JP5480781B2 - Rubber composition for pneumatic tread and pneumatic tire - Google Patents

Description

The present invention relates to a rubber composition for a base tread, and a pneumatic tire having a base tread produced using the rubber composition.

In order to protect the global environment, the fuel efficiency of automobiles is being reduced. For this reason, tires with low rolling resistance are also desired for automobile tires.

The rolling resistance of the tire is largely attributed to energy loss accompanying repeated deformation during traveling. As a method of reducing the rolling resistance, for example, a rubber composition having two layers of rubber in the tread portion having the highest contribution ratio (about 34%) in the rolling resistance and a small energy loss on the inner side (base tread). Have been proposed in which rubber compositions having excellent wear resistance are arranged on the outside (cap tread).

The rubber composition for base tread is designed with a small amount of reinforcing agent added in order to reduce energy loss. However, when the amount of the reinforcing agent is reduced, the rubber rigidity is lowered, and the steering stability (handling performance) tends to be deteriorated. On the other hand, when the amount of the reinforcing agent is increased to improve the rigidity, the rolling resistance increases and the fuel efficiency tends to deteriorate. Further, when the amount of vulcanizing agent such as sulfur is increased to improve the rigidity, the durability tends to deteriorate. Thus, it has been difficult to improve the fuel efficiency, steering stability and durability in a well-balanced manner in the base tread rubber composition.

Patent Document 1 discloses a rubber composition using a specific alkylphenol / sulfur chloride condensate as a method for improving fuel economy, steering stability and durability in a well-balanced manner. However, other methods are also required.

JP 2009-84533 A

An object of the present invention is to solve the above-mentioned problems and provide a rubber composition for a base tread that can improve fuel economy, steering stability and durability in a well-balanced manner, and a pneumatic tire using the rubber composition.

The present invention relates to a rubber composition for a base tread containing a rubber component and a reinforcing agent, wherein the rubber component includes natural rubber, butadiene rubber (1,2) containing 1,2-syndiotactic polybutadiene crystals, and a rare earth. Butadiene rubber (2) synthesized using an elemental catalyst, and the reinforcing agent content is 25 to 35% by mass in the total mass of 100% by mass of the base tread rubber composition. The present invention relates to a rubber composition.

In 100% by mass of the rubber component, the content of the natural rubber is 50% by mass or more, the content of the butadiene rubber (1) is 5 to 30% by mass, and the content of the butadiene rubber (2) is 5 to 30% by mass. % Is preferred.

In the rubber composition, the reinforcing agent content and the acetone extraction amount preferably satisfy the following formula.
3.0 ≦ (Reinforcing agent content / Acetone extraction amount) ≦ 9.0

The natural rubber preferably contains a modified natural rubber having a phosphorus content of 200 ppm or less.

In 100% by mass of the rubber component, the content of the modified natural rubber is preferably 20% by mass or more.

The present invention also relates to a pneumatic tire having a base tread produced using the rubber composition.

According to the present invention, since the rubber composition for base tread contains a rubber component containing natural rubber and a specific butadiene rubber and a predetermined amount of reinforcing agent, the rubber composition is used as a base tread of a pneumatic tire. By using it, it is possible to provide a pneumatic tire with improved fuel economy, steering stability and durability in a well-balanced manner.

The present invention is a rubber composition for a base tread containing a rubber component and a reinforcing agent, wherein the rubber component includes natural rubber and a 1,2-syndiotactic polybutadiene crystal butadiene rubber (1) (hereinafter, SPB-containing BR) and butadiene rubber (2) (hereinafter also referred to as rare earth BR) synthesized using a rare earth element-based catalyst, and the total mass of the rubber composition for base tread is 100% by mass. A rubber composition for a base tread in which the content of the reinforcing agent is 25 to 35% by mass.

