GB2158076A - Rubber compositions - Google Patents

Abstract

A rubber composition having a highly satisfactory balance of wet skid resistance, ice skid resistance and rolling resistance comprises at least 20% by weight of an at least partially coupled styrene-butadiene block copolymer composed of (A) a styrene-butadiene copolymer block and (B) a butadiene homopolymer block and satisfying certain defined requirements of styrene content, vinyl group content and glass-transparent temperatures. Such a rubber composition is particularly intended for use in a tire tread.

Description

SPECIFICATION Rubber compositions This invention relates to a rubber composition which is particularly suitable for use in a tire tread, which composition comprises a block copolymer produced by starting from styrene and butadiene and composed of two different copolymer blocks.

Lately, requirements for running stability and low fuel consumption in automobiles have become severe.

In this connection, it has strongly been desired to develop rubber compositions having not only fracture properties such as wear resistance, which had previously been required in rubber materials for tire treads, but also having good wet skid resistance and ice skid resistance and low rolling resistance. However, it is difficult to simultaneously satisfy the above properties because there is in particular a conflicting relation between wet skid resistance, ice skid resistance and rolling resistance.

As is well known, polymers have a relatively high glass transition temperature (Tg) such as emulsified SBR have a good wet skid resistance, while polymers having a low Tg such as high cis 1,4-BR have good ice skid resistance and rolling resistance. The ice skid resistance is a property required at low temperature and the rolling resistance is a property required at a temperature above room temperature. Therefore, in order to satisfy the severe requirements presently demanded of tires, lowering the Tg is required for providing good ice skid resistance and rolling resistance but this is not sufficient.That is, when the above properties are indicated by viscoelastic index, the wet skid resistance is expressed as tan 5 near 0 C (the larger the value, the better), the ice skid resistance as glass transition temperature (the smaller the value, the better) and the rolling resistance as tan 5 at a temperature of 50-70"C (the smaller the value, the better), so that conflict occurs in attempting to simultaneously satisfy the required properties. Therefore, it has been attempted to harmonize the above conflicting properties by blending different polymers with each other. As a result, some improvements have been obtained but have not yet satisfied all of the presently required performances.

Recently, it has been attempted to modify SBR, BR, and the like produced mainly by using an organolithium initiator. As a result, the balance between the wet skid resistance and the rolling resistance in SBR and BR having a relatively high content of vinyl bonds contained in butadiene (abbreviated as BD hereinafter) units is considerably improved as compared with conventional emulsified SBR, natural rubber and high cis 1,4-BR (Japanese Patent laid open No. 54-62,248, No. 56110,753 and British Patent No.

1,166,832). However, the above polymers have poor ice skid resistance.

The present inventors have made various studies in order to further improve the wet skid resistance and the rolling resistance in styrene-butadiene copolymers produced by using an organolithium initiator and have found that a block copolymer composed of two block portions different from each other in the content of bound styrene and content of vinyl bonds contained in BD units, or a branched copolymer thereof improves the above properties, and also a peak of a temperature-tan 5 dispersion curve as measured by a dynamic measurement becomes broad because the two block portions having different glass transition temperatures become compatible with each other through a vulcanization step (Japanese Patent laid open No. 57-109,817 and No.57-200,439).

The inventors have further made various investigations with respect to the phenomenon that the peak of the temperature - tan 5 dispersion curve becomes broad and have found that the balance of the wet skid resistance, the ice skid resistance and the rolling resistance can be maintained by making the difference in glass transistion temperature (Tg) between the block portions of the polymer at least a predetermined value.

