JP2008207574A - Pneumatic tire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pneumatic tire having excellent wet grip performance, in ABS braking. <P>SOLUTION: In a first test piece extracted from new or unused tread rubber, hardness acquired on the basis of a type-A durometer hardness test of JIS K6253 at 20°C, is represented as H<SB>1</SB>. In a second test piece extracted from new or unused tread rubber, separated from the first test piece, and having a size of 100 mm in length ×5 mm in width ×2 mm in thickness, hardness at the center part of the second test piece acquired on the basis of an M method of JIS K6253, is represented as H<SB>2</SB>. After the acquisition of the H<SB>2</SB>, hardness H<SB>3</SB>at the center part of the second test piece after 100% elongation at 20°C and release of a tensile force is acquired based on the M method of JIS K6253. The pneumatic tire uses tread rubber satisfying expressions (I): 60≤H<SB>1</SB>≤75, and (II): H<SB>2</SB>-H<SB>3</SB>≥4. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a pneumatic tire, and more particularly to a pneumatic tire using a tire tread that exhibits excellent wet grip performance during ABS braking.

  In recent years, anti-lock brakes that maintain directional stability, ensure steering performance, and shorten braking distances by preventing braking of tires that are likely to occur during braking on slippery road surfaces such as sudden braking or snowy roads by controlling the braking force A device (hereinafter referred to as “ABS”) is mounted on many vehicles, and the braking system of the vehicle is based on ABS braking (hereinafter referred to as “ABS braking”) from lock braking in which tires are locked to brake the vehicle. It has moved to. Conventionally, the grip performance of a tire has been evaluated by obtaining a braking distance of a vehicle using the tire.

The tire grip performance on dry and wet road surfaces (hereinafter referred to as “dry grip performance” and “wet grip performance”, respectively) is the tire structure and the characteristics of the constituent materials, particularly the rubber composition (hereinafter referred to as the tread portion). The wet grip performance of a tire, which is closely related to the viscoelastic properties of the “tread rubber composition” and evaluated by rock braking, correlates with the glass transition point (Tg) of the tread rubber composition of the tire, It is known in the art that wet grip performance tends to improve as the glass transition point of the tread rubber composition increases. In general, there is also a correlation between the glass transition point of the tread rubber composition and the loss tangent tan δ (hereinafter referred to as “tan δ (0 ° C.)”) obtained at 0 ° C. by a viscoelasticity measuring machine. It is known in the art that the larger the value of tan δ (0 ° C.), the shorter the wet braking distance, that is, the better the wet grip performance. However, in ABS braking, even when the value of tan δ (0 ° C.) is large, there is no tendency for the wet braking distance to be shortened. As a cause of this, it is conceivable that the slip ratio (also referred to as the slip ratio) is different between lock braking and ABS braking. Here, the slip ratio (%) is the following formula:
Slip rate (%) = 100 × (vehicle running speed−tire circumferential speed) / vehicle running speed. A slip rate of 0% means that the running speed of the vehicle and the circumferential speed of the tire are equal, and the difference between them is zero. A slip rate of 100% means that the tire is in a locked state without rotating, and the tire is slipping on the road surface. When the slip ratio is 100%, the coefficient of friction in the lateral direction of the tire decreases, and the steering wheel operation becomes impossible. In ABS braking, in general, the braking force is controlled by increasing or decreasing the brake hydraulic pressure, the friction coefficient in the traveling direction is high, and the slip rate of 2 to 15% that can secure the cornering force is obtained. The braking force is controlled. In this slip ratio range, the front end of the tread block with respect to the vehicle traveling direction is grounded, but the rear end with respect to the vehicle traveling direction is slightly slipped. In such a situation, even if a tread rubber composition showing the same value of tan δ (0 ° C.) is used, superiority or inferiority in wet grip performance may appear. ) Alone, the wet grip performance of the tread rubber composition during ABS braking cannot be predicted.

