can be used. It can be used, and as commercially available products, Ultrasil 7000GR (manufactured by Evonik), Ultrasil 9100GR (manufactured by Evonik), Zeosil 1165MP (manufactured by Solvay), Zeosil Premium 200MP (manufactured by Solvay), Zeosil Premium 200MP (manufactured by Solvay) ), Ultrasil 5000GR (manufactured by Evonik), Ultrasil VN3GR (manufactured by Evonik), and the like can be used.

First, eight types of rubber compositions for tires (rubbers 1 to 8) having the formulations shown in Table 1 were prepared. In preparing these rubber compositions, the components excluding sulfur and the vulcanization accelerator were weighed, kneaded in a 1.8 L closed mixer at 160 ° C. for 5 minutes, and then the masterbatch was released. This masterbatch was kneaded with an open roll, sulfur and a vulcanization accelerator were added, and the mixture was mixed to obtain each rubber composition. Table 1 shows the measurement using a viscoelastic spectrometer (manufactured by Toyo Seiki Seisakusho) in accordance with JIS K6394 under the conditions of frequency 20 Hz, initial strain 10%, dynamic strain ± 2%, and temperature 20 ° C. The elastic modulus of each rubber composition is also shown.

The types of raw materials used in Table 1 are shown below.
・ SBR: Styrene-butadiene rubber, Nippon Zeon Corporation Nipol 1502
-BR: Butadiene rubber, Nippon Zeon Corporation Nipol BR 1220
・ NR: Natural rubber, STR20
-CB: THAI CARBON BLACK PUBLICK THAIBLACK N550
-Silica: Evonik's Ultrasil VN3GR
・ Oil: Extract No. 4 S manufactured by Showa Shell Sekiyu Co., Ltd.
-Coupling agent: Si69 manufactured by Evonik Industries
・ Stearic acid: NOF beads stearic acid ・ Zinc oxide: Zinc oxide 3 types manufactured by Shodo Chemical Industry Co., Ltd. ・ Anti-aging agent: PILFLEX 13 manufactured by NICOL LIMITED
・ Sulfur: Tsurumi Kagaku Kogyo Co., Ltd. with Jinhua Ink Oil Fine powder sulfur ・ Vulcanization accelerator: FLEXSYS Co., Ltd. SANTOCURE CBS

Next, using the above-mentioned rubber composition for tires (rubbers 1 to 8) for the undertread, the rubber composition having a tire size of 245 / 50R18, having the basic structure shown in FIG. 1, and constituting the undertread. Type, ratio Ec / Eu of the elastic coefficient Ec of the rubber composition constituting the cap tread and the elastic coefficient Eu of the rubber composition constituting the under tread, height Hg from the outer peripheral surface of the reinforcing layer on the bottom surface of the main groove, Height Ht from the outer peripheral surface of the reinforcing layer on the surface of the land, height h from the outer peripheral surface of the reinforcing layer at the boundary between the cap tread and the under tread at the center position in the width direction of the land, under in the groove area. 17 types of pneumatic tires of Conventional Example 1, Comparative Examples 1 to 10, and Examples 1 to 6 in which the tread ratio was set as shown in Table 2 were produced (note that other than "rubber 8" satisfying the conditions of the present invention. Examples 1 to 5 using the above are reference examples) .

In addition, the elastic modulus (listed in Table 1) of each rubber composition is also shown in the column of "Type of under tread" in Tables 2 and 3. Further, as the cap tread, a rubber composition generally used for cap treading in a pneumatic tire was used, and the elastic modulus Ec was 8.0 MPa.

These 17 types of pneumatic tires were evaluated for groove crack resistance, steering stability, and heat generation by the following evaluation methods, and the results are also shown in Tables 2 and 3.

Groove crack resistance Each test tire is assembled on a wheel with a rim size of 18 x 8JJ, filled with an air pressure of 230 kPa, left in a container irradiated with 100 ppmh of ozone at a temperature of 50 ° C. for one day, and then on the bottom surface of the main groove. The crack occurrence state was visually confirmed, and the result of Conventional Example 1 was evaluated on a 5-point scale with 2 points (reference).

Steering stability Each test tire is assembled on a wheel with a rim size of 18 x 8JJ, mounted on a test vehicle with a displacement of 2.5L class with an air pressure of 230kPa, and sensory evaluation is performed by a test driver on a circuit course consisting of a dry road surface. rice field. The evaluation results are shown by an index with Comparative Example 1 as 100. The larger this index value is, the better the steering stability is. If this index value is "97" or more, it means that the conventional level is maintained and sufficiently good steering stability is maintained.