According to the invention, there is provided a rubber composition comprising at least 20% by weight of a styrene-butadiene block copolymer produced by copolymerization of styrene and butadiene and satisfying the following requirements: (i) a copolymer block (A) is a styrene-butadiene copolymer block having a content of bound styrene of 10-80% by weight and an average content of vinyl bonds contained in the butadiene units of 30-70% by weight; (ii) a copolymer block (B) is a polybutadiene block having an average content of vinyl bonds contained in butadiene units of not more than 60%; (iii) each of the copolymer blocks (A) and (B) is included in an amount of at least 20% by weight;; (iv) the average content of vinyl bonds in the copolymer block (A) is higher by at least 5% by weight than the average content of vinyl bonds in the copolymer block (B); (v) the copolymer block (A) has a glass transition temperature higher by at least 30"C than that of the copolymer block (B); (vi) the total content of bound styrene in the said block copolymer is 5-40% by weight; (vii) at least 20% by weight of the block copolymer is a branched block copolymer coupled with a coupling agent having 3-4functionalities; and (viii) a distribution width of the content of vinyl bonds in each of the copolymer blocks (A) and (B) in the block copolymer is at least 10%.

The rubber composition according to the invention is different from those disclosed in Japanese Patent laid open No. 57-102,912 and No. 57-109,817 in the following respects: (1) it contains a block copolymer having a branched block copolymer; and (2) there are differences between certain properties of the respective block portions (A) and (B), and has excellent fracture properties.

In Japanese Patent laid open No. 57-200,439, excellent wet skid resistance, rolling resistance and fracture properties are obtained by including carbon-metal bonds in the block copolymer. However, the rubber composition according to the invention has a much better balance between the ice skid resistance and the wet skid resistance as compared with the block copolymer disclosed in Japanese Patent laid open No.

57-200,439. This balance can be obtained by making the distribution width of the content of vinyl bonds in each of the copolymer blocks (A) and (B) at least 10%.

Moreover, properties similar to those of the rubber composition according to the invention can be obtained by properly selecting the content of bound styrene and the content of vinyl bonds contained in butadiene units in each of the block portions in the block copolymer of Japanese Patent laid open No.

57-200,439, but the balance between the ice skid resistance and the wet skid resistance is not sufficient.

Therefore, in the rubber composition according to the present invention, these properties were improved by giving a certain distribution width of the content of vinyl bonds to each of the block portions. Although the cause of the improving effect is not clear, it is considered that the broadening of the peak of temperature-tan 5 dispersion curve after the vulcanization of the block copolymer is further promoted by extending the temperature range of the Tg region in the copolymer blocks (A) and (B) and the rate of change of modulus of elasticity (E') at the Tg region became apparent, which effectively act to improve the wet skid resistance measured at room temperature and the ice skid resistance measured at about -10 C.

Also, the use of a metal such as Si, Ge or Pb (other than Sn) as the coupling agent does not sufficiently satisfy the ice skid resistance, the rolling resistance and the fracture properties.

That is, it is well-known that the fracture properties are improved by giving a certain distribution width of content of vinyl bonds to at least one of the copolymer blocks in a block copolymer (Japanese Patent laid open No. 57-102,912) or by introducing carbon-metal (Sn) bonds into the molecular chain (Japanese Patent laid open No. 57-200,439). However, it is not readily predictable therefrom that a block copolymer can be obtained having an improved balance of wet skid resistance, rolling resistance and fracture properties.

Furthermore, Japanese Patent laid open No. 57-165,445 discloses a rubber composition having excellent rebound resilience, resistance to heat build-up and wear resistance, comprising a block copolymer composed of two block portions having different contents of bound styrene, a part of which being a branched copolymerformed by a coupling reaction. However, such a rubber composition cannot achieve the balance of properties aimed at by the present invention, particularly the balance between the wet skid resistance and the ice skid resistance.

According to the invention, an excellent balance of wet skid resistance, ice skid resistance and rolling resistance is attained by using a block copolymer in which two copolymers (A) and (B) having different monomer compositions and having a certain difference between their glass transition temperatures are block-polymerized, a part of which being branched by a coupling reaction.

The properties aimed at by the present invention cannot be obtained by simply blending the copolymer (A) with the copolymer (B).