  In pneumatic tires, in order to improve the wet grip performance of the tread, for example, the hardness of the tread surface rubber (cap rubber) is lower than the hardness of the tread internal rubber (base rubber), and the thickness of the tread internal rubber is set to the tire. It has been proposed to increase the thickness from the inner side toward the outer side (Patent Document 1 below). In addition, in order to improve steering stability and ride comfort, the tread part is composed of a base tread and a cap tread, and the base tread is positioned at the center base part located in the center in the width direction where the thickness and rubber hardness are different. Dividing into three parts of the side base part located in an outer edge part is proposed (patent document 2 below). However, the inventions described in these patent documents improve the overall characteristics of the tread portion by using a plurality of rubber members having predetermined different hardnesses, and the wetness during ABS braking of the tread rubber composition is improved. Evaluation methods for predicting grip performance have not been studied.

JP 2003-127614 A Japanese Patent Laid-Open No. 10-297214

  Therefore, in order to shorten the ABS braking distance, it is necessary to find a new physical property of the tread rubber composition that is closely related to the wet grip performance during ABS braking.

  An object of the present invention is to provide a pneumatic tire using a tread rubber exhibiting excellent wet grip performance during ABS braking.

  The present inventor examined the above problems, produced pneumatic tires using tread rubber compositions of various hardnesses, attached the pneumatic tires to a vehicle, and conducted an ABS braking test on a wet road surface. When the hardness of the tread surface when new or not used before the ABS braking test is used as a reference, the difference between the tread rubber hardness before the ABS braking test and the tread rubber hardness after the ABS braking test as the reference increases. And found that the braking distance tends to be shorter. Further, this phenomenon that the hardness of the tread rubber is lowered by the ABS braking test is caused by, for example, the hardness of the vulcanized rubber obtained after the vulcanized rubber is stretched to 100% by the tensile force and the tensile force is released. This is a phenomenon corresponding to a decrease in hardness compared to the hardness obtained in an unstretched state before the braking test, and when a rubber composition in which the degree of hardness decrease due to elongation is a specific value or more is used for the tread. It was found that a pneumatic tire can be obtained that reliably exhibits excellent wet grip performance. The present inventor further considered that the decrease in the hardness of the tread rubber is a phenomenon caused by the deformation that the tread has undergone mainly due to braking in the ABS braking test, and in order to analyze this phenomenon, The vulcanized rubber test piece obtained from the tread rubber composition of the composition was stretched at 100% elongation at a temperature of 20 ° C. and then the tensile force was released, and at a temperature of 20 ° C. before and after stretching, the JIS K6253 M As a result of obtaining the hardness in accordance with the law, the ABS braking distance correlates well with the decrease in tread rubber hardness by the ABS braking test, and the decrease in tread rubber hardness by the ABS braking test can be reproduced by laboratory evaluation. I found.

That is, according to the present invention, a pneumatic tire using a tread rubber, and the first test piece taken from a new or unused tread rubber is JIS K6253 type A durometer hardness at a temperature of 20 ° C. The hardness obtained in accordance with the test is expressed as H _1, and is taken from a new or unused tread rubber and has a length of 100 mm, a width of 5 mm, and a thickness of 2 mm separately from the first test piece. the second test piece, after the hardness has been determined according to method M in JIS K6253 in the central portion of the specimen represented as H _2, was determined H _2, for the second specimen, the temperature 20 ° C. When the hardness obtained in accordance with JIS K6253 M method is expressed as H ₃ at the center of the second test piece after releasing the tensile force by 100% in the following formula (I) And (II):
60 ≦ H ₁ ≦ 75 (I)
H ₂ −H ₃ ≧ 4 (II)
A pneumatic tire using a tread rubber characterized by satisfying the above is provided.

