JP2005325311A - Rubber composition and pneumatic tire obtained using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rubber composition excellent in hysteresis loss property and failure characteristics. <P>SOLUTION: The rubber composition comprises 100 pts. mass of a rubber component (A) comprising a copolymer of a conjugated diene with an aromatic vinyl compound having a weight-average molecular weight of 300,000-3,000,000 in terms of polystyrene and 5-150 pts. mass of a copolymer (B) of the conjugated diene with the aromatic vinyl compound having a weight-average molecular weight of 3,000-90,000 in terms of polystyrene, where the sum of the content of the aromatic vinyl compound and a half of the content of vinyl bonds in the conjugated diene is at least 70 wt.% in the copolymer (B), and the content of the aromatic vinyl compound in the copolymer (B) is at least 40 wt.% and larger than the content of the aromatic vinyl compound in the rubber component (A) by 5 wt.%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a rubber composition and a pneumatic tire using the same.

  With the recent evolution of advanced power performance of passenger cars, there is a need for better handling stability as a tire characteristic. For this reason, several techniques for improving steering stability have been developed so far. It is known that improving the hysteresis loss of the rubber composition at room temperature or higher is effective for ensuring the steering stability.

  As a technique for improving the hysteresis loss, a method using a liquid polymer having a high glass transition temperature Tg is known. The microstructure of the styrene-butadiene copolymer (SBR) obtained by solution polymerization or emulsion polymerization can be changed depending on the purpose. By using this property and using a styrene-butadiene copolymer having a high styrene content or butadiene vinyl bond content in the rubber composition, good hysteresis loss can be obtained (for example, see Patent Document 1). ).

  However, this technique increases the glass transition temperature Tg of the rubber composition. Therefore, especially at room temperature, the tire becomes hard and loses rubber elasticity, which is naturally limited. The styrene content of the current styrene-butadiene copolymer having a high glass transition temperature Tg is limited to about 45% by weight. For example, in solution polymerization, the styrene content is about 40% by weight and the vinyl bond content is about 50% by weight.

  In order to eliminate such drawbacks, the hardness at low temperature is limited by mixing a styrene-butadiene copolymer having a high glass transition temperature Tg and a polybutadiene (BR) having a low glass transition temperature Tg. However, attempts have been made to obtain high hysteresis loss.

Improvement techniques using liquid polymers in rubber compositions have also been used. When a liquid polymer is used for the rubber composition, high hysteresis loss is obtained due to the entanglement effect of low molecules. This technique is excellent in that a high hysteresis loss can be secured regardless of the glass transition temperature Tg.
JP-A-61-203145

  However, the method of mixing polymers having different glass transition temperatures Tg has a problem that the softening point of the softening agent is low when combined with a general softening agent (for example, oil-extended rubber). The effect is not obtained. In addition, it cannot be said that the technique using a liquid polymer can provide sufficient hysteresis loss for the required characteristics associated with the recent high performance of passenger cars. Therefore, introduction of a new method for improving hysteresis loss is desired.

  In addition, as a required characteristic of such high-performance tires, securing steering stability on dry road surfaces and wet road surfaces is an important issue, and securing fracture characteristics typified by wear resistance is also important from the viewpoint of economy. It is a difficult task.

  Then, an object of this invention is to provide the rubber composition which can acquire high hysteresis loss property and fracture characteristics, and a pneumatic tire using the same.

  As a result of intensive studies to achieve the above object, the present inventors surprisingly have a high styrene content that has a glass transition temperature Tg higher than that of the rubber component and is semi-compatible or incompatible with the rubber component. Hysteresis at a very high level by blending a quantity of liquid polymer conjugated diene and aromatic vinyl compound into a rubber component containing a high molecular weight conjugated diene and aromatic vinyl compound copolymer It was found that lossability can be improved. Furthermore, it has also been found that by adding a liquid polymer to the rubber component, high fracture characteristics can be maintained as compared with the case of using oil or the like.

  Specifically, the present inventors attempted to apply a technique of mixing polymers having different glass transition temperatures Tg to a liquid polymer having viscoelastic properties that are semi-compatible or incompatible. There are several studies using liquid polymers in rubber compositions. However, the study examples intentionally set the compatibility range with the rubber component as the microstructure of the liquid polymer for the purpose of suppressing the deterioration of fracture characteristics due to the phase separation phenomenon of low molecules and improving the hysteresis loss property due to the entanglement of molecules. It is limited to what is used.