Heat generation The rubber composition used for the grooved rubber layer in each test tire was press-vulcanized at 160 ° C. for 30 minutes in a predetermined mold to prepare a test piece, and tan δ at 60 ° C. of this test piece was obtained. , A viscoelastic spectrometer (manufactured by Toyo Seiki Seisakusho Co., Ltd.) was used to measure under the conditions of initial distortion of 10%, amplitude of ± 2%, frequency of 20 Hz, and atmospheric temperature of 60 ° C. The evaluation result is shown by an index in which the reciprocal of the measured value of Comparative Example 1 is 100. The larger the index value, the smaller the tan δ at 60 ° C., which means that the heat generation is low. If this index value is "95" or more, it means that the conventional level is maintained and sufficiently good heat generation is maintained.

As is clear from Tables 2 and 3, in each of Examples 1 to 6, the groove crack resistance was improved as compared with Conventional Example 1 while maintaining or reducing steering stability and heat generation.

On the other hand, Comparative Example 1 has a high height Ht, and Comparative Example 2 has a high proportion of undertread but a high height Hg. Therefore, the groove crack resistance is excellent, but the steering stability and heat generation are deteriorated. .. In Comparative Example 3, the effect of improving the groove crack resistance could not be obtained because the ratio of the under tread that can be placed in the groove bottom region was small. In Comparative Example 4, since the height h was too small, the undertread was substantially eliminated in the center of the land portion where the rubber was greatly deformed and generated heat during traveling, and the heat generation was deteriorated. In Comparative Example 5, since the height h was excessive, the land rigidity was lowered and the steering stability was deteriorated. In Comparative Example 6, since the rubber composition (rubber 4) in which the amount of the filler was excessive was used as the under tread, the heat generating property was deteriorated. In Comparative Example 7, since the rubber composition (rubber 5) in which the amount of the filler was too small was used as the under tread, the balance between the elastic modulus and heat generation was deteriorated, and the steering stability and heat generation were deteriorated. In Comparative Example 8, since the elastic modulus Eu of the under tread became excessive, the groove crack resistance deteriorated. In Comparative Example 9, since the ratio of the elastic modulus Ec of the cap tread and the elastic modulus Eu of the under tread became excessively large, the steering stability and heat generation were deteriorated. In Comparative Example 10, since a rubber composition (rubber 8) containing no butadiene rubber containing a butadiene structure was used as the under tread, the heat generating property was deteriorated.

1 tread part 2 sidewall part 3 bead part 4 carcass layer 5 bead core 6 bead filler 7 reinforcement layer 7a belt layer 7b belt reinforcement layer 8 main groove 9 land part 10 tread rubber layer 11 cap tread 12 under tread 20 side rubber layer 30 rim cushion Rubber layer 40 Under-groove rubber layer CL Tire equatorial line

Claims (2)

A tread portion extending in the tire circumferential direction to form an annular shape, a pair of sidewall portions arranged on both sides of the tread portion, and a pair of bead portions arranged inside the tire radial direction of these sidewall portions. A carcass layer mounted between the pair of bead portions, a reinforcing layer arranged on the outer peripheral side of the carcass layer in the tread portion, and a reinforcing layer arranged on the outer peripheral side of the carcass layer and the reinforcing layer in the tread portion. In a pneumatic tire provided with a tread rubber layer and having a main groove extending in the tire circumferential direction and a land portion partitioned by the main groove formed on the outer surface of the tread portion.
The tread rubber layer is composed of a cap tread exposed on the surface of the tread portion and an under tread arranged on the inner peripheral side of the cap tread, and the under tread occupies the subgroove region of the main groove in the meridional cross section. The ratio is 95% or more, the height Ht from the outer peripheral surface of the reinforcing layer to the surface of the land portion is 30 mm or less, and the height Hg from the outer peripheral surface of the reinforcing layer to the bottom surface of the main groove is 5 mm. The height h from the outer peripheral surface of the reinforcing layer to the boundary between the cap tread and the under tread at the center position in the width direction of the land portion is 0.5 mm or more and h <with respect to the height Hg. The rubber composition that satisfies the relationship of Hg and constitutes the undertread contains three types of natural rubber, polybutadiene rubber, and styrene butadiene rubber in the rubber component, and the ratio of the natural rubber in the rubber component is from 15% by weight to 80% by weight, the proportion of polybutadiene rubber is 10% by weight to 60% by weight, and 20 parts by weight to 80 parts by weight of the filler and 2.0 parts by weight to 6. A rubber composition containing 0 parts by weight of sulfur , the filler being carbon black and silica, the rubber composition constituting the under tread having an elasticity Eu of 11 MPa or less, and the cap tread constituting the cap tread. A pneumatic tire, characterized in that the elastic coefficient Ec of the above satisfies the relationship of Ec / Eu ≦ 5.0 with respect to the elastic coefficient Eu. The pneumatic tire according to claim 1, wherein the under tread is exposed to at least a portion of the bottom surface of the inner surface of the main groove.

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