In the pneumatic tire of the present invention, as described above, the tread rubber conforms to the JIS K6253 type A durometer hardness test at a temperature of 20 ° C. for the first test piece collected from the new or unused tread rubber. The second test having a length of 100 mm × width of 5 mm × thickness of 2 mm separately from the first test piece, which is expressed as H ₁ and is taken from the new or unused tread rubber for pieces, the hardness was determined in accordance with method M in JIS K6253 in the central portion of the specimen represented as H _2, after obtaining the H _2, the second test piece, 100% at a temperature 20 ° C. stretching When the hardness obtained in accordance with JIS K6253 M method is expressed as H ₃ at the center of the second test piece after releasing the tensile force, the following formulas (I) and (II) :
60 ≦ H ₁ ≦ 75 (I)
H ₂ −H ₃ ≧ 4 (II)
It is characterized by satisfying. The tread rubber according to the present invention has a hardness determined according to the JIS K6253 type A durometer test at a temperature of 20 ° C. in a new or unused state after vulcanization, that is, not yet stretched. As shown by (I), it exists in the range of 60-75. If the hardness value obtained in accordance with this type A durometer test is less than 60, the rubber obtained after vulcanization becomes too soft, and the handling stability on both wet and dry road surfaces is improved. On the other hand, when it exceeds 75, the rubber obtained after vulcanization becomes too hard, and the followability to the unevenness of the road surface deteriorates, which is not preferable.

  The decrease rate of the hardness of the tread rubber by the ABS braking test is the largest at the surface portion of the tread, and the decrease rate of the hardness decreases from the surface of the tread to the deep portion where it is not easily deformed. The portion located deeper from the surface tends to be closer to the hardness when new or unused, that is, the hardness of the unstretched state before the ABS braking test. It is considered that as the rate of decrease in hardness near the tread surface is larger, the tread is deformed corresponding to the unevenness of the road surface, the contact area is increased, and a stronger grip force can be obtained. Therefore, at the time of ABS braking, the front end of the tread block is grounded with respect to the traveling direction of the tread block, but the rear end of the tread block is slightly slipped even if the rear end is slightly slipped with respect to the traveling direction. It is considered that the braking distance is shortened because the contact area of the end portion of the tread block increases corresponding to the unevenness of the road surface, so that the friction coefficient of the contact portion of the tread block increases.

  As a result of examining the decrease in the hardness of the tread rubber by the ABS braking test from the blending surface, the smaller the particle size of the blended reinforcing filler, the larger the blending amount of the reinforcing filler, or the higher the glass transition point of the rubber component The degree of decrease in hardness after the ABS braking test is large. In general, the reinforcing effect of the reinforcing filler on the rubber is such that the reinforcing filler particles are the same if the compounding amount of the reinforcing filler with respect to the rubber component contained in the rubber composition and the chemical properties of the reinforcing filler surface are the same. It is known that the particle size of the reinforcing filler is smaller, the specific surface area of the reinforcing filler is larger, and the rubber bonded to the reinforcing filler per unit mass of the rubber composition. It is known that the number of molecular chains increases and the interaction at the interface between the rubber and the reinforcing filler increases. When the rubber is deformed by an external force, the rubber molecular chain is deformed in accordance with the external force, and the filler particles bonded to the rubber molecule follow the deformation. It is easy to cut and easy to cut. Even if the external force is released, the cut rubber molecular chain cannot return to its original state, and as a result, it is considered that the hardness decreases. Further, if the particle size of the reinforcing filler with respect to the rubber component contained in the rubber composition and the chemical properties of the reinforcing filler surface are the same, the larger the amount of the reinforcing filler, the more the unit mass of the rubber composition Since the surface area of the reinforcing filler per unit area becomes larger, the number of rubber molecular chains bonded to the reinforcing filler per unit mass of the rubber composition is increased as described above. Although the interaction at the interface increases, the longer the deformation, the shorter the rubber molecular chain is cut, and even if the external force is released, the cut rubber molecular chain cannot return to its original state and the hardness decreases. A rubber component having a high glass transition point has a large hysteresis loss, and in this case as well, it is considered that the return when the deformation is released is poor and the hardness is reduced. However, the hardness of the tread rubber is within the range of 60 to 75 as described above so that a tire having high handling stability and riding comfort and low noise and vibration characteristics can be obtained in addition to the improvement of wet braking performance. It is preferable that it exists in. When using a reinforcing filler with a small particle size or increasing the blending amount, the hardness of the resulting tread rubber can be adjusted using a softening agent such as aroma oil or a plasticizer.

