Rubber compostion and pneumatic tire using the same

Abstract

This invention provides a rubber composition being excellent in the workability during production and the heat resistance and high in the storage elastic modulus (G) and small in the loss tangent (tan ), which is formed by compounding 1-60 parts by mass of a low-molecular weight conjugated diene-based polymer (B) having a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 10,000-200,000 and a vinyl bond content in a conjugated diene compound portion of not less than 40% based on 100 parts by mass of a rubber component (A) comprised of at least one selected from the group consisting of natural rubber, polyisoprene rubber, styrene-butadiene copolymer rubber, polybutadiene rubber and isobutylene isoprene rubber, wherein the rubber component (A) comprises natural rubber and/or polyisoprene rubber and a proportion of styrene units in total of the low-molecular weight conjugated diene-based polymer (B) is less than 5% by mass.

Claims

1. A rubber composition characterized by compounding 1-60 parts by mass of a low-molecular weight conjugated diene-based polymer (B) synthesized through an anionic polymerization and having a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 40,000-200,000 and a vinyl bond content in a conjugated diene compound portion of not less than 55% based on 100 parts by mass of a rubber component (A) having a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of more than 200,000, wherein the rubber component (A) comprises natural rubber and polybutadiene rubber, an amount of the aromatic vinyl compound bonded in the low-molecular weight conjugated diene-based polymer (B) is less than 5% by mass; wherein the rubber composition does not comprise polyisoprene rubber; and wherein a proportion of styrene units in total of the rubber component (A) is less than 15% by mass.

2. A rubber composition according to claim 1, wherein the low-molecular weight conjugated diene-based polymer (B) has a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 40,000-150,000.

3. A rubber composition according to claim 2, wherein the low-molecular weight conjugated diene-based polymer (B) has a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 40,000-100,000.

4. A rubber composition according to claim 1, wherein the low-molecular weight conjugated diene-based polymer (B) is polybutadiene.

5. A rubber composition according to claim 4, wherein the low-molecular weight conjugated diene-based polymer (B) is polybutadiene having a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 40,000-150,000.

6. A rubber composition according to claim 1, wherein carbon black or silica is further compounded.

7. A pneumatic tire characterized by using a rubber composition as claimed in claim 1 in a tread portion.

8. A pneumatic tire characterized by using a rubber composition as claimed in claim 1 in a sidewall portion.

9. A pneumatic tire characterized by using a rubber composition as claimed in claim 1 in at least any one of an innerliner, a ply-coating rubber and a bead filler.

Description

BEST MODE FOR CARRYING OUT THE INVENTION

(1) The invention will be described in detail below. The rubber composition according to the invention is characterized by compounding 1-60 parts by mass of a low-molecular weight conjugated diene-based polymer (B) having a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 10,000-200,000 and a vinyl bond content in a conjugated diene compound portion of not less than 40% based on 100 parts by mass of a rubber component (A) comprised of at least one selected from the group consisting of natural rubber, polyisoprene rubber, styrene-butadiene copolymer rubber, polybutadiene rubber and isobutylene isoprene rubber,

(2) wherein the rubber component (A) comprises natural rubber and/or polyisoprene rubber, and

(3) a proportion of styrene units in total of the low-molecular weight conjugated diene-based polymer (B) is less than 5% by mass.

(4) In general, a diene-based rubber containing a double bond in its main chain such as natural rubber, polyisoprene rubber or the like as a rubber component (A) is easily broken at a higher temperature and is low in the heat resistance. However, the rubber composition according to the invention is compounded with the low-molecular weight conjugated diene-based polymer (B) having a high vinyl bond content in the conjugated diene compound portion, so that there are effects in which a crosslinked sulfur broken by exposing to a higher temperature forms re-crosslinking with carbon in an allyl site existing in the side chain of the low-molecular weight conjugated diene-based polymer (B), and so on. Therefore, the re-crosslinking of the low-molecular weight conjugated diene-based polymer (B) with the crosslinked sulfur can maintain a three-dimensional network structure to improve the heat resistance of the rubber composition. Also, the inventors have made studies and found that when the low-molecular weight conjugated diene-based polymer (B) having a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 10,000-200,000, a vinyl bond content in a conjugated diene compound portion of not less than 40% and a proportion of styrene units in total thereof of less than 5% by mass is compounded into a rubber composition instead of a part or a whole of a softening agent included in the compounding, the storage elastic modulus (G) is improved and the loss tangent (tan ) can be reduced without deteriorating the workability of the rubber composition.