Examples of BR generally used in the tire industry include high-cis BRs such as BR150B manufactured by Ube Industries, Ltd., and by replacing the high-sis BR with SPB-containing BR, rigidity can be improved. By substituting with the system BR, low fuel consumption and durability can be improved. Furthermore, the combined use of SPB-containing BR and rare earth-based BR can enhance the respective improvement effects. And in the rubber composition which used SPB containing BR and rare earth type BR together, a low fuel consumption, steering stability, and durability can be obtained with sufficient balance by mix | blending a predetermined amount of reinforcing agent.

As the SPB-containing BR, those widely used in the tire industry can be used, and those in which 1,2-syndiotactic polybutadiene crystals are chemically bonded to BR and dispersed are preferable. The 1,2-syndiotactic polybutadiene crystal contained can provide a sufficient complex elastic modulus and improve rigidity. Thereby, good steering stability can be obtained.

The melting point of the 1,2-syndiotactic polybutadiene crystal is preferably 180 ° C. or higher, more preferably 190 ° C. or higher. If it is less than 180 ° C., the 1,2-syndiotactic polybutadiene crystals melt at the time of rubber kneading, and the rigidity may be lowered. Moreover, this melting | fusing point becomes like this. Preferably it is 220 degrees C or less, More preferably, it is 210 degrees C or less. When it exceeds 220 degreeC, there exists a tendency for the dispersibility in a rubber composition to deteriorate.

In the SPB-containing BR, the content of 1,2-syndiotactic polybutadiene crystals is preferably 2.5% by mass or more, more preferably 10% by mass or more. If it is less than 2.5% by mass, the rigidity may not be sufficiently improved. Further, the content is preferably 20% by mass or less, more preferably 18% by mass or less. If it exceeds 20% by mass, the SPB-containing BR tends to be difficult to disperse in the rubber composition.

The content of SPB-containing BR in 100% by mass of the rubber component is preferably 5% by mass or more, more preferably 15% by mass or more, and further preferably 20% by mass or more. If it is less than 5% by mass, the rigidity may not be sufficiently improved. The content of the SPB-containing BR is preferably 30% by mass or less, more preferably 28% by mass or less. If it exceeds 30% by mass, the proportion of other rubber components decreases, and there is a possibility that the fuel economy, steering stability and durability cannot be improved in a well-balanced manner.

As the rare earth BR, those widely used in the tire industry can be used. As the rare earth element catalyst used for the synthesis (polymerization) of the rare earth element BR, a known one can be used. For example, a catalyst containing a lanthanum series rare earth element compound, an organoaluminum compound, an aluminoxane, a halogen-containing compound, and a Lewis base as necessary. Is mentioned. Among these, an Nd-based catalyst using a neodymium (Nd) -containing compound as the lanthanum series rare earth element compound is preferable from the viewpoint that BR having a high cis content and a low vinyl content can be obtained.

Examples of the lanthanum series rare earth element compounds include halides, carboxylates, alcoholates, thioalcolates, amides, and the like of rare earth metals having an atomic number of 57 to 71. Of these, as described above, the use of an Nd-based catalyst is preferable in that a BR having a high cis content and a low vinyl content can be obtained.

The organoaluminum compound is represented by AlR ^a R ^b R ^c (wherein R ^a , R ^b and R ^c are the same or different and represent hydrogen or a hydrocarbon group having 1 to 8 carbon atoms). Things can be used. Examples of the aluminoxane include a chain aluminoxane and a cyclic aluminoxane. As the halogen-containing compound, AlX _k R ^d _3-k (wherein X is a halogen, R ^d is an alkyl group having 1 to 20 carbon atoms, an aryl group or an aralkyl group, k is 1, 1.5, 2 or 3) Aluminum halides represented by: Me ₃ SrCl, Me ₂ SrCl ₂ , strontium halides such as MeSrHCl ₂ , MeSrCl ₃ ; metal halides such as silicon tetrachloride, tin tetrachloride, titanium tetrachloride, etc. . The Lewis base is used for complexing a lanthanum series rare earth element compound, and acetylacetone, ketone, alcohol and the like are preferably used.

