JP4391583B2 - Rubber composition and pneumatic tire - Google Patents

Description

TECHNICAL FIELD The present invention relates to a rubber composition and a pneumatic tire using the rubber composition, and more particularly to a rubber composition that can improve handling stability of a tire while ensuring fracture resistance such as wear resistance when used in a tread. Is.

  In recent years, with regard to advanced evolution of passenger car power performance, more excellent handling stability has been demanded as a tire characteristic. In particular, as a required characteristic of a high-performance tire, ensuring steering stability on a dry road surface is an important issue. Until now, various technologies have been developed in response to the demand for improvement in handling stability of these tires. Here, in developing a rubber composition for a tire tread that directly contributes to improvement of tire handling stability, it is generally effective to use a loss tangent (tan δ) at room temperature or higher as an index. By using a rubber composition having a high tan δ at room temperature or higher for the tread, the steering stability of the tire can be improved.

On the other hand, conventionally, as a technique for increasing the tan δ of the rubber composition, a technique of blending a C ₉ aromatic resin having a high glass transition point (Tg) into the rubber composition (see Patent Document 1), and a molecular weight. A technique (see Patent Document 2) in which tens of thousands of liquid styrene-butadiene copolymers are blended into a rubber composition is known. However, when a resin having a high glass transition point is blended in a rubber composition, there is a problem that the fracture characteristics of the rubber composition deteriorate due to the low molecular weight of the resin. Further, when a liquid styrene-butadiene copolymer is blended in a rubber composition, there is a problem that the tan δ of the rubber composition cannot be sufficiently improved because the glass transition point of the liquid copolymer is low. .

  On the other hand, securing the wear characteristics of the tire is also an important issue from the viewpoint of economy. In addition, when an additive such as a resin or a liquid polymer is blended to increase the tan δ of the rubber composition, the blended additive is compatible with the rubber component from the viewpoint of sufficiently securing the fracture characteristics of the tire. Must be sufficiently good.

JP-A-5-9338 JP-A-61-203145

Accordingly, an object of the present invention is to provide a rubber composition that can solve the above-described problems of the prior art and can significantly improve steering stability while ensuring the fracture resistance of a tire. Another object of the present invention is formed by applying the rubber composition in the tread, and fracture resistance, such as steering stability and wear resistance is to provide a pneumatic tire which is highly compatible .

As a result of intensive studies to achieve the above-mentioned object, the present inventors have obtained an acrylate (co) polymer having a good compatibility with the rubber component, a high glass transition point, and a relatively high molecular weight as a rubber composition. By blending with the rubber composition, the tan δ of the rubber composition can be significantly increased, and the fracture resistance of the rubber composition can be ensured. Further, by using the rubber composition in a tread, the fracture resistance of the tire can be secured. As a result, the present inventors have found that the steering stability can be greatly improved while sufficiently securing the performance, and the present invention has been completed.

That is, the rubber composition of the present invention is a (co) polymer obtained by polymerizing at least one alkyl acrylate containing at least tert-butyl acrylate with respect to 100 parts by mass of a rubber component composed of a diene rubber. Further, it is characterized by blending 5 to 150 parts by mass of a (co) polymer having a polystyrene-equivalent weight average molecular weight of 2 × 10 ³ to 50 × 10 ³ . Wherein the copolymer with one or more alkyl acrylates containing tert- butyl acrylate, may be made by copolymerizing a styrene.

In a preferred example of the rubber composition of the present invention, the rubber component includes a copolymer of a conjugated diene compound and an aromatic vinyl compound having a polystyrene-equivalent weight average molecular weight of 300 × 10 ^{3 to} 3,000 × 10 ³ . Here, the copolymer of the conjugated diene compound and the aromatic vinyl compound is preferably a styrene-butadiene copolymer rubber (SBR).

  Furthermore, the pneumatic tire of the present invention is characterized by using the rubber composition in a tread.