  On the other hand, the present inventors have studied intentionally blending a liquid polymer in a semi-compatible to incompatible region with a rubber component for the purpose of maximizing hysteresis loss. As a result, with respect to hysteresis loss, a rubber composition having unprecedented high hysteresis loss that exceeds the intention of the present inventors was obtained. Furthermore, it has been clarified that the deterioration of the fracture characteristics, which is a concern due to the deterioration of compatibility, can be surprisingly minimized. Based on these findings, the inventors have completed the present invention.

  That is, the rubber composition of the present invention has a polystyrene equivalent weight with respect to 100 parts by mass of the rubber component (A) containing a copolymer of a conjugated diene and an aromatic vinyl compound having a polystyrene equivalent weight average molecular weight of 300,000 to 3000000. 5 to 150 parts by mass of a copolymer (B) of a conjugated diene having an average molecular weight of 3000 to 90000 and an aromatic vinyl compound, and the content of the aromatic vinyl compound in the copolymer (B) is ½ × The total of the vinyl bond content of the conjugated diene is 70% by weight or more, the content of the aromatic vinyl compound of the copolymer (B) is 40% by weight or more, and the content of the aromatic vinyl compound of the copolymer (B) Is 5% by weight more than the content of the aromatic vinyl compound of the rubber component (A). According to such a rubber composition, high hysteresis loss and fracture characteristics can be obtained.

  The aromatic vinyl compound of the rubber component (A) and the copolymer (B) is preferably styrene. The conjugated diene of the rubber component (A) is preferably butadiene, and the conjugated diene of the copolymer (B) is preferably butadiene and / or isoprene. The rubber component (A) may contain a styrene-butadiene copolymer and a diene rubber other than the styrene-butadiene copolymer.

  Further, the content of the aromatic vinyl compound in the copolymer of the conjugated diene and the aromatic vinyl compound contained in the rubber component (A) is more preferably 10 to 50% by weight. It is more preferable that the content of the aromatic vinyl compound in the copolymer (B) is more than 15% by weight than the content of the aromatic vinyl compound in the rubber component (A). Moreover, it is preferable that a rubber composition contains a filler and a crosslinking agent.

  The pneumatic tire of the present invention is characterized by using any of the above rubber compositions in a tread portion. According to such a pneumatic tire, by using a rubber composition having high hysteresis loss and fracture characteristics for the tread portion, it is possible to ensure extremely high and safe driving stability and wear resistance. The fracture characteristics represented by can be secured.

  ADVANTAGE OF THE INVENTION According to this invention, the rubber composition which can acquire high hysteresis loss property and fracture characteristics, and a pneumatic tire using the same can be provided.

  The rubber composition according to this embodiment includes a rubber component (A) containing a copolymer of a conjugated diene and an aromatic vinyl compound, and a copolymer (B) of a conjugated diene and an aromatic vinyl compound. That is, the rubber composition of the present embodiment comprises a conjugated diene, which is a liquid polymer having a specific molecular structure, with respect to the rubber component (A) serving as a matrix containing a copolymer of a conjugated diene and an aromatic vinyl compound. A copolymer (B) with an aromatic vinyl compound is blended as a softening agent.

  As the conjugated diene of the rubber component (A), for example, butadiene, isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene, 2-phenyl-1,3-butadiene, 1,3-hexadiene and the like are used. it can. These conjugated butadienes may be used alone or in a combination of two or more. In particular, the conjugated diene of the rubber component (A) is preferably butadiene.

  Examples of the aromatic vinyl compound of the rubber component (A) include styrene, α-methylstyrene, 1-vinylnaphthalene, 3-vinyltoluene, ethylvinylbenzene, divinylbenzene, 4-cyclohexylstyrene, 2,4,6- Trimethylstyrene or the like can be used. In particular, as the aromatic vinyl compound of the rubber component (A), styrene is preferable due to availability.

  Furthermore, when copolymerization is performed using a conjugated diene monomer and an aromatic vinyl compound monomer, the monomer is readily available and excellent in practicality, and anionic polymerization characteristics are excellent in terms of living properties and the like. Therefore, it is particularly preferable to use butadiene as the conjugated diene monomer and styrene as the aromatic vinyl compound.