The rare earth element-based catalyst may be used in the state of being dissolved in an organic solvent (n-hexane, cyclohexane, n-heptane, toluene, xylene, benzene, etc.) during the polymerization of butadiene, or may be silica, magnesia, magnesium chloride, etc. It may be used by being supported on an appropriate carrier. The polymerization conditions may be either solution polymerization or bulk polymerization. The preferable polymerization temperature is -30 to 150 ° C, and the polymerization pressure may be arbitrarily selected depending on other conditions.

The Mooney viscosity ML _{1 + 4} (100 ° C.) of the rare earth BR is preferably 35 or more, more preferably 40 or more. If it is less than 35, the viscosity of the unvulcanized rubber composition is low, and there is a possibility that an appropriate thickness cannot be ensured after vulcanization. The Mooney viscosity is preferably 55 or less, more preferably 50 or less. If it exceeds 55, the unvulcanized rubber composition becomes too hard and it may be difficult to extrude with a smooth edge.
The Mooney viscosity is measured according to ISO 289 and JIS K6300.

The ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the rare earth BR is preferably 1.2 or more, more preferably 1.5 or more. If it is less than 1.2, the workability tends to deteriorate. The Mw / Mn is preferably 5 or less, more preferably 4 or less. If it exceeds 5, the improvement effect of fuel efficiency tends to be reduced.

The Mw of the rare earth BR is preferably 300,000 or more, more preferably 480,000 or more, preferably 1,000,000 or less, more preferably 800,000 or less. The Mn of the rare earth BR is preferably 100,000 or more, more preferably 150,000 or more, preferably 600,000 or less, more preferably 400,000 or less. If Mw or Mn is less than the lower limit, fuel economy and durability tend to deteriorate. When the upper limit is exceeded, workability tends to deteriorate.
In the present invention, Mw and Mn are values converted from standard polystyrene using a gel permeation chromatograph (GPC).

The cis content of the rare earth BR is preferably 90% by mass or more, more preferably 93% by mass or more, and still more preferably 95% by mass or more. If it is less than 90% by mass, fuel economy may not be sufficiently improved.

The vinyl content of the rare earth BR is preferably 0.9% by mass or less, more preferably 0.8% by mass or less. If it exceeds 0.9 mass%, fuel economy and durability tend to deteriorate.
The vinyl content (1,2-bonded butadiene unit amount) and cis content (cis-1,4-bonded butadiene unit amount) of BR can be measured by infrared absorption spectrum analysis.

The content of the rare earth BR in 100% by mass of the rubber component is preferably 5% by mass or more, more preferably 10% by mass or more, and further preferably 13% by mass or more. If it is less than 5% by mass, fuel economy may not be sufficiently improved. The content of the SPB-containing BR is preferably 30% by mass or less, more preferably 20% by mass or less, and still more preferably 18% by mass or less. If it exceeds 30% by mass, the proportion of other rubber components decreases, and there is a possibility that the fuel economy, steering stability and durability cannot be improved in a well-balanced manner.

As the natural rubber (NR), those widely used in the tire industry can be used, and examples thereof include RSS # 3 and TSR20.

The content of NR in 100% by mass of the rubber component is preferably 50% by mass or more, more preferably 55% by mass or more, and further preferably 58% by mass or more. If it is less than 50% by mass, good durability may not be ensured. The NR content is preferably 80% by mass or less, more preferably 70% by mass or less. If it exceeds 80% by mass, the proportion of other rubber components decreases, and there is a possibility that the fuel efficiency, steering stability and durability cannot be improved in a well-balanced manner.

A preferred example of NR is modified natural rubber (HPNR) having a phosphorus content of 200 ppm or less. Since HPNR in which phospholipids contained in NR are reduced and removed (preferably HPNR from which protein is also removed) has the property of not generating heat, further reduction in fuel consumption can be achieved compared to the use of normal NR. it can. In addition, when SPB-containing BR and rare earth-based BR are used in combination, the viscosity of the unvulcanized rubber composition increases, and there is a concern about deterioration of kneading processability and extrusion processability. However, by blending HPNR, By reducing the viscosity of the vulcanized rubber composition, this concern can be resolved.