The block copolymer of the rubber composition according to the invention can be produced through adiabatic or exothermic (temperature-increasing) polymerization in the presence of a polar compound such as ether or a tertiary amine using an organolithium initiator. First, either the copolymer block (A) or the copolymer block (B) is polymerized, after which the remaining block is polymerized and then a coupling agent is added thereto to produce a block copolymer including a branched copolymer. The content of vinyl bonds contained in the butadiene units can be controlled by varying the amount of the polar compound in the polymerization system or the polymerization temperature in adiabatic or exothermic polymerization.

Further, the distribution width of the content of vinyl bonds contained in the butadiene units in each block of the block copolymer can be controlled by the polymerization temperature range in the adiabatic or exothermic polymerization.

According to the invention, there are used coupling agents having 3-4 functionalities, and among them a tin halide is particularly preferred. In the case of using a tin halide, it is most preferable that the bonds in the molecular chain are butadienyl-Sn bonds. Suitable tin halides are tin tetrachloride, tin methyl trichloride, tin butyl trichloride, tin phenyl trichloride, tin diphenyl chloride, tin triphenyl chloride and compounds replacing chlorine by fluorine or iodine. In the block copolymer of the rubber composition according to the invention, the content of the branched block copolymer is not less than 20% by weight, preferably not less 30% by weight. When the content of the branched block copolymer is less than 20% by weight, the fracture properties and ice skid resistance of the resulting rubber composition are adversely affected.

The upper limit of the content of the branched block copolymer in the block copolymer of the rubber composition according to the invention is not particularly restricted, but it is industrially very difficult to be not less than 80% by weight. In order to produce more butadienyl-Sn bonds in the production of the block copolymer of (BR-SBR)n - Sn type, it is preferable to add a small amount of butadiene prior to the coupling reaction.

The content of bound styrene in the copolymer block (A) is 1080% by weight, preferably 20-80% by weight, more preferably 25-75% by weight, and the total content of bound styrene in the block copolymer is 5-40% by weight, preferably 10-40% by weight, more preferably 1535% by weight. When the content of bound styrene in the copolymer block (A) is less than 10% by weight, the wet skid resistance becomes insufficient. On the other hand, when the content of bound styrene in the copolymer block (A) exceeds 80% by weight, the rolling resistance and fracture properties are adversely affected. The copolymer block (B) contains no bound styrene.If the copolymer block (B) contains bound styrene, the balance between the wet skid resistance and the ice skid resistance becomes poor. When the total content of bound styrene in the block copolymer is less than 5% by weight, the fracture properties and the ice skid resistance are poor, while when the total content exceeds 40% by weight, the rolling resistance and ice skid resistance are unfavourably poor.

In the copolymer block (A) of the block copolymer of the rubber composition according to the invention, when a polystyrene block consisting of a styrene sequence of 9 or more styrene units is 2-35% by weight, more preferably 3-25% by weight, of the content of bound styrene in the copolymer block (A), the balance between the wet skid resistance and the ice skid resistance becomes particularly good.

Also, when the bonding site between the copolymer block (A) and the copolymer block (B) in the block copolymer of the rubber composition according to the invention contains a polystyrene block, the wet skid resistance and the ice skid resistance are improved.

The term "polystyrene block" means a block composed of styrene sequence of 9 or more styrene units and can be determined by means of the following measurement. A solution of the block copolymer in carbon tetrachloride (20 mg/0.4 mll Cm14) is prepared and then the styrene peak area thereof is measured by means of a 1H-NMR apparatus (made by JEOL, Ltd., FX-1 00), from which a weight fraction of the polystyrene block is calculated according to the following equation.

Weight fraction of Peak area at 6.5 ppm polystyrene block = --- --- - x 100 Peak area at 6.5 ppm + Peak area at 7.1 ppm In order to produce the above polystyrene block, there is adopted a method wherein the amount of randomizing agent such as ether or a tertiary amine is controlled in accordance with the charging ratio of styrene/butadiene in the polymerization of the copolymer block (A), or a method wherein styrene is polymerized at the latter half or after the completion of polymerization of the copolymer block (A) and then polymerization of the copolymer block (B) is performed. In view of industrial production, however, the former method is more preferable.