(5) The rubber component (A) in the rubber composition according to the invention is at least one selected from the group consisting of natural rubber, polyisoprene rubber (IR), styrene-butadiene copolymer rubber (SBR), polybutadiene rubber (BR) and isobutylene isoprene rubber (IIR) and is required to comprise at least natural rubber and/or polyisoprene rubber. When the rubber component (A) comprises styrene-butadiene copolymer rubber, a proportion of styrene units in total of the rubber component (A) is preferably less than 30% by mass, more preferably less than 20% by mass, even preferably less than 15% by mass. When the proportion of styrene units in total of the rubber component (A) is less than 30% by mass, the rubber component (A) is excellent in the compatibility with the low-molecular weight conjugated diene-based polymer (B). Consequently, the fracture strength of the rubber composition is increased and the effects of improving the storage elastic modulus (G) and of reducing the loss tangent (tan ) can be stably obtained. In addition, when the low-molecular weight conjugated diene-based polymer (B) comprises styrene-butadiene copolymer, a proportion of styrene units in total of the low-molecular weight conjugated diene-based polymer (B) is required to be less than 5% by mass. Moreover, the rubber component (A) may be used alone or in a blend of two or more.

(6) The rubber component (A) of the rubber composition according to the invention is preferable to have a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of more than 200,000. When the weight average molecular weight as converted to polystyrene is not more than 200,000, there is a fear of deteriorating the fracture properties of the rubber composition.

(7) The rubber composition according to the invention is required to comprise 1-60 parts by mass, preferably 5-50 parts by mass, more preferably 5-40 parts by mass of the low-molecular weight conjugated diene-based polymer (B) having a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 10,000-200,000, a vinyl bond content in a conjugated diene compound portion of not less than 40% and a proportion of styrene units in total thereof of less than 5% by mass based on 100 parts by mass of the rubber component (A). When the amount of the low-molecular weight conjugated diene-based polymer (B) is less than 1 part by mass, the effect of imparting the workability to the rubber composition is weak, while when it exceeds 60 parts by mass, there is a tendency of deteriorating the fracture properties of the vulcanized rubber.

(8) The low-molecular weight conjugated diene-based polymer (B) is required to have a weight average molecular weight as measured through a gel permeation chromatography and converted to polystyrene of 10,000-200,000, preferably 20,000-200,000, more preferably 30,000-150,000, even preferably 30,000-100,000. When the weight average molecular weight as converted to polystyrene is less than 10,000, the effect of improving the heat resistance is not obtained, while when it exceeds 200,000, the workability of the rubber composition is deteriorated.

(9) The low-molecular weight conjugated diene-based polymer (B) is required to have a vinyl bond content in a conjugated diene compound portion of not less than 40%, preferably not less than 45%, more preferably not less than 50%. When the vinyl bond content in the conjugated diene compound portion is not less than 40%, the effect of improving the heat resistance is obtained. While, when the vinyl bond content in the conjugated diene compound portion is less than 40%, it is difficult to ensure the heat resistance of the rubber composition.

(10) The low-molecular weight conjugated diene-based polymer (B) is not particularly limited as long as it has the specified weight average molecular weight and vinyl bond content in the conjugated diene compound portion and suppresses the proportion of styrene units contained to less than a certain value, but it is preferably a homopolymer of a conjugated diene compound or a copolymer of an aromatic vinyl compound and a conjugated diene compound. As the conjugated diene compound as a monomer are mentioned 1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene, 2-phenyl-1,3-butadiene, 1,3-hexadiene and so on. Among them, 1,3-butadiene and isoprene are preferable. On the other hand, as the aromatic vinyl compound as a monomer are mentioned styrene, p-methylstyrene, m-methylstyrene, p-tert-butyl styrene, -methylstyrene, chloromethyl styrene, vinyl toluene and so on. Therefore, as the low-molecular weight conjugated diene-based polymer (B) are preferable polybutadiene and/or polyisoprene, and polybutadiene is more preferable. Moreover, these monomers may be used alone or in a combination of two or more.

(11) When the low-molecular weight conjugated diene-based polymer (B) is an aromatic vinyl compound-conjugated diene compound copolymer, the amount of the aromatic vinyl compound bonded is preferably less than 5% by mass. When the amount of the aromatic vinyl compound bonded is not less than 5% by mass, the compatibility with the rubber component (A), particularly the rubber component (A) having the proportion of styrene units in total thereof of less than 30% by mass, for example, natural rubber (NR) or polyisoprene rubber (IR) is deteriorated and sufficient heat resistance and fracture properties can not be obtained.

(12) The low-molecular weight conjugated diene-based polymer (B) is not particularly limited, but it can be obtained, for example, by polymerizing a conjugated diene compound alone as a monomer or a mixture of an aromatic vinyl compound and a conjugated diene compound as monomers in a hydrocarbon solvent inactive to a polymerization reaction. In case of introducing at least one functional group in the molecule of the low-molecular weight conjugated diene-based polymer (B), it can be attained by (1) a method wherein the monomer(s) is (co)polymerized with a polymerization initiator to produce a (co)polymer having a polymerization active site and then the polymerization active site is modified with various modifying agents, or (2) a method of (co)polymerizing the monomer(s) with a polymerization initiator having a functional group, for example, a polymerization initiator having SnLi, CLi or NLi bond.