According to the present invention, at least tert-butyl acrylate was used as a monomer, and a (co) polymer having a relatively high molecular weight, good compatibility with a rubber component and a sufficiently high glass transition point was blended, A rubber composition having a high tan δ and sufficient fracture characteristics can be provided. Furthermore, it is possible to provide a pneumatic tire using the rubber composition in a tread and having sufficient fracture characteristics and excellent handling stability.

The present invention is described in detail below. The rubber composition of the present invention, the rubber component 100 parts by mass consisting of diene rubber, a formed by polymerizing one or more alkyl acrylates containing at least tert- butyl acrylate (co) polymers, polystyrene It is formed by blending 5 to 150 parts by mass of a (co) polymer having a reduced weight average molecular weight of 2 × 10 ³ to 50 × 10 ³ . Since the (co) polymer has a high glass transition point, the tan δ of the rubber composition can be increased. In addition, since the (co) polymer has a relatively high molecular weight and relatively good compatibility with the rubber component, it is possible to sufficiently ensure the fracture resistance of the rubber composition. Therefore, the rubber composition of the present invention in which the above (co) polymer is blended has a high tan δ and sufficient fracture resistance. By applying the rubber composition to a tire tread, The steering stability and fracture characteristics of the tire can be highly compatible.

  Examples of the diene rubber used as a rubber component in the rubber composition of the present invention include natural rubber (NR), conjugated styrene-butadiene copolymer rubber (SBR), styrene-isoprene copolymer rubber (SIR), and the like. Copolymers of diene compounds and aromatic vinyl compounds, homopolymers of conjugated diene compounds such as polyisoprene rubber (IR) and polybutadiene rubber (BR), butyl rubber (IIR), ethylene-propylene copolymers and mixtures thereof Synthetic rubber such as In addition, a part of these diene rubbers may be modified with a polyfunctional modifier, for example, tin tetrachloride and the like, and may have a branched structure. These diene rubbers may be used alone or in a blend of two or more.

Among the above-mentioned diene rubbers, a conjugated diene compound-aromatic vinyl compound copolymer is preferable, and SBR is particularly preferable from the viewpoint of compatibility of generally available rubber. The conjugated diene compound-aromatic vinyl compound copolymer preferably has a polystyrene-equivalent weight average molecular weight of 300 × 10 ^{3 to} 3,000 × 10 ³ . When a conjugated diene compound-aromatic vinyl compound copolymer having a weight average molecular weight of less than 300 × 10 ³ is used, the fracture characteristics of the rubber composition tend to decrease, whereas the weight average molecular weight exceeds 3,000 × 10 ³ . The conjugated diene compound-aromatic vinyl compound copolymer has poor productivity because the viscosity of the polymerization solution becomes high. In addition, when using SBR as a rubber component, NR, BR, etc. can be blended, However, It is preferable that the ratio of SBR in a rubber component shall be 40 mass% or more.

The (co) polymer used in the rubber composition of the present invention is obtained by polymerizing at least one alkyl acrylate containing at least tert-butyl acrylate, and has a polystyrene-equivalent weight average molecular weight of 2 × 10 ³ to 50 × 10 ³ . is there. If the weight average molecular weight of the acrylate- based (co) polymer is less than 2 × 10 ³ , the fracture characteristics of the rubber composition tend to decrease, and if it exceeds 50 × 10 ³ , the processability of the rubber composition decreases. Tend. Depending on the purpose, a (co) polymer having a polystyrene-equivalent weight average molecular weight of 5 × 10 ³ to 50 × 10 ³ is blended in the rubber composition, and the rubber composition is used for the tread. Best performance.

  The (co) polymer preferably has a glass transition point of 70 ° C. or higher. By blending a (co) polymer having a glass transition point of 70 ° C. or higher into the rubber composition, tan δ of the rubber composition can be sufficiently increased.