The average content of vinyl bonds contained in butadiene units is 30-70% by weight, preferably 35-70% by weight, in the copolymer block (A), and not more than 60% by weight, preferably not more than 55%by weight, in the copolymer block (B).

The distribution width of vinyl bonds in each block is essential for improving the wet skid resistance, the ice skid resistance and the fracture properties. The distribution width is rrequired to be not less than 10%, preferably not less than 15%. In this case, the distribution width of vinyl bonds can be obtained by extending the temperature range of adiabatic polymerization utilizing the fact that the content of vinyl bonds contained in butadiene units is varied by the polymerization temperature in the production of a butadiene-series copolymer in the presence of a catalyst consisting of an organolithium compound and a conventional polar compound such as ether.When adiabatic polymerization is carried out within a range 40 C-80 C as shown in the accompanying drawing, which is a graph showing the relation between the polymerization temperature and the microstructure in the polymerization of the block copolymer sample Exp-1 described subsequently, the difference between the contents of vinyl bonds at temperatures of 40"C and 80"C provides a distribution width of vinyl bonds.

Further, the copolymer block (A) has an average content of vinyl bonds higher by at least 5% by weight than that of the copolymer block (B).

A content of vinyl bonds in the copolymer block (A) of less than 30% weight unfavourably affects the difference in glass transition temperatures between the copolymer blocks (A) and (B), and unsatisfactorily affects the balance between the rolling resistance and the wet skid resistance. On the other hand, when the content of vinyl bonds exceeds 70% by weight, the fracture properties become poor. The total content of vinyl bonds in the block copolymer of the rubber composition according to the invention is not particularly critical, but it is preferably within a range of 25-60% by weight. Outside this range, the balance between the wet skid resistance and the rolling resistance becomes poor.

When the difference in the average content of vinyl bonds between the copolymer blocks (A) and (B) is outside the range defined in the invention, the broadening of the temperature distribution peak for tan 5 is degraded and also the balance of physical properties becomes poor.

In the block copolymer of the rubber composition according to the invention, it is required that the copolymer block (A) has a glass transition temperature (Tg) higher by at least 30"C, preferably not less than 40"C, than that of the copolymer block (B). When the Tg difference is less than 30"C, the balance between the wet skid resistance and the ice skid resistance cannot be maintained.

The block copolymer of the rubber composition according to the invention contains at least 20% by weight, preferably not less than 30% by weight, of each of the copolymer block (A) and the copolymer block (B). When the content is less than 20% by weight, the balance between the wet skid resistance and the ice skid resistance becomes unsatisfactorily poor.

The Mooney viscosity (ML1 $4) of the block copolymer of the rubber composition according to the invention is preferably within a range of 20-150. When this value is less than 20, the rolling resistance may become poor, while when the value exceeds 150, the processability may become poor.

The block copolymer is blended with at least one rubber suitably selected from natural rubber, cis 1,4-polyisoprene rubber, polybutadiene rubber, other styrene-butadiene copolymer rubbers, ethylenepropylene-diene (EPDM) terpolymer rubber, butyl rubber, halogenated EPDM and halogenated butyl rubber.

The content of the block copolymer in the rubber composition according to the invention is required to be at least 20% by weight, preferably not less than 30% by weight. When the content is less than 20% by weight, the desired properties of wet skid resistance, ice skid resistance and rolling resistance cannot be simultaneously satisfied.

The rubber composition containing the block copolymer produced in accordance with the invention has good wet skid resistance, ice skid resistance and rolling resistance and excellent fracture properties, so that it can preferably be used as rubber material for tire treads. In this case, there may be used various additives conventionally used, for example carbon black, process oils (aromatic oil, naphthenic oil, paraffinic oil and the like), filiers, vulcanization accelerators and vulcanizing agents.

The invention will be further described with reference to the following illustrative Examples.

Examples 1-10, Comparative Examples 1-15 Samples shown in the following Table 1 were obtained in accordance with the polymerizing conditions shown in the following Table 2.