(13) As the polymerization initiator used for the synthesis of the polymer (B) is preferable a lithium compound, and further a hydrocarbyl lithium and a lithium amide compound are preferable. When the lithium compound is used as the polymerization initiator, the aromatic vinyl compound and the conjugated diene compound are polymerized through an anionic polymerization. When the hydrocarbyl lithium is used as the polymerization initiator, there is obtained a polymer having a hydrocarbyl group at a polymerization starting terminal and a polymerization active site at another terminal. On the other hand, when the lithium amide compound is used as the polymerization initiator, there is obtained a polymer having a nitrogen-containing functional group at a polymerization starting terminal and a polymerization active site at another terminal, which can be used as the low-molecular weight conjugated diene-based polymer (B) having at least one functional group in the invention without being modified with a modifying agent. Moreover, the amount of the polymerization initiator used is preferably within a range of 0.2-20 mmol per 100 g of the monomer.

(14) As the hydrocarbyl lithium are mentioned ethyl lithium, n-propyl lithium, isopropyl lithium, n-butyl lithium, sec-butyl lithium, tert-octyl lithium, n-decyl lithium, phenyl lithium, 2-naphthyl lithium, 2-butyl-phenyl lithium, 4-phenyl-butyl lithium, cyclohexyl lithium, cyclopentyl lithium, a reaction product of diisopropenylbenzene and butyl lithium and so on. Among them, alkyl lithiums such as ethyl lithium, n-propyl lithium, isopropyl lithium, n-butyl lithium, sec-butyl lithium, tert-octyl lithium, n-decyl lithium and so on are preferable, and n-butyl lithium is particularly preferable.

(15) The method for producing the conjugated diene-based polymer with the above polymerization initiator is not particularly limited as mentioned above. For example, the polymer (B) can be produced by polymerizing the monomer(s) in a hydrocarbon solvent inactive to the polymerization reaction. As the hydrocarbon solvent inactive to the polymerization reaction are mentioned propane, n-butane, isobutane, n-pentane, isopentane, n-hexane, cyclohexane, propene, 1-butene, isobutene, trans-2-butene, cis-2-butene, 1-pentene, 2-pentene, 1-hexene, 2-hexene, benzene, toluene, xylene, ethylbenzene and so on. They may be used alone or in a combination of two or more.

(16) The polymerization reaction is required to be carried out in the presence of a randomizer. The randomizer can control the microstructure of the conjugated diene compound portion in the (co)polymer, and more concretely has an action of controlling the vinyl bond content of the conjugated diene compound portion in the (co)polymer, randomizing the conjugated diene compound unit and aromatic vinyl compound unit in the copolymer, or the like. As the randomizer are mentioned dimethoxybenzene, tetrahydrofuran, dimethoxyethane, diethylene glycol dibutyl ether, diethylene glycol dimethyl ether, bistetrahydrofuryl propane, triethylamine, pyridine, N-methylmorpholine, N,N,N,N-tetramethyl ethylenediamine, 1,2-dipiperidinoethane, potassium-t-amylate, potassium-t-butoxide, sodium-t-amylate and so on. The amount of the randomizer used is preferably within a range of 0.1-100 molar equivalent per 1 mole of the polymerization initiator.

(17) The anionic polymerization is preferably carried out through a solution polymerization. The concentration of the monomer in the polymerization reaction solution is preferably within a range of 5-50% by mass, more preferably 10-30% by mass. Moreover, when the conjugated diene compound and the aromatic vinyl compound are used together, the content of the aromatic vinyl compound in the mixture of the monomers can be properly selected depending on the aromatic vinyl compound content of the target copolymer. Also, the polymerization system is not particularly limited and may be a batch system or a continuous system.

(18) The polymerization temperature in the anionic polymerization is preferably within a range of 0-150 C., more preferably 20-130 C. Also, the polymerization may be carried out under a generating pressure, but it is preferable to be usually conducted under a pressure enough to keep the monomers used at substantially a liquid phase. When the polymerization reaction is carried out under a pressure higher than the generating pressure, the reaction system is preferable to be pressurized with an inert gas. Moreover, the starting materials used in the polymerization such as the monomers, polymerization initiator, solvent and the like are preferable to be used after the reaction obstructing substances such as water, oxygen, carbon dioxide, protonic compound and the like are previously removed.

(19) When the polymerization active site of the (co)polymer having the polymerization active site is modified with a modifying agent, a nitrogen-containing compound, a silicon-containing compound and a tin-containing compound are preferably used as the modifying agent. In this case, a nitrogen-containing functional group, a silicon-containing functional group or a tin-containing functional group can be introduced through the modification reaction.

(20) The modification reaction of the polymerization active site with the modifying agent is preferable to be carried out by a solution reaction. In such a solution may be included the monomer(s) used in the polymerization. Also, the reaction system of the modification reaction is not particularly limited and may be a batch system or a continuous system. Further, the reaction temperature of the modification reaction is not particularly limited as long as the reaction proceeds, and the reaction temperature in the polymerization reaction may be adopted as it is. The amount of the modifying agent used is preferably within a range of 0.25-3.0 mol, more preferably 0.5-1.5 mol per 1 mol of the polymerization initiator used for the production of the (co)polymer.