  The rubber component used in the tire tread of the present invention is composed of one or more crosslinkable diene rubbers. Examples of the crosslinkable diene rubber include natural rubber, butadiene rubber, isoprene rubber, styrene-butadiene rubber, butyl rubber, isobutylene-diene rubber, and ethylene-propylene rubber. When a blend of two or more rubber components is used, the blend ratio is not particularly limited.

  The tread rubber of the present invention typically contains a reinforcing filler capable of reinforcing a rubber composition such as silica and carbon black in a proportion of 40 to 120 parts by weight with respect to 100 parts by weight of the rubber component.

Silica typically has a nitrogen adsorption specific surface area (N ₂ SA) of 90-250 m ² / g. When N ₂ SA is less than 90 m ² / g, the filling amount can be increased, but the effect of improving the reinforcing property is poor, and the wet grip performance cannot be improved. Silica having N ₂ SA exceeding 250 m ² / g has high cohesiveness and is difficult to disperse in the rubber component. In the present specification, “nitrogen adsorption specific surface area (N ₂ SA)” means a specific surface area (unit m ² / g) measured according to JIS K6217-2. Silica is typically blended at 40 to 120 parts by weight with respect to 100 parts by weight of the rubber component. Typically, when silica is less than 40 parts by weight with respect to 100 parts by weight of the rubber component, a tire using the resulting rubber composition in the tread part exhibits insufficient wet grip performance. The silica used in the tread rubber of the present invention is not particularly limited to its production method.

  Examples of the carbon black that can be used in the rubber composition for a tire tread of the present invention include SAF, HAF, FEF, and ISAF grades that are usually used in a tire tread. Carbon black is typically preferably used in an amount of 40 to 120 parts by weight with respect to 100 parts by weight of the rubber component.

Carbon black typically has 80-200 m ² / g N ₂ SA. If the N ₂ SA of the carbon black is less than 80 m ² / g, there is a drawback that the rubber has insufficient reinforcement, and if it exceeds 200 m ² / g, the miscibility with other components at the time of blending deteriorates. There is a drawback.

  Examples of reinforcing fillers other than silica and carbon black include talc, calcium carbonate, aluminum hydroxide, and magnesium carbonate.

  The compounding amount of zinc oxide, which is a vulcanization acceleration aid, is typically 1 to 5 parts by weight with respect to 100 parts by weight of the rubber component.

  The tread rubber of the present invention may contain any of the silane coupling agents known in the art depending on the type of silica used. By blending the silane coupling agent with the rubber composition of the present invention, it is possible to further improve the processability when not vulcanized and the wet grip performance after vulcanization. The amount of the silane coupling agent is typically 6 to 10% by weight based on the total amount of the silica.

The tread rubber satisfying the above formula (I) can be obtained by combining these compounding agents. Since silica or carbon black, which is a reinforcing agent, increases in hardness when the amount is increased, it can be adjusted by adding a plasticizer such as aroma oil. When obtaining a tread rubber satisfying the above formula (II), the higher the glass transition point of the resulting rubber composition, the greater the decrease in hardness, and the greater the N ₂ SA of silica or carbon black as the reinforcing agent, The greater the compounding amount of the agent, the greater the degree of hardness reduction. The condition of formula (II) can be satisfied by combining a plurality of these characteristics.

  The tread rubber exhibits excellent wet grip performance when the requirement of the above formula (II) is satisfied in addition to the requirement of the above formula (I).

  In addition to the reinforcing filler, zinc oxide and silane coupling agent, the tread rubber of the present invention includes a vulcanization or cross-linking agent, a vulcanization or cross-linking accelerator, an anti-aging agent known in the art. Various compounding agents and additives such as a plasticizer and a softening agent can be blended by a general blending method in an amount generally used for the rubber component.