The modified natural rubber (HPNR) has a phosphorus content of 200 ppm or less. If it exceeds 200 ppm, the amount of gel increases during storage, and the tan δ of the vulcanized rubber increases, resulting in poor fuel economy, and the Mooney viscosity of the unvulcanized rubber tends to increase, resulting in poor processability. . The phosphorus content is preferably 150 ppm or less, and more preferably 110 ppm or less. Here, the phosphorus content can be measured by a conventional method such as ICP emission analysis. Phosphorus is derived from phospholipids (phosphorus compounds).

In the modified natural rubber, the nitrogen content is preferably 0.3% by mass or less, and more preferably 0.15% by mass or less. When the nitrogen content exceeds 0.3% by mass, the Mooney viscosity increases during storage and the processability tends to deteriorate, and there is a possibility that the fuel efficiency cannot be improved sufficiently. The nitrogen content can be measured by a conventional method such as Kjeldahl method. Nitrogen is derived from protein.

The modified natural rubber is preferably substantially free of phospholipids. “Substantially no phospholipid is present” represents a state in which a natural rubber sample is extracted with chloroform and a peak due to phospholipid does not exist at −3 ppm to 1 ppm in ³¹ P NMR measurement of the extract. The peak of phosphorus present at -3 ppm to 1 ppm is a peak derived from the phosphate structure of phosphorus in the phospholipid.

As a method for producing the modified natural rubber, for example, the production method described in JP 2010-138359 A, that is, the step (A) of obtaining a saponified natural rubber latex by saponifying the natural rubber latex, and the obtained Examples of the method include a step (B) of washing the saponified natural rubber latex until the phosphorus content in the rubber is 200 ppm or less. Specifically, first, a natural rubber latex is saponified with an alkali to prepare a saponified natural rubber latex, and then the agglomerated rubber obtained by agglomerating the saponified natural rubber latex contains phosphorus to the rubber content. A modified natural rubber (saponified natural rubber) can be produced by a method of repeatedly washing with water until the rate becomes 200 ppm or less and drying.

According to the above production method, the phosphorus compound separated by saponification is washed away, so that the phosphorus content of the natural rubber can be suppressed. Moreover, since the protein in natural rubber is decomposed by the saponification treatment, the nitrogen content of the natural rubber can be suppressed.

The content of the modified natural rubber in 100% by mass of the rubber component is preferably 20% by mass or more, more preferably 40% by mass or more, and further preferably 50% by mass or more. If it is less than 20% by mass, the effect of blending the modified natural rubber may not be sufficiently obtained. The content of the modified natural rubber is preferably 80% by mass or less, more preferably 70% by mass or less. If it exceeds 80% by mass, the proportion of other rubber components decreases, and there is a possibility that the fuel efficiency, steering stability and durability cannot be improved in a well-balanced manner.

As the reinforcing agent contained in the rubber composition of the present invention, reinforcing fillers widely used in the tire industry can be used, and carbon black and silica can be preferably used.
Carbon black and silica are not particularly limited, and common ones can be used. When silica is used, it is preferably used in combination with a known silane coupling agent in order to promote dispersion of silica.

Content of a reinforcing agent is 25-35 mass% in the total mass of 100 mass% of the rubber composition for base treads of this invention. If it is in the said range, low-fuel-consumption property, steering stability, and durability will be obtained with good balance. The lower limit of the content is preferably 28% by mass or more, more preferably 30% by mass or more.

The nitrogen adsorption specific surface area (N ₂ SA) of carbon black is preferably 50 m ² / g or more, more preferably 70 m ² / g or more. If it is less than 70 m < ² > / g, there exists a possibility that sufficient reinforcement may not be obtained. The nitrogen adsorption specific surface area of carbon black is preferably 130 m ² / g or less, more preferably 110 m ² / g or less. If it exceeds 130 m ² / g, the fuel efficiency tends to deteriorate.
In addition, the nitrogen adsorption specific surface area of carbon black is calculated | required by A method of JISK6217.