(21) In the invention, the reaction solution containing the polymer (B) may be dried to separate the polymer (B) and then the resulting polymer (B) may be compounded into the rubber component (A), or the reaction solution containing the polymer (B) may be mixed with a rubber cement of the rubber component (A) at a solution state and then dried to obtain the mixture of the rubber component (A) and the polymer (B).

(22) In the rubber composition according to the invention, it is more preferable to compound a filler in an amount of 30-90 parts by mass based on 100 parts by mass of the rubber component (A). When the amount of the filler compounded is less than 30 parts by mass, the fracture properties and wear resistance of the vulcanized rubber are insufficient, while when it exceeds 90 parts by mass, the workability tends to be deteriorated. As the filler are preferable carbon black and silica. Moreover, as the carbon black are preferable FEF, SRF, HAF, ISAF and SAF grade carbon blacks, and HAF, ISAF and SAF grade carbon blacks are more preferable. On the other hand, as the silica are preferable precipitated silica, fumed silica and the like, and the precipitated silica is more preferable. These reinforcing fillers may be used alone or in a combination of two or more.

(23) The rubber composition according to the invention may further contain a softening agent. As the softening agent are mentioned process oils such as a paraffinic oil, a naphthenic oil, an aromatic oil and so on. The aromatic oil is preferable in view of the fracture properties and wear resistance, and the naphthenic oil and paraffinic oil are preferable in view of the low heat build-up and low-temperature characteristics. The amount of the softening agent compounded is not particularly limited, but a total amount of the low-molecular weight conjugated diene-based polymer (B) and the softening agent compounded is preferable to be 1-80 parts by mass based on 100 parts by mass of the rubber component (A). When the total amount of the low-molecular weight conjugated diene-based polymer (B) and the softening agent compounded exceeds 80 parts by mass, the fracture properties of the vulcanized rubber tend to be deteriorated.

(24) The rubber composition according to the invention can be properly compounded with additives usually used in the rubber industry such as an antioxidant, a silane coupling agent, a vulcanization accelerator, an accelerator activator, a vulcanizing agent and the like within a scope of not damaging the object of the invention in addition to the above rubber component (A), low-molecular weight conjugated diene-based polymer (B), filler and softening agent. As these additives can be preferably used commercially available ones. The rubber composition can be produced by compounding the rubber component (A) with the low-molecular weight conjugated diene-based polymer (B), and, if necessary, the properly selected various additives and milling, warming, extruding and so on.

(25) The pneumatic tire according to the invention is characterized by using the aforementioned rubber composition in at least any one of a tread portion, a sidewall portion, an innerliner, a ply-coating rubber and a bead filler. The tire using the rubber composition in at least any one of a tread portion, a sidewall portion, an innerliner, a ply-coating rubber and a bead filler is excellent in the workability during production, heat resistance, low fuel consumption and steering stability. The pneumatic tire according to the invention is not particularly limited as long as the above rubber composition is used in at least any one of a tread portion, a sidewall portion, an innerliner, a ply-coating rubber and a bead filler, and can be produced by the usual method. Moreover, as a gas filled into the tire can be used usual air or air having a regulated partial oxygen pressure but also inert gases such as nitrogen, argon, helium and so on.

EXAMPLES

(26) The following examples are given in illustration of the invention and are not intended as limitations thereof.

Production Example of Polymer (B-1)

(27) Into a pressure glass vessel of 800 mL dried and replaced with nitrogen are charged 300 g of cyclohexane, 40 g of 1,3-butadiene and 0.16 mmol of ditetrahydrofurylpropane, and further 1.32 mmol of n-butyl lithium (n-BuLi) is added thereto and then polymerization reaction is conducted at 50 C. for 1.5 hours. In this case, the polymerization conversion is approximately 100%. Thereafter, 0.5 mL of a solution of 2,6-di-t-butyl-p-cresol (BHT) in isopropanol (BHT concentration: 5% by mass) is added to the polymerization system to stop the polymerization reaction and further the drying is carried out according to a usual manner to obtain a polymer (B-1).

Production Example of Polymer (B-2)

(28) Into a pressure glass vessel of 800 mL dried and replaced with nitrogen are charged 300 g of cyclohexane, 40 g of 1,3-butadiene, 10 g of styrene and 0.53 mmol of ditetrahydrofurylpropane, and further 1.32 mmol of n-butyl lithium (n-BuLi) is added thereto and then the polymerization reaction is conducted at 50 C. for 1.5 hours. In this case, the polymerization conversion is approximately 100%. Thereafter, 0.5 mL of a solution of 2,6-di-t-butyl-p-cresol (BHT) in isopropanol (BHT concentration: 5% by mass) is added to the polymerization system to stop the polymerization reaction and further the drying is carried out according to a usual manner to obtain a polymer (B-2).