The raw material monomer of the (co) polymer includes an alkyl acrylate, and the alkyl group of the alkyl acrylate is preferably an alkyl group having 1 to 12 carbon atoms, more preferably an alkyl group having 1 to 8 carbon atoms. Examples include methyl group, ethyl group, n-propyl group, isopropyl group, isobutyl group, tert-butyl group and the like. As the alkyl acrylates, specifically, methyl acrylate, ethyl acrylate, n- propyl acrylate, isopropyl acrylate, isobutyl acrylate, etc. tert- butyl acrylate. Here, the raw material monomer of the (co) polymer of the present invention, required to include tert- butyl acrylate, other alkyl acrylates may include one or more as required.

The starting monomers of the copolymer of the present invention may contain in addition to styrene of the above alkyl acrylates. The copolymer is preferably a random copolymer. Here, the styrene content in the copolymer is preferably 90% by mass or less, and more preferably 50% by mass or less.

The (co) polymer of the present invention can be produced by a general olefin polymerization method using alkyl acrylate and optionally styrene as monomers , and can be produced, for example, by anionic polymerization. When the (co) polymer of the present invention is produced by anionic polymerization, usually, an organic lithium compound is used as a polymerization initiator, and one or more monomers are (co) polymerized in an inert organic solvent. In order to activate the polymerization initiator and suppress the termination reaction, it is preferable to add an organoaluminum compound as a ligand of the polymerization initiator to the polymerization system. Further, as described above, a random copolymerized copolymer is preferable as the copolymer. Therefore, it is preferable to use a randomizer as necessary.

  Examples of the organic lithium compound used as the polymerization initiator include alkyl lithium such as ethyl lithium, n-propyl lithium, i-propyl lithium, n-butyl lithium, sec-butyl lithium, tert-octyl lithium, and n-decyl lithium, phenyl Lithium, 2-naphthyllithium, aryllithium such as 2-butyl-phenyllithium, aralkyllithium such as 4-phenyl-butyllithium, cycloalkyllithium such as cyclohexyllithium and 4-cyclopentyllithium, lithium hexamethyleneimide, lithium pyrrolidide , Lithium piperidide, lithium heptamethylene imide, lithium dodecamethylene imide, lithium dimethylamide, lithium diethylamide, lithium dipropylamide, lithium dibutylamide, lithium dihexyl Amide, Lithium diheptylamide, Lithium dioctylamide, Lithymdi-2-ethylhexylamide, Lithium didecylamide, Lithium-N-methylpiperazide, Lithium ethylpropylamide, Lithium ethylbutyramide, Lithium methylbutyramide, Lithium ethylbenzylamide, Lithium Examples include lithium amide compounds such as methylphenethylamide, and among these, n-butyllithium is preferable. The amount of these organolithium compounds used is preferably in the range of 0.2 to 20 mmol per 100 g of monomer.

  Examples of the inert organic solvent include propane, n-butane, i-butane, n-pentane, i-pentane, n-hexane, propene, 1-butene, i-butene, trans-2-butene, and cis-2- Examples include aliphatic hydrocarbons such as butene, 1-pentene, 2-pentene, 1-hexene, and 2-hexene, aromatic hydrocarbons such as benzene, toluene, xylene, and ethylbenzene, and alicyclic hydrocarbons such as cyclohexane. Of these, cyclohexane is preferred. Moreover, the said inert organic solvent may be used individually by 1 type, or may mix and use 2 or more types.

  Examples of the organoaluminum compound used as the ligand for the polymerization initiator include triethylaluminum, tripropylaluminum, tributylaluminum, ethyldimethylaluminum, 2-ethylhexylaluminum, cyclohexylaluminum, and diphenylaluminum. Among these, triethylaluminum is used. preferable. The amount of these organoaluminum compounds used is preferably in the range of 0.01 to 20 mol with respect to 1 mol of the organic lithium compound as a polymerization initiator.