The content of carbon black is preferably 10 parts by mass or more, more preferably 40 parts by mass or more with respect to 100 parts by mass of the rubber component. If the amount is less than 10 parts by mass, sufficient reinforcement may not be obtained. The carbon black content is preferably 75 parts by mass or less, more preferably 65 parts by mass or less. If it exceeds 75 parts by mass, the fuel efficiency tends to deteriorate.

The nitrogen adsorption specific surface area (N ₂ SA) of silica is preferably 80 m ² / g or more, more preferably 100 m ² / g or more. If it is less than 80 m < ² > / g, there exists a possibility that sufficient reinforcement may not be obtained. The nitrogen adsorption specific surface area of carbon black is preferably 180 m ² / g or less, more preferably 150 m ² / g or less. If it exceeds 180 m ² / g, the fuel efficiency tends to deteriorate.

The content of silica is preferably 20 parts by mass or more, more preferably 40 parts by mass or more with respect to 100 parts by mass of the rubber component. If the amount is less than 20 parts by mass, sufficient reinforcement may not be obtained. The content of silica is preferably 75 parts by mass or less, more preferably 65 parts by mass or less. If it exceeds 75 parts by mass, the fuel efficiency tends to deteriorate.

In addition to the above components, the rubber composition of the present invention contains compounding agents commonly used in the production of rubber compositions such as zinc oxide, stearic acid, anti-aging agents, softeners, waxes, sulfur, and vulcanization acceleration. An agent or the like can be appropriately blended.

The rubber composition of the present invention is produced by a general method. That is, it can be produced by a method of kneading the above components with a Banbury mixer, a kneader, an open roll or the like and then vulcanizing.

The rubber composition for base tread of the present invention thus obtained has an acetone extraction amount of preferably 10% by mass or less, more preferably 8% by mass or less. If it exceeds 10% by mass, the rigidity tends to decrease and the steering stability tends to deteriorate. The amount of acetone extracted is preferably 1% by mass or more, more preferably 3% by mass or more. If it is less than 1% by mass, the rubber becomes too hard and the durability tends to deteriorate.
In addition, the acetone extraction amount can be measured by the method described in Examples described later.

In the rubber composition for a base tread of the present invention, the content (mass%) of the reinforcing agent and the acetone extraction amount (mass%) are “3.0 ≦ (content of reinforcing agent / acetone extraction amount) ≦ 9. It is preferable to satisfy the formula represented by “0”. Thereby, low fuel consumption, handling stability, and durability can be improved in a well-balanced manner. The lower limit of the above formula is preferably 4.0 or more, more preferably 4.5 or more, and the upper limit is preferably 8.8 or less.

A tread composed of a cap tread and a base tread can be produced by a known method. For example, a rubber composition formed into a sheet is pasted into a predetermined shape, or two or more extruders are charged with the rubber composition. The method of forming in two layers at the head exit of an extruder is mentioned.

The pneumatic tire of the present invention is produced by a usual method using the rubber composition. That is, if necessary, a rubber composition containing various additives is extruded in accordance with the shape of the base tread at an unvulcanized stage, molded by a normal method on a tire molding machine, Bonding together with the tire member forms an unvulcanized tire. This unvulcanized tire can be heated and pressurized in a vulcanizer to produce a pneumatic tire.

The pneumatic tire of the present invention is suitably used as a tire for passenger cars and a tire for trucks and buses.

The present invention will be specifically described based on examples, but the present invention is not limited to these examples.

Hereinafter, various chemicals used in the production examples will be described together. In addition, the chemical | medical agent refine | purified according to the usual method as needed.
Natural rubber latex: Field latex obtained from Taitex Co., Ltd. Surfactant: Emal-E (Polyoxyethylene lauryl ether sodium sulfate) manufactured by Kao Corporation
NaOH: NaOH manufactured by Wako Pure Chemical Industries, Ltd.