Production Example of Polymer (B-3)

(29) Into a pressure glass vessel of 800 mL dried and replaced with nitrogen are charged 300 g of cyclohexane, 40 g of 1,3-butadiene and 0.53 mmol of ditetrahydrofurylpropane, and further 1.32 mmol of n-butyl lithium (n-BuLi) is added thereto and then the polymerization reaction is conducted at 50 C. for 1.5 hours. In this case, the polymerization conversion is approximately 100%. Thereafter, 0.5 mL of a solution of 2,6-di-t-butyl-p-cresol (BHT) in isopropanol (BHT concentration: 5% by mass) is added to the polymerization system to stop the polymerization reaction and further the drying is carried out according to a usual manner to obtain a polymer (B-3).

Production Example of Polymer (B-4)

(30) Into a pressure glass vessel of 800 mL dried and replaced with nitrogen are charged 300 g of cyclohexane, 40 g of 1,3-butadiene and 0.79 mmol of ditetrahydrofurylpropane, and further 1.32 mmol of n-butyl lithium (n-BuLi) is added thereto and then the polymerization reaction is conducted at 50 C. for 1.5 hours. In this case, the polymerization conversion is approximately 1.00%. Thereafter, 0.5 mL of a solution of 2,6-di-t-butyl-p-cresol (BHT) in isopropanol (BHT concentration: 5% by mass) is added to the polymerization system to stop the polymerization reaction and further the drying is carried out according to a usual manner to obtain a polymer (B-4).

Production Example of Polymers (B-5)-(B-11) and (B-17)

(31) Polymers (B-5)-(B-11) and (B-17) are synthesized in the same manner as in Production Example of Polymer (B-3) except that the amount of n-butyl lithium (n-BuLi) used is changed.

Production Example of Polymers (B-12)-(B-13)

(32) Polymers (B-12)-(B-13) are synthesized in the same manner as in Production Example of Polymer (B-1) except that the amount of ditetrahydrofurylpropane used is changed.

Production Example of Polymers (B-14)-(B-15)

(33) Polymers (B-14)-(B-15) are synthesized in the same manner as in Production Example of Polymer (B-2) except that the amount of 1,3-butadiene and styrene used is changed.

Production Example of Polymer (B-16)

(34) A polymer (B-16) is synthesized in the same manner as in Production Example of Polymer (B-4) except that isoprene is used instead of 1,3-butadiene and the amount of ditetrahydrofurylpropane used is changed from 0.79 mmol to 0.026 mmol and the polymerization reaction is conducted for 7 hours.

(35) The weight average molecular weight (Mw) and microstructure of the polymers (B-1)-(B-17) produced as mentioned above are measured according to the following methods. The results are shown in Table 1.

(36) (1) Weight Average Molecular Weight (Mw)

(37) The weight average molecular weight (Mw) of each polymer as converted to polystyrene is measured through a gel permeation chromatography [GPC: HLC-8020 manufactured by TOSOH, column: GMH-XL (two series columns) manufactured by TOSOH, detector: differential refractometer (RI)] as a standard of monodisperse polystyrene.

(38) (2) Microstructure

(39) The microstructure of the polymer is determined by an infrared method (Morello method).

(40) TABLE-US-00001 TABLE 1 Weight average Bound Vinyl molecular weight styrene content bond content (Mw) (10.sup.3) (% by mass) (%) Polymer (B-1) 80 0 35 Polymer (B-2) 80 20 45 Polymer (B-3) 80 0 45 Polymer (B-4) 80 0 55 Polymer (B-5) 3 0 45 Polymer (B-6) 10 0 45 Polymer (B-7) 25 0 45 Polymer (B-8) 40 0 45 Polymer (B-9) 120 0 45 Polymer (B-10) 180 0 45 Polymer (B-11) 220 0 45 Polymer (B-12) 80 0 40 Polymer (B-13) 80 0 50 Polymer (B-14) 80 3 45 Polymer (B-15) 80 7 45 Polymer (B-16) 80 0 45 Polymer (B-17) 100 0 45

(41) Then, a rubber composition having a compounding recipe as shown in Tables 2-6 is prepared and the Mooney viscosity [ML1+4 (130 C.)] and heat resistance of the rubber composition are measured according to the following methods. Further, the rubber composition is vulcanized on usual conditions to obtain a vulcanized rubber, and the storage elastic modulus (G), loss tangent (tan ) and fracture strength of the thus vulcanized rubber are measured according to the following methods. The results are shown in Tables 2-6.

(42) (3) Mooney Viscosity

(43) The Mooney viscosity [ML1+4 (130 C.)] is measured at 130 C. according to JIS K 6300-1: 2001, and represented by an index on the basis that the Mooney viscosity in the rubber compositions of Comparative Examples 1 and 7 is 100 in Tables 2-5, the Mooney viscosity in the rubber composition of Comparative Example 11 is 100 for Example 20 and Comparative Example 11 in Table 6, the Mooney viscosity in the rubber composition of Comparative Example 12 is 100 for Example 21 and Comparative Example 12 in Table 6, the Mooney viscosity in the rubber composition of Comparative Example 13 is 100 for Example 22 and Comparative Example 13 in Table 6, and the Mooney viscosity in the rubber composition of Comparative Example 14 is 100 for Example 23 and Comparative Example 14 in Table 6. The smaller the index value, the better the workability.