  Examples of the randomizer include dimethoxybenzene, tetrahydrofuran, dimethoxyethane, diethylene glycol dibutyl ether, diethylene glycol dimethyl ether, bistetrahydrofurylpropane, triethylamine, pyridine, N-methylmorpholine, N, N, N ′, N′-tetramethylethylenediamine, 1 , 2-dipiperidinoethane, potassium-t-amylate, potassium-t-butoxide, sodium-t-amylate and the like. The amount of these randomizers used is preferably in the range of 0.01 to 20 mol with respect to 1 mol of the organic lithium compound as a polymerization initiator.

  In the production of the (co) polymer of the present invention, the polymerization temperature is preferably in the range of about -80 to 150 ° C, more preferably in the range of -20 to 100 ° C. The polymerization can be carried out under generated pressure, but it is usually preferred to carry out the polymerization under a pressure sufficient to keep the monomer used in a substantially liquid phase. Although the pressure of the polymerization reaction depends on the raw materials such as monomers and initiators used and the polymerization temperature, the polymerization reaction can be carried out at a pressure higher than the generated pressure if desired, and the polymerization reaction is performed at a pressure higher than the generated pressure. When carried out below, the reaction system is preferably pressurized with an inert gas. Further, it is preferable to previously remove water, oxygen, carbon dioxide and other catalyst poisons from all the raw materials such as monomers, polymerization initiators and solvents used in the polymerization.

  The blending amount of the (co) polymer in the rubber composition of the present invention is 5 to 150 parts by mass with respect to 100 parts by mass of the rubber component. When the blending amount of the (co) polymer is less than 5 parts by weight with respect to 100 parts by weight of the rubber component, the effect of blending the (co) polymer is small, and the tan δ of the rubber composition can be sufficiently improved. Moreover, when it exceeds 150 mass parts, the fracture strength of a rubber composition will fall.

  The rubber composition of the present invention preferably contains a filler, and is not particularly limited, but preferably contains carbon black and / or silica.

  Examples of the silica include, but are not limited to, wet silica (hydrous silicic acid), dry silica (anhydrous silicic acid), calcium silicate, aluminum silicate, and the like. Wet silica is preferred because it is excellent in the effect of achieving both wet grip properties and low rolling resistance. The rubber composition of the present invention may contain only silica as a filler. In this case, the amount of silica is in the range of 10 to 250 parts by mass with respect to 100 parts by mass of the rubber component. From the viewpoint of reinforcing properties and the improvement efficiency of various physical properties thereby, a range of 20 to 150 parts by mass is preferable. When the compounding amount of silica is less than 10 parts by mass with respect to 100 parts by mass of the rubber component, fracture characteristics and the like are not sufficient, and when it exceeds 250 parts by mass, the processability of the rubber composition is lowered.

  In the rubber composition of the present invention, when silica is used as a filler, it is preferable to add a silane coupling agent at the time of blending from the viewpoint of further improving the reinforcing property. Examples of the silane coupling agent include bis (3-triethoxysilylpropyl) tetrasulfide, bis (3-triethoxysilylpropyl) trisulfide, bis (3-triethoxysilylpropyl) disulfide, and bis (2-triethoxysilyl). Ethyl) tetrasulfide, bis (3-trimethoxysilylpropyl) tetrasulfide, bis (2-trimethoxysilylethyl) tetrasulfide, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyltri Methoxysilane, 2-mercaptoethyltriethoxysilane, 3-trimethoxysilylpropyl-N, N-dimethylthiocarbamoyl tetrasulfide, 3-triethoxysilylpropyl-N, N-dimethylthiocarbamoyl tetrasulfide, 2-triethoxysilyl Ethyl-N, N-dimethylthiocarbamoyl tetrasulfide, 3-trimethoxysilylpropylbenzothiazole tetrasulfide, 3-triethoxysilylpropylbenzothiazole tetrasulfide, 3-triethoxysilylpropyl methacrylate monosulfide, 3-trimethoxysilylpropyl Methacrylate monosulfide, bis (3-diethoxymethylsilylpropyl) tetrasulfide, 3-mercaptopropyldimethoxymethylsilane, dimethoxymethylsilylpropyl-N, N-dimethylthiocarbamoyl tetrasulfide, dimethoxymethylsilylpropylbenzothiazole tetrasulfide, etc. Among these, bis (3-triethoxysilylpropyl) tetrasulfide and 3-trimethoxysilyl from the viewpoint of reinforcing effect Propyl benzothiazole tetrasulfide are preferable. These silane coupling agents may be used alone or in combination of two or more.