Production Example 1
After adjusting the solid rubber latex solids concentration (DRC) to 30% (w / v), 1000 g of natural rubber latex was added with 10 g of Emal-E and 20 g of NaOH, followed by saponification at room temperature for 48 hours. Natural rubber latex was obtained. Water was added to the latex to dilute to DRC 15% (w / v), and then formic acid was added with slow stirring to adjust the pH to 4.0 to 4.5 to cause aggregation. The agglomerated rubber was pulverized, washed repeatedly with 1000 ml of water, and then dried at 110 ° C. for 2 hours to obtain a solid rubber (saponified natural rubber).

Production Example 2
After adjusting the solid content concentration (DRC) of the natural rubber latex to 30% (w / v), the formic acid was added with slow stirring to adjust the pH to 4.0 to 4.5 and agglomerate. The agglomerated rubber was pulverized and dried at 110 ° C. for 2 hours to obtain a solid rubber (untreated natural rubber).

About the solid rubber obtained by the manufacture example and TSR20 used by evaluation of the rubber composition mentioned later, nitrogen content and phosphorus content were measured by the method shown below. The results are shown in Table 1.

(Measurement of nitrogen content)
The nitrogen content was measured using CHN CORDER MT-5 (manufactured by Yanaco Analytical Industries). For the measurement, first, a calibration curve for determining the nitrogen content was prepared using antipyrine as a standard substance. Next, about 10 mg of natural rubber obtained in the production example was weighed, and an average value was obtained from the measurement results of three times to obtain the nitrogen content of the sample.

(Measurement of phosphorus content)
The phosphorus content was determined using an ICP emission spectrometer (ICPS-8100, manufactured by Shimadzu Corporation).
In addition, ³¹ P-NMR measurement of phosphorus uses an NMR analyzer (400 MHz, AV400M, manufactured by Nippon Bruker Co., Ltd.), and uses a measurement peak of P atom in an 80% aqueous phosphoric acid solution as a reference point (0 ppm), and raw rubber with chloroform. More extracted components were purified, dissolved in CDCl ₃ and measured.

As shown in Table 1, saponified natural rubber (HPNR) had a reduced nitrogen content and phosphorus content compared to untreated natural rubber and TSR20. Moreover, in the ³¹ P NMR measurement of the extract extracted from HPNR obtained in the production example, no peak due to phospholipid was detected at -3 ppm to 1 ppm.

Hereinafter, various chemicals used in Examples and Comparative Examples will be described.
NR1: TSR20
NR2 (HPNR): Production Example 1 above
NR3 (untreated natural rubber): Production Example 2 above
BR1 (Hisys BR): BR150B manufactured by Ube Industries, Ltd. (BR synthesized using a Co-based catalyst)
BR2 (SPB-containing BR): VCR617 (1,2-syndiotactic polybutadiene crystal dispersion, 1,2-syndiotactic polybutadiene crystal content manufactured by Ube Industries, Ltd .: 17% by mass, 1,2-sindi (Melting point of tactic polybutadiene crystal: 200 ° C)
BR3 (rare earth-based BR): BUNA-CB24 manufactured by LANXESS (BR synthesized using an Nd-based catalyst, cis content: 96% by mass, vinyl content: 0.7% by mass, ML _{1 + 4} (100 ° C.): 45, (Mw / Mn: 2.69, Mw: 500,000, Mn: 186,000)
Carbon black: Seast NH manufactured by Mitsubishi Chemical Corporation (N351, N ₂ SA: 74 m ² / g)
Silica: 115 Gr made by Rhodia (average primary particle size: 22 nm, N ₂ SA: 112 m ² / g)
Silane coupling agent: Si266 manufactured by Evonik Degussa
Oil: Process X-140 manufactured by Japan Energy Co., Ltd.
Wax: Sunnock N manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.
Anti-aging agent: Antigen 6C (N- (1,3-dimethylbutyl) -N′-phenyl-p-phenylenediamine) manufactured by Sumitomo Chemical Co., Ltd.
Stearic acid: Stearic acid “椿” manufactured by NOF Corporation
Zinc oxide: Zinc Hana No. 1 manufactured by Mitsui Mining & Smelting Co., Ltd. Sulfur: Seimisulfur vulcanization accelerator manufactured by Nihon Kiboshi Kogyo Co., Ltd .: Noxeller NS (N-tert-butyl manufactured by Ouchi Shinsei Chemical Co., Ltd.) -2-Benzothiazolylsulfenamide)