  From the above, the copolymer of the conjugated diene and the aromatic vinyl compound contained in the rubber component (A) is preferably a styrene-butadiene copolymer (SBR). Any styrene-butadiene copolymer (SBR) may be used as long as it is generally available, and the effects of the present invention can be obtained by using any styrene-butadiene copolymer (SBR). The rubber component (A) may contain a styrene-butadiene copolymer (SBR) and a diene rubber or natural rubber (NR) other than the styrene-butadiene copolymer (SBR).

  As this diene rubber, for example, polybutadiene (BR), polyisoprene (IR), butyl rubber (IIR), ethylene-propylene copolymer (EPDM), acrylonitrile-butadiene copolymer (NBR) or the like can be used. . These diene rubbers may be used alone or in a combination of two or more. Further, as the copolymer of the conjugated diene and the aromatic vinyl compound, a copolymer having a branched structure by partially using a polyfunctional modifier such as tin tetrachloride may be used. The diene rubber and natural rubber supplementarily blended with the styrene-butadiene copolymer (SBR) are preferably 60 parts by weight or less of 100 parts by weight of the rubber component (A).

  The copolymer of the conjugated diene and the aromatic vinyl compound contained in the rubber component (A) is preferably randomly copolymerized. As the polymerization method, a general diene polymerization method such as anionic polymerization or emulsion polymerization can be used.

  The polystyrene-converted weight average molecular weight of the copolymer of the conjugated diene and the aromatic vinyl compound contained in the rubber component (A) can be selected from the molecular region of the copolymer of the conjugated diene and the aromatic vinyl compound. It is preferable that it is 300,000-3 million. When the molecular weight is less than 300,000, the fracture resistance and wear resistance are lowered. When the molecular weight exceeds 3000000, the viscosity of the polymerization solution increases and the productivity is poor.

  The molecular weight of the copolymer of the conjugated diene and the aromatic vinyl compound contained in the rubber component (A) is the sum of the blending amount of the copolymer (B) and the blending amount of the softening agent other than the copolymer (B). It is preferable to select based on the total amount of the softening agent and / or the blending amount of the filler. Specifically, when the total amount of the softening agent is small, the blending amount of the filler is reduced, and a copolymer of a conjugated diene and an aromatic vinyl compound having a low molecular weight within a molecular weight range of 300000 to 3000000 is used as a rubber component. It is preferable to use for (A). On the other hand, when the total amount of the softener and the blending amount of the filler are large, a copolymer of a conjugated diene having a high molecular weight and an aromatic vinyl compound having a molecular weight in the range of 300000 to 3000000 may be used for the rubber component (A). preferable.

  The content of the aromatic vinyl compound in the copolymer of the conjugated diene and the aromatic vinyl compound contained in the rubber component (A) is preferably 10 to 50% by weight. If the content of the aromatic vinyl compound is less than 10% by weight, the fracture characteristics are lowered. When the content of the aromatic vinyl compound exceeds 50% by weight, the wear resistance decreases.

  As the conjugated diene of the copolymer (B), for example, butadiene, isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene, 2-phenyl-1,3-butadiene, 1,3-hexadiene, and the like are used. Can do. These conjugated butadienes may be used alone or in a combination of two or more. In particular, the conjugated diene of the copolymer (B) is preferably butadiene, isoprene, or a mixture of butadiene and isoprene.

  Examples of the aromatic vinyl compound of the copolymer (B) include styrene, α-methylstyrene, 1-vinylnaphthalene, 3-vinyltoluene, ethylvinylbenzene, divinylbenzene, 4-cyclohexylstyrene, 2,4,6 -Trimethylstyrene or the like can be used. In particular, as the aromatic vinyl compound of the copolymer (B), styrene is preferable due to availability.

  Furthermore, when copolymerization is performed using a conjugated diene monomer and an aromatic vinyl compound monomer, the monomer is readily available and excellent in practicality, and anionic polymerization characteristics are excellent in terms of living properties and the like. Therefore, it is particularly preferable to use butadiene as the conjugated diene monomer and styrene as the aromatic vinyl compound.

  From the above, the copolymer of the conjugated diene and the aromatic vinyl compound contained in the copolymer (B) is preferably a styrene-butadiene copolymer (SBR). Any styrene-butadiene copolymer (SBR) may be used as long as it is generally available, and the effects of the present invention can be obtained by using any styrene-butadiene copolymer (SBR). Furthermore, it is preferable that the copolymer (B) is randomly copolymerized. As the polymerization method, a general diene polymerization method such as anionic polymerization or emulsion polymerization can be used.