  On the other hand, the carbon black is not particularly limited, and examples thereof include FEF, SRF, HAF, ISAF, and SAF grades. The carbon black is preferably carbon black having an iodine adsorption amount (IA) of 60 mg / g or more and a dibutyl phthalate (DBP) oil absorption of 80 mL / 100 g or more. By blending carbon black, various physical properties of the rubber composition can be improved, but those of HAF, ISAF, and SAF grades are more preferable from the viewpoint of improving wear resistance. The rubber composition of the present invention may contain only carbon black as a filler. In this case, the amount of carbon black is in the range of 10 to 250 parts by mass with respect to 100 parts by mass of the rubber component. From the viewpoint of reinforcing properties and efficiency of improving various physical properties thereby, a range of 20 to 150 parts by mass is preferable. When the compounding amount of carbon black is less than 10 parts by mass with respect to 100 parts by mass of the rubber component, the fracture characteristics and the like are not sufficient, and when it exceeds 250 parts by mass, the processability of the rubber composition decreases.

  A general rubber crosslinking system can be used for the rubber composition of the present invention, and it is preferable to use a combination of a crosslinking agent and a vulcanization accelerator. Here, as a crosslinking agent, sulfur etc. are mentioned, The usage-amount of a crosslinking agent has the preferable range of 0.1-10 mass parts as a sulfur content with respect to 100 mass parts of said rubber components, The range of 1-5 mass parts Is more preferable. If the compounding amount of the crosslinking agent is less than 0.1 parts by mass as a sulfur component with respect to 100 parts by mass of the rubber component, the rupture strength, wear resistance and low heat build-up of the vulcanized rubber will be reduced. Is lost.

  On the other hand, the vulcanization accelerator is not particularly limited, but 2-mercaptobenzothiazole (M), dibenzothiazyl disulfide (DM), N-cyclohexyl-2-benzothiazylsulfenamide (CZ). ), Thiazole vulcanization accelerators such as Nt-butyl-2-benzothiazolylsulfenamide (NS), and guanidine vulcanization accelerators such as diphenylguanidine (DPG). The amount of the vulcanization accelerator used is preferably in the range of 0.1 to 5 parts by mass, more preferably in the range of 0.2 to 3 parts by mass with respect to 100 parts by mass of the rubber component. These vulcanization accelerators may be used alone or in combination of two or more.

  In the rubber composition of the present invention, process oil or the like can be used as a softening agent, and examples of the process oil include paraffinic oil, naphthenic oil, and aromatic oil. Among these, aromatic oils are preferable from the viewpoint of tensile strength and wear resistance, and naphthenic oils and paraffinic oils are preferable from the viewpoint of hysteresis loss and low-temperature characteristics. The amount of these process oils used is preferably in the range of 0 to 100 parts by mass with respect to 100 parts by mass of the rubber component. If the amount of process oil used exceeds 100 parts by mass with respect to 100 parts by mass of the rubber component, the tensile strength and low heat build-up of the vulcanized rubber tend to deteriorate.