(44) (4) Heat Resistance (Vulcanization Reversion)

(45) The torque of the rubber composition is measured by using a rheometer at 175 C. for 15 minutes to evaluate a vulcanization reversion according to the following equation, which is represented by an index on the basis that an inverse number of the vulcanization reversion in the rubber compositions of Comparative Examples 1 and 7 is 100 in Tables 2-5, an inverse number of the vulcanization reversion in the rubber composition of Comparative Example 11 is 100 for Example 20 and Comparative Example 11 in Table 6, an inverse number of the vulcanization reversion in the rubber composition of Comparative Example 12 is 100 for Example 21 and Comparative Example 12 in Table 6, an inverse number of the vulcanization reversion in the rubber composition of Comparative Example 13 is 100 for Example 22 and Comparative Example 13 in Table 6, and an inverse number of the vulcanization reversion in the rubber composition of Comparative Example 14 is 100 for Example 23 and Comparative Example 14 in Table 6. The larger the index value, the better the heat resistance.
Vulcanization reversion(%)=[(FmaxF)/(FmaxFmin)]100

(46) In the equation, Fmax represents the maximum value of torque, Fmin represents the minimum value of torque, and F represents a value of torque after 15 minutes of the measurement.

(47) (5) Storage Elastic Modulus (G) and Loss Tangent (tan )

(48) The storage elastic modulus (G) and loss tangent (tan ) are measured at a temperature of 50 C., a frequency of 15 Hz and a strain of 5% by using a viscoelasticity measuring device manufactured by RHEOMETRICS Corporation, and represented by an index on the basis that the storage elastic modulus (G) and loss tangent (tan ) in the rubber compositions of Comparative Examples 1 and 7 are each 100 in Tables 2-5, the storage elastic modulus (G) and loss tangent (tan ) in the rubber composition of Comparative Example 11 are each 100 for Example 20 and Comparative Example 11 in Table 6, the storage elastic modulus (G) and loss tangent (tan ) in the rubber composition of Comparative Example 12 are each 100 for Example 21 and Comparative Example 12 in Table 6, the storage elastic modulus (G) and loss tangent (tan ) in the rubber composition of Comparative Example 13 are each 100 for Example 22 and Comparative Example 13 in Table 6, and the storage elastic modulus (G) and loss tangent (tan ) in the rubber composition of Comparative Example 14 are each 100 for Example 23 and Comparative Example 14 in Table 6. With respect to the storage elastic modulus (G), the lager the index value, the higher the storage elastic modulus. With respect to the loss tangent (8), the smaller the index value, the better the low heat buildup.

(49) (6) Fracture Strength

(50) A JIS No. 3 dumbbell specimen is prepared and a tensile test is conducted according to JIS K 6251-1993 to measure the fracture strength of the rubber composition, which is represented by an index on the basis that the fracture strength in the rubber compositions of Comparative Examples 1 and 7 is 100 in Tables 2-5, the fracture strength in the rubber composition of Comparative Example 11 is 100 for Example 20 and Comparative Example 11 in Table 6, the fracture strength in the rubber composition of Comparative Example 12 is 100 for Example 21 and Comparative Example 12 in Table 6, the fracture strength in the rubber composition of Comparative Example 13 is 100 for Example 22 and Comparative Example 13 in Table 6, and the fracture strength in the rubber composition of Comparative Example 14 is 100 for Example 23 and Comparative Example 14 in Table 6. The larger the index value, the better the fracture strength.

(51) TABLE-US-00002 TABLE 2 Comparative Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Example 1 Example 2 Example 5 Type of low-molecular weight conjugated diene- B-1 B-2 B-3 B-4 B-5 based polymer (B) Compounding Rubber Natural parts by 60 60 60 60 60 60 60 component (A) rubber *1 mass Butadiene 40 40 40 40 40 40 40 rubber *2 Low-molecular weight 10 10 10 10 10 conjugated diene-based polymer (B) Aromatic oil 5 15 5 5 5 5 5 Carbon black (N220) 60 60 60 60 60 60 60 Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Zinc white 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Antioxidant 6C *3 1 1 1 1 1 1 1 Vulcanization accelerator 0.2 0.2 0.2 0.2 0.2 0.2 0.2 DM *4 Vulcanization accelerator 0.5 0.5 0.5 0.5 0.5 0.5 0.5 CZ *5 Sulfur 1 1 1 1 1 1 1 Physical Workability [ML.sub.1+4 (130 C.)] (index) 100 78 91 90 92 91 78 properties Loss tangent (tan ) (index) 100 118 87 96 85 84 114 Storage elastic modulus (G) (index) 100 86 114 116 115 118 88 Heat resistance (vulcanization reversion) 100 88 105 118 139 145 95 (index) Fracture strength (index) 100 82 102 91 130 137 93