  Although the polystyrene conversion weight average molecular weight of a copolymer (B) can be selected from the molecular area | region of the copolymer of a conjugated diene and an aromatic vinyl compound, it is preferable that it is 3000-90000. This molecular weight region 3000 to 90000 is an optimum range for obtaining the effect of improving the hysteresis loss, and the effect of improving the hysteresis loss is reduced outside this molecular weight region. Further, when the molecular weight is less than 3000, the fracture characteristics are also lowered. When the molecular weight exceeds 900,000, the processability of the rubber composition is also lowered. Although it depends on the purpose, the weight average molecular weight in terms of police styrene of the copolymer (B) is more preferably 5,000 to 50,000. According to this, when this rubber composition is used for a tread portion, good performance can be expressed.

  The content of the aromatic vinyl compound in the copolymer (B) is preferably determined based on the relationship with the rubber component (A). The content of the aromatic vinyl compound in the copolymer (B) is preferably 5% by weight more than the content of the aromatic vinyl compound in the rubber component (A). When the content of the aromatic vinyl compound in the copolymer (B) is less than 5% by weight from the content of the aromatic vinyl compound in the rubber component (A), the molecules of the copolymer (B) and the rubber component (A ) Are not sufficiently separated from the molecule, and the effect of improving the hysteresis loss cannot be sufficiently obtained. Furthermore, it is more preferable that the content of the aromatic vinyl compound in the copolymer (B) is more than 15% by weight than the content of the aromatic vinyl compound in the rubber component (A).

  Furthermore, it is preferable that the total of the content of the aromatic vinyl compound of the copolymer (B) and the vinyl bond content of 0.5 × conjugated diene is 70% by weight or more. Moreover, it is preferable that content of the aromatic vinyl compound of a copolymer (B) is 40 weight% or more. By using the copolymer (B) having a large content of such an aromatic vinyl compound, the copolymer (B) may be semi-compatible or incompatible with the rubber component (A). it can.

  The vinyl bond content of the conjugated diene of the copolymer (B) is not limited, but is preferably 10 to 70% by weight. According to this, a copolymer (B) obtained by anionic polymerization or emulsion polymerization using an easily available low molecular weight polymer can be used.

  It is preferable that a copolymer (B) contains 5-150 weight part with respect to 100 weight part of rubber components (A). When the copolymer (B) is less than 5 parts by weight, the effect of improving the hysteresis loss is reduced. When the copolymer (B) exceeds 150 parts by weight, the softener becomes excessively mixed, and the fracture characteristics of the rubber composition are not sufficient.

  The rubber composition of this embodiment preferably further contains a filler (filler). As the filler, it is preferable to use a reinforcing filler such as carbon black or silica. These fillers may be used alone or in a combination of two or more.

  There is no restriction | limiting in particular in carbon black, For example, FEF, SRF, HAF, ISAF, SAF etc. can be used. Carbon black preferably has an iodine adsorption amount (IA) of 60 mg / g or more and a dibutyl phthalate oil absorption (DBP) of 80 ml / 100 g or more. By using carbon black as a filler, the characteristics of the rubber composition are greatly improved. In particular, the use of HAF, ISAF, or SAF is preferable because it can improve wear resistance.

  When carbon black is used alone as the filler, the rubber composition preferably contains 10 to 250 parts by weight of carbon black with respect to 100 parts by weight of the rubber component (A). When the carbon black is less than 10 parts by weight, the effect of improving the fracture characteristics and the like decreases. When carbon black exceeds 250 weight part, the workability of a rubber composition will be inferior. More preferably, the rubber composition contains 20 to 150 parts by weight of carbon black with respect to 100 parts by weight of the rubber component (A). According to this, the reinforcement effect by carbon black and the improvement efficiency of the characteristic of a rubber composition by it can be improved.

  Silica is not particularly limited, and for example, wet silica (hydrous silicic acid), dry silica (anhydrous silicic acid), calcium silicate, aluminum silicate, and the like can be used. In particular, the use of wet silica is preferable because it has a high effect of improving fracture characteristics, and can achieve both high wet grip properties (braking performance and handling stability on wet road surfaces) and low rolling resistance.