  In addition to the rubber component, (co) polymer, filler such as carbon black and silica, silane coupling agent, crosslinking agent, vulcanization accelerator, softener, the rubber composition of the present invention includes, for example, aging Additives usually used in the rubber industry such as an inhibitor, zinc oxide, stearic acid, an antioxidant, an ozone deterioration inhibitor and the like can be appropriately selected and blended within a range that does not impair the object of the present invention.

  The rubber composition of the present invention is obtained by kneading using a kneading machine such as a roll or an internal mixer, and after molding, vulcanization is performed, and the tire tread, undertread, carcass, sidewall, It can be used for tire applications such as beads, anti-vibration rubber, belts, hoses, and other industrial products, but is particularly suitable as a tire tread.

  The pneumatic tire of the present invention is characterized by using the rubber composition in a tread. Since the tire is formed by applying a rubber composition having a high tan δ and good fracture characteristics to the tread, the tire has excellent handling stability and good fracture resistance. The tire of the present invention is not particularly limited except that the above rubber composition is used for the tread, and can be produced according to a conventional method. Moreover, as gas with which this tire is filled, inert gas, such as nitrogen, argon, helium other than normal or the air which adjusted oxygen partial pressure, can be used.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

(Synthesis Example 3)
500 g of cyclohexane and 100 g of tert-butyl acrylate that had been dried in advance were added to a 2 L stainless steel pressure-resistant reaction vessel with a temperature adjustment jacket that had been dried and purged with nitrogen, respectively. After adjusting the jacket temperature and adjusting the internal temperature to 40 ° C., 10 mmol of triethylaluminum was added, and further a hexane solution (n-BuLi: 10 mmol) of n-butyllithium (n-BuLi) was added to carry out a polymerization reaction. . The polymerization reaction was carried out while adjusting the temperature so that the temperature of the polymerization system became 75 ° C. after 25 minutes. Further, after maintaining the temperature of the polymerization system for 15 minutes, 0.5 mL of an isopropanol solution (BHT concentration = 5%) of 2,6-di-tert-butylparacresol (BHT) was added to the polymerization system to stop the polymerization reaction. The polymer C was obtained by drying according to a conventional method. When the molecular weight was measured with gel permeation chromatography [GPC: Tosoh HLC-8020, column: Tosoh GMH-XL (two in series), detector: differential refractometer (RI)] based on monodisperse polystyrene. , polymer C thus obtained had a polystyrene reduced weight-average molecular weight (Mw) of Tsu 10 × 10 ³ der. Further, the polymer C had a glass transition point of 107 ° C.

(Synthesis Example 4)
500 g of cyclohexane, 80 g of tert-butyl acrylate and 20 g of styrene, which had been dried in advance, were added to a 2 L stainless steel pressure-resistant reaction vessel with a temperature control jacket that had been dried and purged with nitrogen, respectively. After adjusting the jacket temperature and adjusting the internal temperature to 40 ° C., 10 mmol of triethylaluminum was added, and further a hexane solution (n-BuLi: 10 mmol) of n-butyllithium (n-BuLi) was added to carry out a polymerization reaction. . The polymerization reaction was carried out while adjusting the temperature so that the temperature of the polymerization system became 75 ° C. after 25 minutes. Further, after maintaining the temperature of the polymerization system for 15 minutes, 0.5 mL of an isopropanol solution (BHT concentration = 5%) of 2,6-di-tert-butylparacresol (BHT) was added to the polymerization system to stop the polymerization reaction. The copolymer D was obtained by drying according to a conventional method. In the obtained copolymer D, tert-butyl acrylate units were 80% by mass, styrene units were 20% by mass, and styrene was randomly connected. The copolymer D had a polystyrene equivalent weight average molecular weight (Mw) of 10 × 10 ³ and a glass transition point of 105 ° C.

  Next, using the above (co) polymer, rubber compositions having the compounding formulations shown in Tables 1 and 2 were prepared according to a conventional method, and the resulting rubber composition had the following fracture resistance and handling stability. The method was evaluated. The results are shown in Tables 1 and 2.