First of all, polymerization was conducted under the polymerization condition for the copolymer block (A) by using cyclohexane as a solvent and n-butyl lithium as a polymerization initiator. TThe temperature condition was adiabatic without heating or cooling. Then, after the degree of conversion reached 95-100%, a predetermined amount of monomer was additionally charged to perform the adiabatic polymerization under the polymerization condition for the copolymer block (B). After the completion of polymerization, a given coupling agent was added. With respect to the sample Exp-19, polymerization was conducted in the order of the copolymer block (B) - the copolymer block (A), after which 5 g of butadiene was added to conduct the coupling reaction.The sample Exp. 22 was produced in the same manner as Exp-1 9, except that 5 g of butadiene was not added.

The contents of bound styrene and vinyl bond in the polymer were measured by means of an infrared spectrophotometer.

The content of styrene sequence of 9 or more styrene units in the copolymer block (A) was measured from the sample produced in the first polymerization step by means of 'H-NMR. Moreover, when the degree of conversion at the first step was 80%, the ratio of the styrene sequence of 9 or more styrene units in the styrene-butadiene copolymer was 0 in all samples and the polystyrene block was included in the bonding site between the copolymer block (A) and the copolymer block (B). In the polymer of sample Exp-1 9, the polystyrene block was formed in the latter half of the second polymerization step.

The content of the branched copolymer was measured by means of gel permeation chromatography (GPC) [GPC 200 type made by WATERS Inc., column: STyRAGEL-106, 106,106, 104(4feet x 4), solvent: tetrahydrofuran].

The drawing shows the relation between the polymerization temperature and the microstructure in the polymerization for the sample Exp-1. As seen from this figure, the distribution width of the content of vinyl bond in the copolymer block (A) is 20% and the distribution width of the content of vinyl bond in the copolymer block (B) is 19%. Simiiarly, the distribution width of the content of vinyl bond in the other samples were measured.

Each of samples Exp-l -20 was used to prepare a rubber composition in accordance with the compounding recipe shown in the following Table 3. In this case, the vulcanization was under conditions of a temperature of 145"C and a time of 30 minutes. As a result, the properties of the resulting vulcanizate are shown in the following Table 4, wherein wet skid resistance value measured (at room temperature) by a wet skid resistance meter made by Stanley Works Corp. and tan 8 are used as the indication of the wet skid resistance, ice skid resistance value measured (at temperature of -10"C) by the above skid resistance meter is used as the indication of the ice skid resistance, tan Sat 50"C is used as the indication of the rolling resistance, and tensile strength is used as the indication of the fracture properties.

The samples of Examples 1-10 have an excellent balance of wet skid resistance, ice skid resistance and rolling resistance as compared with Comparative Examples 1-15, and have good fracture properties and wear resistance.