  The tread rubber of the present invention can be produced by a general mixing or kneading method and operating conditions using a commonly used mixing or kneading apparatus such as a Banbury mixer or a kneader. The rubber composition of the present invention is kneaded together with one or more kinds of rubber, a predetermined amount of reinforcing filler, a predetermined amount of zinc oxide, and other general rubber compounding agents, or a specific component rubber in advance. It may be produced by preparing a mixture (masterbatch) and then mixing or kneading with a given component.

  The present invention will be described in more detail with reference to the following examples and comparative examples, but it goes without saying that the technical scope of the present invention is not limited by these examples.

Test Methods The performances of the rubber compositions obtained in the following examples and comparative examples were determined by the following test methods.

(1) Hardness before stretching:
For a test piece having a diameter of 29 mm × thickness of 12.5 mm as fabricated, the hardness before elongation is measured at a temperature of 20 ° C. in accordance with a JIS K6253 type A durometer hardness test. Asked. This hardness corresponds to the hardness H ₁ and is represented as H _{1 in} Tables 1 to 3 below.

(2) Hardness difference before and after 100% elongation:
About a strip-shaped rubber test piece having a size of length 100 mm × width 5 mm × thickness 2 mm,
The hardness of the test piece was determined in accordance with JIS K6253 M method, and then both ends in the longitudinal direction of the rubber test piece were fixed to an extension jig, and the temperature was 20 ° C. to 100% for 30 to 60 seconds. The tensile force is released after 1 second from 100% elongation, the test piece is removed from the extension jig, and the hardness at 20 ° C. is measured at the center of the test piece in accordance with JIS K6253 M method. Asked. The hardness before stretching corresponds to the hardness H ₂ , and the hardness after 100% stretching corresponds to the hardness H ₃ . After obtaining the hardness H ₂ and H ₃ , the difference between H ₂ and H ₃ , that is, H ₂ −H ₃ was obtained.

(3) tan δ (0 ° C.):
For a test piece having a length of 20 mm, a width of 5 mm, and a thickness of 2 mm, using a rheograph solid manufactured by Toyo Seiki Seisakusho, in accordance with JIS K6394, frequency 20 Hz, initial elongation 10%, dynamic strain 2 %, The loss tangent tan δ (0 ° C.) was determined at a temperature of 0 ° C.

(4) Wet braking distance:
Tires of size 215 / 45R17 using the rubber compositions of Examples and Comparative Examples in the tread portion are mounted on a vehicle equipped with 2000 cc of ABS, the rim size is 17 × 7 JJ, and the air pressure of the front tire and the rear tire are both The braking stop distance from a speed of 100 km is measured on an asphalt surface sprayed at a water depth of 1 to 2 mm at 200 kPa. The braking stop distance of the comparative example A1 is measured in the example A group, and the braking stop distance of the comparative example B1 in the example B group. In the C example group, the braking stop distances obtained in the comparative examples and examples of the A to C example groups are represented by an index when the braking stop distance of the comparative example C1 is 100. The smaller the value, the shorter the braking distance and the better the wet grip performance.

Example A group
Comparative Examples A1-A3 and Examples A1-A3
Preparation of rubber composition:
For Comparative Examples A1 to A3 and Examples A1 to A3, according to Table 1 below, using a 1.8 liter closed Banbury mixer, materials other than the vulcanization accelerator and sulfur were mixed for 5 minutes, and the mixer was mixed at 150 ° C. After discharging from the reactor, it was cooled to room temperature. Thereafter, the mixture was again mixed for 3 minutes using a 1.8 liter closed Banbury mixer, released at 150 ° C., and then the vulcanization accelerator and sulfur were mixed with an open roll. Each of the obtained unvulcanized rubber compositions was vulcanized at 150 ° C. for 30 minutes using a predetermined mold to obtain test pieces for the above respective tests. Table 1 also shows the test results of the hardness H ₁ before stretching, the hardness difference H ₂ −H ₃ before and after 100% stretching, tan δ (0 ° C.), and the wet braking distance.