(52) TABLE-US-00003 TABLE 3 Comparative Example 3 Example 4 Example 5 Example 6 Example 7 Example 6 Type of low-molecular weight conjugated diene-based B-6 B-7 B-8 B-9 B-10 B-11 polymer (B) Compounding Rubber Natural rubber *1 parts 60 60 60 60 60 60 component (A) Butadiene rubber *2 by 40 40 40 40 40 40 Low-molecular weight conjugated mass 10 10 10 10 10 10 diene-based polymer (B) Aromatic oil 5 5 5 5 5 5 Carbon black (N220) 60 60 60 60 60 60 Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 Zinc white 1.5 1.5 1.5 1.5 1.5 1.5 Antioxidant 6C *3 1 1 1 1 1 1 Vulcanization accelerator DM *4 0.2 0.2 0.2 0.2 0.2 0.2 Vulcanization accelerator CZ *5 0.5 0.5 0.5 0.5 0.5 0.5 Sulfur 1 1 1 1 1 1 Physical Workability [ML.sub.1+4 (130 C.)] (index) 80 82 86 97 103 112 properties Loss tangent (tan ) (index) 103 96 92 83 86 95 Storage elastic modulus (G) (index) 98 103 109 114 116 115 Heat resistance (vulcanization reversion) (index) 104 109 124 131 124 115 Fracture strength (index) 100 109 116 127 120 114 *1 Weight average molecular weight as converted to polystyrene = 1,500,000. *2 BR01 manufactured by JSR Corporation, weight average molecular weight as converted to polystyrene = 550,000. *3 N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine. *4 di-2-benzothiazolyl disulfide. *5 N-cyclohexyl-2-benzothiazoly sulfenamide.

(53) As seen from Tables 2-3, the rubber compositions of Examples 1-7 using the low-molecular weight conjugated diene-based polymer (B) having a weight average molecular weight of 10,000-200,000, a vinyl bond content in a conjugated diene compound portion of not less than 40% and a proportion of styrene units in total thereof of less than 5% by mass have highly balanced workability, heat resistance, storage elastic modulus (G), loss tangent (tan ) and fracture strength as compared with the rubber compositions of Comparative Examples 1-6.

(54) TABLE-US-00004 TABLE 4 Com- Com- Ex- Ex- Ex- Ex- Ex- Com- parative parative ample ample ample ample ample parative Comparative Example 7 Example 8 8 9 10 11 12 Example 9 Example 10 Type of low-molecular weight conjugated diene- B-1 B-12 B-3 B-13 B-4 B-14 B-15 B-2 based polymer (B) BR BR BR BR BR SBR SBR SBR Weigh average molecular weight 80 80 80 80 80 80 80 80 (10.sup.3) Bound styrene content (% by mass) 0 0 0 0 0 3 7 20 Vinyl bond content (%) 35 40 45 50 55 45 45 45 Compounding Rubber NR *6 parts 60 60 60 60 60 60 60 60 60 component (A) BR *7 by 40 40 40 40 40 40 40 40 40 SBR1 *8 mass SBR2 *9 Low-molecular weight conjugated 10 10 10 10 10 10 10 10 diene-based polymer (B) Aromatic oil 5 5 5 5 5 5 5 5 5 Carbon black (N220) 60 60 60 60 60 60 60 60 60 Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Zinc white 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Antioxidant 6C *10 1 1 1 1 1 1 1 1 1 Vulcanization accelerator DM *11 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Vulcanization accelerator CZ *12 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Sulfur 1 1 1 1 1 1 1 1 1 Physical Workability [ML.sub.1+4 (130 C.)] (index) 100 91 91 92 91 91 90 91 90 properties Loss tangent (tan ) (index) 100 87 86 85 84 84 86 92 96 Storage elastic modulus (G) (index) 100 114 113 115 117 118 114 114 116 Heat resistance (vulcanization 100 105 131 139 143 145 135 123 118 reversion) (index) Fracture strength (index) 100 102 125 130 136 137 131 102 91

(55) TABLE-US-00005 TABLE 5 Example Example Example Example Example Example Example 13 14 15 16 17 18 19 Type of low-molecular weight conjugated diene-based polymer (B) B-16 B-8 B-17 B-9 B-17 B-17 B-17 IR BR BR BR BR BR BR Weigh average molecular weight (10.sup.3) 80 40 100 120 100 100 100 Bound styrene content (% by mass) 0 0 0 0 0 0 0 Vinyl bond content (%) 45 45 45 45 45 45 45 Compounding Rubber (component A) NR *6 parts by 60 60 60 60 60 60 60 BR *7 mass 40 40 40 40 40 40 40 SBR1 *8 SBR2 *9 Low-molecular weight conjugated 10 10 10 10 5 40 50 diene-based polymer (B) Aromatic oil 5 5 5 5 5 5 5 Carbon black (N220) 60 60 60 60 60 60 60 Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Zinc white 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Antioxidant 6C *10 1 1 1 1 1 1 1 Vulcanization accelerator DM *11 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Vulcanization accelerator CZ *12 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Sulfur 1 1 1 1 1 1 1 Physical Workability [ML.sub.1+4 (130 C.)] (index) 92 86 93 97 96 79 70 properties Loss tangent (tan ) (index) 89 92 85 83 93 68 68 Storage elastic modulus (G) (index) 121 109 116 114 107 122 120 Heat resistance (vulcanization reversion) (index) 141 124 136 131 126 141 143 Fracture strength (index) 136 116 129 127 122 134 130 *6 Natural rubber, weight average molecular weight (Mw) as converted to polystyrene = 1,500,000. *7 Polybutadiene rubber, BR01 manufactured by JSR Corporation, weight average molecular weight (Mw) as converted to polystyrene = 550,000. *8 Styrene-butadiene copolymer rubber, SL 563 type manufactured by JSR Corporation, weight average molecular weight (Mw) as converted to polystyrene = 600 10.sup.3, bound styrene content = 20% by mass, vinyl bond content = 55%. *9 Styrene-butadiene copolymer rubber, T 0150 manufactured by JSR Corporation, weight average molecular weight (Mw) as converted to polystyrene = 900 10.sup.3, bound styrene content = 45% by mass, vinyl bond content = 19%. *10 N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine. *11 di-2-benzothiazolyl disulfide. *12 N-cyclohexyl-2-benzothiazoly sulfenamide.