Examples and Comparative Examples In accordance with the formulation shown in Table 2, chemicals other than sulfur and a vulcanization accelerator were kneaded using a 1.7 L Banbury mixer manufactured by Kobe Steel. Next, using an open roll, sulfur and a vulcanization accelerator were added to the kneaded product and kneaded to obtain an unvulcanized rubber composition.
The obtained unvulcanized rubber composition was press vulcanized with a 2 mm thick mold at 150 ° C. for 30 minutes to obtain a vulcanized rubber composition (vulcanized rubber sheet).
Further, the obtained unvulcanized rubber composition was molded into a base tread shape on a tire molding machine, bonded to another tire member, and vulcanized at 150 ° C. for 30 minutes, so that a test tire (tire size: 195 / 65R15).

The obtained unvulcanized rubber composition, vulcanized rubber composition and test tire were evaluated as follows. The results are shown in Table 2.

(Acetone extraction amount)
According to the method for measuring the amount of acetone extracted according to JIS K 6229, the amount of the substance extracted with acetone contained in the vulcanized rubber composition was measured and expressed in mass%. The amount of acetone extracted is an indicator of the concentration of an organic low molecular compound such as oil and wax contained in the vulcanized rubber composition. JIS K6229 (1998) defines two methods, Method A and Method B, but the acetone extraction amount according to JIS K6229 referred to in this application means mass% obtained by adopting Method A. It shall be.

(Hs)
The hardness (Hs) of the vulcanized rubber sheet was measured by a method based on JIS K6253. The measurement temperature was 30 ° C.

(Viscoelasticity test)
Using a viscoelastic spectrometer manufactured by Iwamoto Seisakusho Co., Ltd., loss tangent tan δ and complex viscoelasticity E ^{* of the} vulcanized rubber sheet at 70 ° C. under conditions of frequency 10 Hz, initial strain 10% and dynamic strain 2% ^. The tan δ index of Comparative Example 1 was set to 100, the E ^* index was set to 100, and tan δ and E ^* of each formulation were indicated by an index according to the following calculation formula. In addition, it shows that heat_generation | fever is small and it is excellent in low-fuel-consumption property, so that a tan-delta index is small, E ^* is high and a rubber | gum is hard, so that a complex viscoelastic index is large.
(Tan δ index) = (tan δ of each formulation) / (tan δ of Comparative Example 1) × 100
^{(E *} index = each formulation ^E *) / (Comparative Example 1 ^{E *)} × 100

(Dematcher flex crack growth test)
The vulcanized rubber sheet was evaluated by a method based on JIS K6260. In Table 2, the number of bendings required for a crack to extend by 1 mm was calculated and expressed. The larger the value, the higher the crack growth resistance and the better the durability.

(Maneuvering stability (handling performance))
The vehicle equipped with the test tire was run, and the response when the steering wheel was turned was evaluated by sensory evaluation of the test driver. Evaluation was performed according to the following criteria.
◎: The response of the car is very fast when the handle is turned ○: The response of the car is fast when the handle is turned △: The response of the car is slightly slow when the handle is turned ×: When the handle is turned Slow car response

(Durability)
A scratch having a width of 5 mm and a depth of 1 mm was formed on the groove bottom of the test tire, and the degree of progress of the scratch after running 30000 km on the table was evaluated. Evaluation was performed according to the following criteria.
○: scar extension is hardly observed (crack extension of less than 10 mm)
Δ: Slight extension is observed slightly (crack extension of about 10 to 50 mm)
X: Scratches extend and peeling occurs in the base tread layer