Table 1(a) Block (A) Styrene Glass Content Block (B) Wholle polymer Content Distri- sequence transi- of Distri- Glass Content Content of bution of 9 or tion vinyl Bound bution transi- of of Mooney Ratio Coupl vinyl Bound width of more temper- bon sty- width of tion vinyl Bound branched viscosity of ing bond sty- vinyl styrene ature in BD rene vinyl temper- bond sty- polymer ML 100 C (A) (B) agent in BD rene bond in units ( C) units (%) bond in ature in BD rene (%) 1+4) units (%) BD units (%) (%) BD units ( C) units (%) (%) (%) (%) (%) Exp-1 55 40 20 9 -30 40 0 19 -87 46 20 61 58 1/1 SnCl4 Exp-2 50 30 20 6 -46 38 0 12 -88 47 20 59 57 2/1 SnCl4 Exp-3 65 35 19 5 -28 52 0 19 -77 57 18 63 64 1/1 " Exp-4 45 47 21 16 -34 37 0 18 -89 40 23 62 59 1/1 " Exp-5 50 40 20 0 -31 40 0 19 -87 44 20 65 60 1/1 " Exp-6 55 40 20 10 -30 39 0 19 -88 46 20 60 59 1/1 SiCl4 Exp-7 54 40 19 9 -31 40 0 19 -87 46 20 0 63 1/1 Exp-8 55 40 28 10 -30 39 0 5 -88 53 36 62 60 9/1 SnCl4 Exp-9 54 41 3 9 -31 40 0 32 -88 40 4 65 59 1/9 " Exp-10 41 82 13 35 0 34 0 12 -92 35 41 60 63 1/1 " Exp-11 54 8 16 0 -68 38 0 15 -87 46 4 61 60 1/1 " Exp-12 55 25 20 3 -50 37 20 17 -71 45 25 58 60 1/1 " Table 1(b) Block (A) Styrene Glass Content Block (B) Wholle polymer Content Distri- sequence transi- of Distri- Glass Content Content of bution of 9 or tion vinyl Bound bution transi- of of Mooney Ratio Coupl vinyl Bound width of more temper- bon sty- width of tion vinyl Bound branched viscosity of ing bond sty- vinyl styrene ature in BD rene vinyl temper- bond sty- polymer ML 100 C (A) (B) agent in BD rene bond in units ( C) units (%) bond in ature in BD rene (%) 1+4) units (%) BD units (%) (%) BD units ( C) units (%) (%) (%) (%) (%) Exp-13 25 40 19 18 -59 40 0 20 -87 34 20 61 58 1/1 SnCl4 Exp-14 75 31 21 2 -20 55 0 23 -75 63 16 64 62 1/1 " Exp-15 61 40 18 6 -23 75 0 28 -51 70 20 58 61 1/1 " Exp-16 41 60 14 25 -20 32 40 16 -52 36 50 56 59 1/1 " Exp-17 54 41 0 8 -32 38 0 0 -90 44 21 60 59 1/1 " Exp-18 55 40 19 11 230 55 0 16 -75 50 20 57 56 1/1 " Exp-19 55 40 20 10 -30 40 0 17 -87 46 20 60 59 1/1 " Exp-20 55 40 20 9 -30 - - - - 55 40 61 58 1/0 " Exp-21 - - - - - 39 0 18 -88 39 0 58 57 0/1 " Exp-22 55 40 20 10 -30 40 0 17 -87 46 20 59 58 1/1 " Exp-23 53 40 17 7 -31 39 0 0 -90 44 20 59 58 1/1 " Table 2(a) Solvent : cyclohexane 2,000g Block (A) Block (B) Coupling Polymerization condition Polymerization condition agent Experi- n-Butyl Butadienel Tetra- Potassium Polymeri- Polymeri- (A) (B) mental lithium styrene hydrofuran dodecyl- zation Butadienel Tetra- zation amount (weight No. (g) (g) (g) benzene tempera- styrene hydrofuran tempera- Kind added ratio) sulfonate ture (g) (g) ture (g) (g) ( C) ( C) Exp-1 0.35 150/100 15 - 35-65 250/0 - 65-100 SnCl4 0.22 1/1 Exp-2 0.33 200/133 15 - 40-80 167/0 - 80-103 " 0.23 2/1 Exp-3 0.35 162/88 15 - 15-45 250/0 - 45-80 " 0.22 1/1 Exp-4 0.37 132/118 12 - 35-65 250/0 - 65-100 " 0.23 1/1 Exp-5 0.38 150/100 15 0.15 40-70 250/0 - 70-105 " 0.21 1/1 Exp-6 0.33 150/100 15 - 35-65 250/0 - 65-100 SiCl4 0.14 1/1 Exp-7 0.26 150/100 15 - 35-65 250/0 - 65-100 - - 1/1 Exp-8 0.35 270/180 15 - 25-75 50/0 - 75-82 SnCl4 0.22 9/1 Exp-9 0.34 30/20 15 - 46-52 450/0 - 52-115 " 0.22 1/9 Exp-10 0.37 45/205 10 - 50-75 250/0 - 75-110 " 0.22 1/1 Exp-11 0.32 230/20 15 - 37-72 250/0 - 70-105 " 0.21 1/1 Table 2(b) Solvent : cyclohexane 2,000g Block (A) Block (B) Coupling Polymerization condition Polymerization condition agent Experi- n-Butyl Butadienel Tetra- Potassium Polymeri- Polymeri- (A) (B) mental lithium styrene hydrofuran