(56) As seen from Tables 4-5, the rubber composition of Comparative Example 8 using the low-molecular weight conjugated diene-based polymer (B-1) having a vinyl bond content in a conjugated diene compound portion of less than 40% cannot obtain sufficient heat resistance and fracture strength. Also, the rubber compositions of Comparative Examples 9-10 using the low-molecular weight conjugated diene-based polymers (B-14)-(B-15) having a proportion of styrene units in total thereof of not less than 5% by mass cannot obtain sufficient heat resistance and fracture strength. Further, as seen from the comparison of Examples 15 and 17-19, the effects of improving the workability, heat resistance, storage elastic modulus (G), loss tangent (tan ) and fracture strength are dramatically improved by increasing the amount of the low-molecular weight conjugated diene-based polymer (B-7) compounded.

(57) TABLE-US-00006 TABLE 6 Ex- Ex- Ex- Ex- Comparative ample Comparative ample Comparative ample Comparative ample Example 11 20 Example 12 21 Example 13 22 Example 14 23 Type of low-molecular weight conjugated diene-based B-17 B-17 B-17 B-17 polymer (B) BR BR BR BR Weigh average molecular weight 100 100 100 100 (10.sup.3) Bound styrene content (% by mass) 0 0 0 0 Vinyl bond content (%) 45 45 45 45 Bound styrene content in rubber component (A) 14 14 18 18 29 29 36 36 (% by mass) Compounding Rubber NR *6 parts 30 30 10 10 35 35 20 20 component (A) BR *7 by SBR1 *8 mass 70 70 90 90 SBR2 *9 65 65 80 80 Low-molecular weight conjugated 10 10 10 10 diene-based polymer (B) Aromatic oil 5 5 5 5 5 5 5 5 Carbon black (N220) 60 60 60 60 60 60 60 60 Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Zinc white 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Antioxidant 6C *10 1 1 1 1 1 1 1 1 Vulcanization accelerator DM *11 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Vulcanization accelerator CZ *12 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Sulfur 1 1 1 1 1 1 1 1 Physical Workability [ML.sub.1+4 (130 C.)] (index) 100 88 100 87 100 91 100 92 properties Loss tangent (tan ) (index) 100 86 100 87 100 89 100 97 Storage elastic modulus (G) (index) 100 117 100 114 100 112 100 110 Heat resistance (vulcanization reversion) 100 140 100 140 100 130 100 111 (index) Fracture strength (index) 100 133 100 130 100 121 100 103 *6 Natural rubber, weight average molecular weight (Mw) as converted to polystyrene = 1,500,000. *7 Polybutadiene rubber, BR01 manufactured by JSR Corporation, weight average molecular weight (Mw) as converted to polystyrene = 550,000. *8 Styrene-butadiene copolymer rubber, SL 563 type manufactured by JSR Corporation, weight average molecular weight (Mw) as converted to polystyrene = 600 10.sup.3, bound styrene content = 20% by mass, vinyl bond content = 55%. *9 Styrene-butadiene copolymer rubber, T 0150 manufactured by JSR Corporation, weight average molecular weight (Mw) as converted to polystyrene = 900 10.sup.3, bound styrene content = 45% by mass, vinyl bond content = 19%. *10 N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine. *11 di-2-benzothiazolyl disulfide. *12 N-cyclohexyl-2-benzothiazoly sulfenamide.

(58) As seen from Table 6, the rubber compositions of Examples 20-23 formed by compounding the low-molecular weight conjugated diene-based polymer (B-17) into a rubber component (A) containing styrene units improve the workability, heat resistance, storage elastic modulus (G), loss tangent (tan ) and fracture strength as compared with the corresponding rubber compositions of Comparative Examples 11-14. Also, as seen from the results of Examples 20-23, when the proportion of styrene units in total of the rubber component (A) is less than 30% by mass, the workability, heat resistance, storage elastic modulus (G), loss tangent (tan ) and fracture strength of the rubber composition are dramatically improved.