(Rolling resistance index)
Using a rolling resistance tester, rolling resistance was measured when the test tire was run under the conditions of a rim (15 × 6JJ), internal pressure (230 kPa), load (3.43 kN), and speed (80 km / h). . The results were expressed as an index according to the following formula, with the rolling resistance index of Comparative Example 1 as 100. The smaller the rolling resistance index, the lower the rolling resistance and the better the fuel economy.
(Rolling resistance index) = (Rolling resistance of each formulation) / (Rolling resistance of Comparative Example 1) × 100

(Mooney viscosity index)
The Mooney viscosity of the unvulcanized rubber composition was measured at 130 ° C. according to JIS K6300. The results were expressed as an index according to the following formula, with the Mooney viscosity index of Comparative Example 1 being 100. The larger the Mooney viscosity index, the lower the viscosity of the unvulcanized rubber composition, indicating that the processing is easier.
(Mooney viscosity index) = (Mooney viscosity of Comparative Example 1) / (Mooney viscosity of each formulation) × 100

(Kneading processability)
The state of the unvulcanized rubber composition was visually observed, and kneadability was evaluated according to the following criteria.
◎: Unvulcanized rubber composition has a lump and gloss ○: Uncured rubber composition has a lump but has no gloss Δ: Unvulcanized rubber composition has a lump and gloss

(Extrudability)
In the process of extruding the unvulcanized rubber composition into the base tread shape, the temperature (discharge temperature) of the unvulcanized rubber composition extruded with a constant extrusion rate (kneaded rubber discharge amount) was measured. And the temperature difference between the blending temperatures (“temperature of each blending” − “temperature of Comparative Example 1”), and extrusion processability was evaluated according to the following criteria. It is judged that the extrudability deteriorated as the temperature increased.
A: Temperature difference is 0 ° C. or more and less than 10 ° C. ○: Temperature difference is 10 ° C. or more and less than 20 ° C. Δ: Temperature difference is 20 ° C. or more

From Table 2, the examples containing natural rubber, rubber components including SPB-containing BR and rare earth-based BR, and a predetermined amount of reinforcing agent are lower in fuel consumption, driving stability and durability than the comparative examples. Improved in a well-balanced manner.

Claims (7)

A rubber composition for a base tread containing a rubber component and a reinforcing agent,
The rubber component includes natural rubber, butadiene rubber (1,2) containing 1,2-syndiotactic polybutadiene crystals, and butadiene rubber (2) synthesized using a rare earth element-based catalyst,
The total weight 100 weight% of the rubber composition for a base tread, Ri is 25 to 35% by mass content of the reinforcing agent,
In 100% by mass of the rubber component, the content of the natural rubber is 50% by mass or more, the content of the butadiene rubber (1) is 5 to 30% by mass, and the content of the butadiene rubber (2) is 5 to 30% by mass. %
A rubber composition for a base tread in which the reinforcing agent content (% by mass) and the acetone extraction amount (% by mass) satisfy the following formula .
3.0 ≦ (Reinforcing agent content / Acetone extraction amount) ≦ 9.0 The natural rubber, base tread rubber composition for phosphorus content claim 1 Symbol placing comprises the following modified natural rubber 200 ppm. The rubber composition for a base tread according to claim 2 , wherein the content of the modified natural rubber is 20% by mass or more in 100% by mass of the rubber component. The rubber composition for base treads according to any one of claims 1 to 3, comprising 20 to 75 parts by mass of silica with respect to 100 parts by mass of the rubber component. Nitrogen adsorption specific surface area of 50-130m ² The rubber composition for base treads in any one of Claims 1-4 containing carbon black of / g. The rubber composition for base treads according to any one of claims 1 to 5, wherein the amount of acetone extracted is 1 to 10% by mass. The pneumatic tire which has a base tread produced using the rubber composition in any one of Claims 1-6 .