From Table 1, it can be seen that Examples A1 to A3 using SAF grade carbon black or silica having a large N ₂ SA have a large hardness difference and a small wet braking distance (index). Further, the FEF class carbon black having the smallest N ₂ SA has a large wet braking distance (index) even if the blending amount is increased.

Group B cases
Comparative Examples B1-B2 and Examples B1-B4
Preparation of rubber composition:
For Comparative Examples B1 to B2 and Examples B1 to B4, according to Table 2 below, using a 1.8 liter closed Banbury mixer, materials other than the vulcanization accelerator and sulfur were mixed for 5 minutes, and the mixer was mixed at 150 ° C. After discharging from the reactor, it was cooled to room temperature. Thereafter, the mixture was again mixed for 3 minutes using a 1.8 liter closed Banbury mixer, released at 150 ° C., and then the vulcanization accelerator and sulfur were mixed with an open roll. Each of the obtained unvulcanized rubber compositions was vulcanized at 150 ° C. for 30 minutes using a predetermined mold to obtain test pieces for the above respective tests. Table 2 also shows the test results of the hardness H ₁ before stretching, the hardness difference H ₂ −H ₃ before and after 100% stretching, tan δ (0 ° C.), and the wet braking distance.

From Table 2, it can be seen that in the combination of HAF grade carbon black with small N ₂ SA and SBR with low Tg, the hardness difference is small even if the blending amount of carbon black is increased (Comparative Example B1, Comparative Example B2). However, even when HAF grade carbon black is used, the hardness difference is large in combination with SBR having a high Tg (Example B1). It can be seen that silica with a large N ₂ SA provides a large hardness difference regardless of the type of polymer.

Example C group
Comparative Examples C1-C2 and Examples C1-C5
Preparation of rubber composition:
For Comparative Examples C1 to C2 and Examples C1 to C5, according to Table 2 below, a 1.8 liter closed Banbury mixer was used to mix materials other than the vulcanization accelerator and sulfur for 5 minutes, and the mixer at 150 ° C. After discharging from the reactor, it was cooled to room temperature. Thereafter, the mixture was again mixed for 3 minutes using a 1.8 liter closed Banbury mixer, released at 150 ° C., and then the vulcanization accelerator and sulfur were mixed with an open roll. Each of the obtained unvulcanized rubber compositions was vulcanized at 150 ° C. for 30 minutes using a predetermined mold to obtain test pieces for the above respective tests. Table 3 also shows the test results of the hardness H ₁ before stretching, the hardness difference H ₂ -H ₃ before and after 100% stretching, tan δ (0 ° C.), and the wet braking distance.

From Table 3, the combination of HAF grade carbon black with a small N ₂ SA and SBR with a low Tg has a small hardness difference (Comparative Example C1, Comparative Example C2), and when SBR with a high Tg is blended or replaced entirely, the hardness difference is large. (Example C3, Example C4). SAF grade carbon black having a large N ₂ SA has a large hardness difference even in combination with SBR having a low Tg (Example C1, Example C2). It can also be seen that the hardness difference is large even when blending SBR with low Tg and natural rubber.

Claims (1)

A pneumatic tire using a tread rubber, which was obtained according to a JIS K6253 type A durometer hardness test at a temperature of 20 ° C. for a first test piece taken from a new or unused tread rubber. A second test piece having a length of 100 mm, a width of 5 mm, and a thickness of 2 mm separately from the first test piece, the hardness of which is expressed as H ₁ and collected from the new or unused tread rubber, the hardness was determined in accordance with method M in JIS K6253 in the central portion of the specimen represented as H _2, after obtaining the H _2, the second test piece, the tensile force is stretched 100% at a temperature 20 ° C. When the hardness determined in accordance with JIS K6253 method M at the center of the second test piece is expressed as H ₃ after the release of the following formulas (I) and (II):
60 ≦ H ₁ ≦ 75 (I)
H ₂ −H ₃ ≧ 4 (II)
A pneumatic tire using a tread rubber characterized by satisfying