dodecyl- zation Butadienel Tetra- zation amount (weight No. (g) (g) (g) benzene tempera- styrene hydrofuran tempera- Kind added ratio) sulfonate ture (g) (g) ture (g) (g) ( C) ( C) Exp-12 0.33 187/63 15 - 35-65 200/50 - 65-97 SnCl4 0.22 1/1 Exp-13 0.34 150/100 10 - 70-110 250/0 - 60-100 " 0.22 1/1 Exp-14 0.33 175/75 15 - 5-35 250/0 - 35-75 " 0.22 1/1 Exp-15 0.35 160/100 15 - 20-50 250/0 - 10-50 " 0.22 1/1 Exp-16 0.36 100/150 15 - 55-81 150/0 - 81-116 " 0.23 1/1 Exp-17 0.33 150/100 15 - 50 250/0 - 82 " 0.22 1/1 Exp-18 0.32 150/100 15 - 35-65 250/0 - 35-75 " 0.21 1/1 Exp-19 0.35 150/100 15 - 33-63 250/0 - 67-103 " 0.22 1/1 Exp-20 0.34 300/200 15 - 33-65 - - - " 0.22 1/0 Exp-21 0.32 - - - - 500/0 - 65-100 " 0.21 0/1 Exp-22 0.35 150/100 15 - 33-63 250/0 - 67-103 " 0.22 1/1 Exp-23 0.34 150/100 15 - 40-79 250/0 - 83 " 0.22 1/1 Table 3 Parts by weight Polymer 100 Carbon black HAF 50 Aromatic oil 10 Stearic acid 2 Zinc white 3 Antioxidant 810 NA*1 Vulcanization accelerator CZ *2 0.6 Vulcanization accelerator M *3 0.6 Vulcanization accelerator D *4 0.4 Sulfur 1.5 *1 N-phenyl-N'-isopropyl-p-phenylenediamine *2 N-cyclohexyl-2-benzothiazolylsulfeneamide *3 2-mercaptobenzothiazole *4 1,3-diphenylguanidine Table 4(a) Polymer *2 *1 Emulsion *3 *3 *4 *4 *4 NR cis BR polymer- Elonga- Tensile *5 Wet lce Lanbourn tan # (Part) (part) ized SBR tion strength tans # skid skid wear at 0 C Sample (part) (part) (%) (kgflcm) at 50 C (index) (index) (index) Example 1 Exp-1 50 50 - - 510 280 0.110 110 110 110 0.25 Example 2 Exp-2 " " - - 515 272 0.108 109 109 112 0.24 Example 3 Exp-3 " " - - 505 275 0.115 112 106 105 0.29 Example 4 Exp-4 " " - - 510 278 0.111 108 115 108 0.28 Example 5 Exp-5 " " - - 505 277 0.102 105 105 108 0.25 Example 6 Exp-1 " 25 25 - 480 265 0.113 107 112 109 0.25 Example 7 Exp-1 " - 25 25 480 262 0.114 108 104 104 0.28 Example 8 Exp-6 " 50 - - 510 266 0.125 110 99 100 0.27 Comparative Exp-7 " " - - 505 262 0.128 108 100 93 0.28 Example 1 Comparative Exp-8 " " - - 500 280 0.138 145 60 100 0.58 Example 2 Comparative Exp-9 " " - - 480 241 0.109 80 120 90 0.16 Example 3 Comparative Exp-10 " " - - 450 250 0.139 128 85 91 0.47 Example 4 Table 4(b) Polymer *2 *1 Emulsion *3 *3 *4 *4 *4 NR cis BR polymer- Elonga- Tensile *5 Wet lce Lanbourn tan # (Part) (part) ized SBR tion strength tans # skid skid wear at 0 C Sample (part) (part) (%) (kgflcm) at 50 C (index) (index) (index) Comparative Exp-11 50 50 - - 475 250 0.106 84 116 97 0.18 Example 5 Comparative Exp-12 " " - - 510 275 0.110 100 100 100 0.22 Example 6 Comparative Exp-13 " " - - 500 270 0.105 94 110 98 0.27 Example 7 Comparative Exp-14 " " - - 475 255 0.113 125 88 92 0.29 Example 8 Comparative Exp-15 " " - - 470 258 0.114 135 87 85 0.48 Example 9 Comparative Exp-16 " " - - 460 252 0.140 140 71 88 0.47 Example 10 Comparative Exp-17 " " - - 490 260 0.112 108 100 103 0.24 Example 11 Comparative Exp-18 " " - - 495 272 0.109 114 96 105 0.29 Example 12 Example 9 Exp-19 " " - - 510 268 0.103 108 105 107 0.27 Table 4(c) Polymer *2 *1 Emulsion *3 *3 *4 *4 *4 NR cis BR polymer- Elonga- Tensile *5 Wet lce Lanbourn tan # (Part) (part) ized SBR tion strength tans # skid skid wear at 0 C Sample (part) (part) (%) (kgflcm) at 50 C (index) (index) (index) Comparative Exp-20/ 25/25 50 - - 470 254 0.114 112 97 97 0.35 Example 13 21 Comparative - - " - 50 520 285 0.140 85 115 115 0.17 Example 14 Example 10 Exp-22 50 " - - 500 264 0.115 108 104 105 0.27 Comparative Exp-23 " " - - 495 274 0.110 109 101 104 0.24 Example 15 *1 BR01 made by Japan Synthetic Rubber Co., Ltd.