  When silica is used alone as the filler, the rubber composition preferably contains 10 to 250 parts by weight of silica with respect to 100 parts by weight of the rubber component (A). When the silica is less than 10 parts by weight, the effect of improving the fracture characteristics and the like is reduced. When silica exceeds 250 weight part, the processability of a rubber composition will be inferior. More preferably, the rubber composition contains 20 to 150 parts by weight of silica with respect to 100 parts by weight of the rubber component (A). According to this, the reinforcement effect by silica and the improvement efficiency of the characteristic of a rubber composition by it can be improved.

  Furthermore, when silica is used as the filler, the rubber composition preferably contains a silane coupling agent. According to this, the reinforcing effect by silica can be further improved. For example, as a silane coupling agent, bis (3-triethoxysilylpropyl) tetrasulfide, bis (3-triethoxysilylpropyl) trisulfide, bis (3-triethoxysilylpropyl) disulfide, bis (2-triethoxy) Silylethyl) tetrasulfide, bis (3-trimethoxysilylpropyl) tetrasulfide, bis (2-trimethoxysilylethyl) tetrasulfide, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyl Trimethoxysilane, 2-mercaptoethyltriethoxysilane, 3-trimethoxysilylpropyl-N, N-dimethylthiocarbamoyl tetrasulfide, 3-triethoxysilylpropyl-N, N-dimethylthiocarba Yl tetrasulfide, 2-triethoxysilylethyl-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-methylthiocarbamoyl tetrasulfide, dimethoxymethyl Silylpropylbenzothiazole tetrasulfide and the like can be blended. In particular, bis (3-triethoxysilylpropyl) polysulfide, 3-trimethoxysilylpropylbenzothiazole tetrasulfide and the like are preferable because they have a high reinforcing effect.

  The rubber composition of the present embodiment preferably further contains a crosslinking agent. Sulfur can be used as the crosslinking agent, and vulcanization is performed. The rubber composition preferably contains 0.1 to 10 parts by weight of sulfur with respect to 100 parts by weight of the rubber component (A). If the sulfur is less than 0.1 part by weight, the rupture strength, abrasion resistance, and low heat build-up of the vulcanized rubber composition may be reduced. When sulfur exceeds 10 weight part, a rubber composition will lose rubber elasticity. The rubber composition more preferably contains 1 to 5 parts by weight of sulfur with respect to 100 parts by weight of the rubber component (A).

  When sulfur is used as the crosslinking agent, the rubber composition preferably contains a vulcanization accelerator. The vulcanization accelerator is not particularly limited, but is a thiazole-based compound such as M (2-lucaptobenzothiazole), DM (dibenzothiazyl sulfide), CZ (N-cyclohexyl-2-benzothiazylsulfenamide), etc. Vulcanization accelerators and guanidine vulcanization accelerators such as DPG (diphenylguanidine) are preferred. The rubber composition preferably contains 0.1 to 5 parts by weight, more preferably 0.2 to 3 parts by weight, of the vulcanization accelerator with respect to 100 parts by weight of the rubber component (A).

  The rubber composition of this embodiment may further contain a softening agent other than the copolymer (B). As the softening agent, for example, paraffinic, naphthenic, and aromatic process oils can be used. When the rubber composition is used for applications in which tensile strength and wear resistance are important, an aromatic system is preferable. When the rubber composition is used for applications in which low temperature characteristics are important, naphthenic or paraffinic compounds are preferable. The softening agent other than the copolymer (B) is preferably 100 parts by weight or less with respect to 100 parts by weight of the rubber component (A). If the softening agent other than the copolymer (B) exceeds 100 parts by weight, the tensile strength and low heat buildup of the rubber composition may be deteriorated.

  In addition to the above, the rubber composition of the present embodiment is a compounding agent usually used in the rubber industry, such as anti-aging agent, zinc oxide such as zinc white, stearic acid, antioxidant, ozone degrading agent, etc. Can be included.

  The rubber composition of the present embodiment is produced by blending the copolymer (B) and other blending components with the rubber component (A) and kneading them using a kneader such as a roll or an internal mixer. it can. Vulcanization is performed after molding and processing. The rubber composition of the present embodiment can be suitably used for a tread portion, an under tread portion, a carcass, a sidewall, a bead portion, and a shoulder portion of a pneumatic tire. The rubber composition can be suitably used particularly for the tread portion. Furthermore, the rubber composition of the present embodiment can be used for industrial products such as vibration-proof rubbers, belts, hoses, and the like, and good performance can be obtained as well.