(1) Fracture resistance A tensile test was performed in accordance with JIS K6301-1995, and the tensile strength (Tb) of the vulcanized rubber composition was measured. In Table 1, the tensile strength of Comparative Example 1 was taken as 100. An index is displayed, and in Table 2, the tensile strength of Comparative Example 4 is set to 100 and the index is displayed. The larger the index value, the better the fracture resistance.

(2) Steering stability Using a rheometrics mechanical spectrometer, tan δ was measured at a shear strain of 5%, a temperature of 60 ° C., and a frequency of 15 Hz. In 2, the tan δ of Comparative Example 4 was taken as 100 and indicated as an index. The larger the index value, the greater the hysteresis loss and the better the steering stability.

* 1 JSR Co., Ltd., SBR1500, styrene content = 23.5 mass%, vinyl bond content = 18%, polystyrene equivalent weight average molecular weight = 444 × 10 ³ .
* 2 ISAF, Tokai Carbon Co., Ltd., Seest 3H.
* 3 N- (1,3-dimethylbutyl) -N'-phenyl-p-phenylenediamine, "NOCRACK 6C" manufactured by Ouchi Shinsei Chemical Industry.
* 4 1,3-Diphenylguanidine, “Noxeller D” manufactured by Ouchi Shinsei Chemical Industry.
* 5 Nt-Butyl-2-benzothiazolylsulfenamide, “Noxeller NS” manufactured by Ouchi Shinsei Chemical Industry.
* 6 Liquid styrene-butadiene copolymer, styrene content = 20 mass%, vinyl bond content = 65%, polystyrene equivalent weight average molecular weight = 10 x 10 ³ .
* 7 Nippon Petrochemical's Neopolymer 140 (product side), polystyrene equivalent weight average molecular weight = 2 × 10 ³ .
* 10 Polymer C obtained in Synthesis Example 3.
* 11 Copolymer D obtained in Synthesis Example 4.

* 2 to * 7 and * 10 to * 11 are the same as Table 1.
* 12 SBR synthesized by solution polymerization, styrene content = 38 mass%, vinyl bond content = 35%, polystyrene equivalent weight average molecular weight = 400 x 10 ³ .

From the results of Comparative Examples 2 and 5 in Table 1 and Table 2, by blending liquid SBR, the steering stability can be improved while improving the fracture resistance of the rubber composition, but the improvement range of the steering stability is I know that it is small. Further, from the results of Comparative Examples 3 and 6, it can be seen that, by adding C ₉ aromatic petroleum resin, the handling stability can be improved, but the fracture resistance is greatly reduced.

On the other hand, from the results of Examples 3, 4, 7 and 8, poly tert- butyl acrylate, by blending a styrene · tert- butyl acrylate copolymer, while ensuring the fracture resistance of the rubber composition, steering stability It can be seen that the sex can be greatly improved.

Claims (5)

A (co) polymer obtained by polymerizing at least one alkyl acrylate containing at least tert-butyl acrylate with respect to 100 parts by mass of a rubber component composed of a diene rubber, and having a polystyrene-equivalent weight average molecular weight of 2 × 10 A rubber composition comprising 5 to 150 parts by mass of a (co) polymer characterized by ³ to 50 × 10 ³ . The (co) polymer, and one or more alkyl acrylates containing at least tert- butyl acrylate, rubber composition according to claim 1, characterized in that a copolymer obtained by copolymerizing a styrene . 2. The rubber composition according to claim 1 , wherein the rubber component contains a copolymer of a conjugated diene compound having a polystyrene-equivalent weight average molecular weight of 300 × 10 ^{3 to} 3,000 × 10 ³ and an aromatic vinyl compound. . The rubber composition according to claim 3 , wherein the copolymer of the conjugated diene compound and the aromatic vinyl compound is a styrene-butadiene copolymer rubber. A pneumatic tire using the rubber composition according to claim 1 for a tread.