*2 SBR1500 made by Japan Synthetic Rubber Co., Ltd.

*3 According to JISK 6301 *4 Representing an index on a basis that a value of Comparative Example 6 was 100. The larger the index value, the better the properties *5 Using a viscoelastic spectrometer made by lwamoto Seisakusho frequency : 10 Hz static strain : 5 % dynamic strain : 0.2-0.5 %

Claims (6)

1. A rubber composition comprising at least 20% by weight of a styrene-butadiene block copolymer produced by copolymerization of styrene and butadiene and satisfying the following requirements: (i) a copolymer block (A) is a styrene-butadiene copolymer block having a content of bound styrene of 10-80% by weight and an average content of vinyl bonds contained in butadiene units of 30-70% by weight; (ii) a copolymer block (B) is a polybutadiene block having an average content of vinyl bonds contained in butadiene units of not more than 60%; (iii) each of the copolymer blocks (A) and (B) is included in an amount of at least 20% by weight; (iv) the average content of vinyl bonds in the copolymer block (A) is higher by at least 5% by weight than the average content of vinyl bonds in the copolymer block (B);; (v) the copolymer block (A) has a glass transition temperature higher by at least 30"C than that of the copolymer block (B); (vi) the total content of bound styrene in the said block copolymer is 5-40% by weight; (vii) at least 20% by weight of the said block copolymer is a branched block copolymer coupled with a coupling agent having 3-4 functionalities; and (viii) a distribution width of the content of vinyl bonds in each of the copolymer blocks (A) and (B) in the said block copolymer is at least 10%.

2. A rubber composition as claimed in claim 1, wherein in the bound styrene portion of the said copolymer block (A) the content of polystyrene block composed of styrene sequences of 9 or more styrene units is 2-35% by weight.

3. A rubber composition as claimed in claim 1 or 2, wherein in the said block copolymer the bonding site between the copolymer block (A) and the copolymer block (B) includes a polystyrene block.

4. A rubber composition as claimed in any of claims 1 to 3, wherein the Mooney viscosity (ML 5 v) of the said block copolymer is 20-150.

5. A rubber composition according to claim 1, substantially as herein described in any of the foregoing Examples.

6. A pneumatic tire the tread of which is at least partly composed of a rubber composition as claimed in any of claims 1 to 5.