  According to such a rubber composition according to the present embodiment, high hysteresis loss and fracture characteristics can be obtained. This is because, in order to obtain higher hysteresis loss, the rubber component (A) has a partial phase separation rather than the liquid polymer and the rubber component molecules being mixed and intertwined with each other. This is probably because the molecules of the copolymer (B), which is a liquid polymer, are entangled with each other. In addition, regarding the point that the degradation of fracture characteristics can be minimized, by using a diene copolymer as the copolymer (B) which is a liquid polymer, the sulfur vulcanization phenomenon is partially caused at the phase separation interface. Presumed to have occurred.

  The pneumatic tire according to the present embodiment uses the rubber composition for a tread. The pneumatic tire is not particularly limited except that the rubber composition is used, and has a normal structure and is manufactured by a normal method. For example, as the gas filled in the pneumatic tire, normal air, air with adjusted oxygen partial pressure, or an inert gas such as nitrogen, argon, or helium can be used. According to such a pneumatic tire according to the present embodiment, by using a rubber composition having high hysteresis loss and fracture characteristics for the tread portion, it is possible to ensure extremely high and safe driving stability, In addition, it is possible to ensure fracture characteristics represented by wear resistance. Therefore, the pneumatic tire can achieve both steering stability and economy.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited to the following Example at all. The physical properties of the copolymer (B) and the rubber composition were measured by the following methods.

[Method for measuring physical properties of copolymer (B)]
The styrene content and the vinyl bond content of butadiene were calculated by measuring their ¹ H-NMR spectra and integrating their ratios. The weight average molecular weight in terms of polystyrene is measured by gel permeation chromatography <GPC: HLC-8020 manufactured by Tosoh Corporation, column: GMH-XL (two in series) manufactured by Tosoh Corporation>, and a differential refractometer (RI) is used. Using monodisperse polystyrene as a standard, this was performed in terms of polystyrene. The microstructure was analyzed by Morero method with infrared spectroscopy.

[Method for measuring physical properties of rubber composition]
The compatibility of the copolymer (B) with the rubber component (A) was relatively evaluated by visually checking the transparency. The higher the transparency of the rubber composition, the higher the compatibility. Therefore, the evaluation criteria are good compatibility when transparent (indicated as “◎” in this case), and semi-compatible when there are opaque parts (the opaque parts are scattered small). In this case, it was indicated as “◯”, and when the opaque part was large, it was indicated as “Δ”), and when it was almost entirely opaque, it was incompatible (in this case, indicated as “x”). The fracture characteristics were measured in accordance with JIS K6301-1995 for tensile strength (hereinafter referred to as “Tb”). The measured value was shown as an index with the fracture characteristic (Tb) of Comparative Example 1 as 100. Steering stability was evaluated by hysteresis loss. Hysteresis loss property was evaluated by tan δ measured using a viscoelasticity measuring device (Rheometric Mechanical Spectrometer) at a shear strain of 5%, a temperature of 60 ° C., and a frequency of 15 Hz. The measured value was shown as an index with the steering stability (tan δ) of Comparative Example 1 as 100. It shows that hysteresis loss property is so high that a numerical value is large, and steering stability is favorable.

[Production Example of Copolymer (B)]
In advance, a 2 L stainless steel pressure-resistant reaction vessel with a temperature adjusting jacket was dried and purged with nitrogen to dry cyclohexane, butadiene and styrene. First, 500 g of the prepared cyclohexane, 65 g of butadiene, and 35 g of styrene were injected into the prepared stainless steel pressure-resistant reaction vessel. Next, after adjusting the internal temperature of the jacket to 40 ° C., 4 mmol of bistetrahydrofurylpropane was added. Thereafter, 18 mmol of a hexane solution containing n-butyllithium (n-BuLi) as an initiator was added to perform polymerization. At this time, 25 minutes after the addition of the initiator, the polymerization was carried out while adjusting the temperature so that the polymerization temperature became 75 ° C. After the completion of polymerization, in this example, after another 35 minutes from 25 minutes, 0.12 mmol of silicon tetrachloride (STC) was added as a coupling agent. 15 minutes after the addition of the coupling agent, 0.5 ml of a 5% solution of isopropanol containing 2.6 ditertiarybutyl paracresol (BHT) was added to stop the reaction. Furthermore, it dried in accordance with the normal method, and obtained the copolymer (B) which is a liquid styrene-butadiene copolymer.

  When the obtained copolymer (B) was analyzed, it was a random copolymer having a styrene content of 35% by weight, a butadiene vinyl bond content of 53%, and a polystyrene equivalent weight average molecular weight of 9000.

Next, by changing the blending amounts of butadiene and styrene in the production example of the copolymer (B), as the copolymer (B), the liquid styrene of Comparative Examples 1 and 2 and Examples 1 to 4 were used. A butadiene copolymer was produced. And these styrene content, the vinyl bond content of butadiene, and the polystyrene conversion weight average molecular weight were measured. Furthermore, according to the composition shown in Table 1, the rubber compositions of Comparative Examples 1 and 2 and Examples 1 to 4 were produced, and the compatibility, fracture characteristics, and steering stability were measured. As the styrene-butadiene copolymer (SBR) of the rubber component (A) serving as a matrix, the measurement results when SBR (a) is used are shown in Table 2, and the measurement results when SBR (b) is used are shown in Table 3. Shown in In Tables 1 to 3, “St / Vi” represents the styrene content (St) and the vinyl bond content (Vi) of butadiene. “Liq.SBR” indicates a liquid styrene-butadiene copolymer.

  The rubber compositions of Comparative Examples 1 and 3 using a liquid styrene-butadiene copolymer having a low styrene content as a softening agent showed high compatibility, but are copolymers (B) having a high styrene content. The rubber compositions of Examples 1 to 4 using a liquid styrene-butadiene copolymer as a softening agent became semi-compatible or incompatible. And the rubber composition of Examples 1-4 using the softening agent with high styrene content had high hysteresis loss property compared with the comparative examples 1 and 2 with low styrene content, and the steering stability improved. In addition, the rubber compositions of Examples 1 to 4 were suppressed to a minimum with almost no decrease in fracture characteristics (Tb). Therefore, according to the rubber compositions of Examples 1 to 4, it was possible to achieve both extremely high handling stability and fracture characteristics.

  In addition, according to the rubber compositions of Examples 2 and 4 in which the styrene content of the softening agent was higher, higher steering stability could be obtained. Further, according to the rubber compositions of Examples 3 and 4 in which the rubber component (A) serving as a matrix has a higher styrene content and higher vinyl bond content, higher handling stability and fracture characteristics (Tb) can be obtained. did it.

Claims (10)

  A conjugated diene having a polystyrene-equivalent weight average molecular weight of 3000 to 90000 with respect to 100 parts by mass of a rubber component (A) containing a copolymer of a conjugated diene and an aromatic vinyl compound having a polystyrene-equivalent weight average molecular weight of 300000 to 3000000. 5 to 150 parts by mass of a copolymer (B) of an aromatic vinyl compound, and the content of the aromatic vinyl compound of the copolymer (B) and the vinyl bond content of ½ × conjugated diene 70% by weight or more in total, the content of the aromatic vinyl compound in the copolymer (B) is 40% by weight or more, and the content of the aromatic vinyl compound in the copolymer (B) is the rubber component (A). A rubber composition characterized by being 5% by weight more than the content of the aromatic vinyl compound.   The rubber composition according to claim 1, wherein the aromatic vinyl compound of the rubber component (A) and the copolymer (B) is styrene.   The rubber composition according to claim 1 or 2, wherein the conjugated diene of the rubber component (A) is butadiene.   The rubber composition according to any one of claims 1 to 3, wherein the conjugated diene of the copolymer (B) is butadiene and / or isoprene.   The rubber composition according to any one of claims 1 to 4, wherein the rubber component (A) includes a styrene-butadiene copolymer and a diene rubber other than the styrene-butadiene copolymer. Stuff.   The content of an aromatic vinyl compound in a copolymer of a conjugated diene and an aromatic vinyl compound contained in the rubber component (A) is 10 to 50% by weight. The rubber composition according to item 1.   7. The content of the aromatic vinyl compound in the copolymer (B) is more than 15% by weight than the content of the aromatic vinyl compound in the rubber component (A). The rubber composition according to item.   The rubber composition according to any one of claims 1 to 7, further comprising a filler.   The rubber composition according to any one of claims 1 to 8, further comprising a cross-linking agent. A pneumatic tire using the rubber composition according to claim 1 in a tread portion.