JP5977083B2 - Rubber composition for case topping and pneumatic tire - Google Patents

Description

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

Conventionally, the fuel consumption of a vehicle has been reduced by reducing tire rolling resistance and suppressing heat generation. In recent years, the demand for lower fuel consumption of tires has become stronger, and in addition to treads, lower fuel consumption (lower heat generation) is also required by improving the composition of internal members such as sidewalls (such as rubber compositions for case toppings). It has become.

Known methods for reducing the heat generation of a rubber composition include a method using a low reinforcing filler, a method for reducing the amount of filler, and a method using silica as a filler. However, in these methods, since the reinforcing property of the rubber composition is lowered, the breaking performance is lowered. Thus, it has been difficult to improve the fuel efficiency and the destruction performance at the same time.

On the other hand, natural rubber is widely used in case topping rubber compositions, but natural rubber has a Mooney viscosity and poor processability compared to other synthetic rubbers. It is used after masticating and reducing Mooney viscosity. Therefore, productivity is reduced when natural rubber is used. Further, since the molecular chain of natural rubber is cut by mastication, there is also a problem that the characteristics (low fuel consumption, rubber strength, resistance to flex crack growth, etc.) of the inherent high molecular weight polymer are lost.

In Patent Document 1, it is proposed to use natural rubber deproteinized so that the total nitrogen content is 0.1% by weight or less. However, processability, fuel efficiency, and flex crack growth resistance are proposed. There is still room for improvement in terms of improving the balance.

JP-A-6-329838

An object of the present invention is to solve the above-mentioned problems and to provide a rubber composition for case topping and a pneumatic tire that can improve workability, fuel efficiency and bending crack growth resistance in a well-balanced manner.

The present invention includes a modified natural rubber having a phosphorus content of 200 ppm or less, carbon black and / or a white filler, and the content of the modified natural rubber in 100% by mass of the rubber component is 5% by mass or more. The present invention relates to a case topping rubber composition.

The modified natural rubber preferably has a nitrogen content of 0.3% by mass or less and a gel content measured as a toluene insoluble content of 20% by mass or less. The modified natural rubber is preferably obtained by saponifying natural rubber latex.

The modified natural rubber is a step of saponifying natural rubber latex to prepare a saponified natural rubber latex (A), and a step of alkali treating the agglomerated rubber obtained by agglomerating the saponified natural rubber latex ( B) and the step (C) of washing until the phosphorus content contained in the rubber is 200 ppm or less are preferred.

The white filler is preferably silica.

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

According to the present invention, since the rubber composition for case topping includes a modified natural rubber having a predetermined phosphorus content of 200 ppm or less and carbon black and / or white filler, processability, fuel efficiency and Bending crack growth resistance can be improved in a balanced manner.

The rubber composition for case topping of the present invention contains a modified natural rubber having a phosphorus content of 200 ppm or less, and carbon black and / or a white filler.

By using the modified natural rubber in which the phospholipids contained in the natural rubber are reduced and removed, the fuel efficiency can be improved. In addition, the unvulcanized rubber composition containing the modified natural rubber is excellent in processability and can be sufficiently kneaded without performing a special mastication process. It is possible to suppress the decrease in the fuel efficiency, and to effectively improve the fuel economy, fracture performance, flex crack growth resistance, and the like. In addition to reducing not only phospholipids but also proteins and gels, these performances can be further improved.
Furthermore, the modified natural rubber does not contain trash components such as pebbles and wood chips, and does not require a step for removing the components, so that it is excellent in productivity.
From these, in this invention, workability, low fuel consumption, and bending crack growth resistance can be balanced.

The modified natural rubber has a phosphorus content of 200 ppm or less. If it exceeds 200 ppm, the Mooney viscosity of the unvulcanized rubber tends to increase and the processability tends to deteriorate. Further, tan δ increases and fuel efficiency tends to deteriorate. The phosphorus content is preferably 150 ppm or less, and more preferably 100 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, or the fuel efficiency tends to deteriorate. The nitrogen content can be measured by a conventional method such as Kjeldahl method. Nitrogen is derived from protein.

The gel content in the modified natural rubber is preferably 20% by mass or less, more preferably 10% by mass or less, and still more preferably 7% by mass or less. When it exceeds 20 mass%, the dispersibility of the filler tends to deteriorate. Moreover, there exists a tendency for workability to deteriorate. The gel content means a value measured as an insoluble content with respect to toluene which is a nonpolar solvent, and may be simply referred to as “gel content” or “gel content” below. The measuring method of the content rate of a gel part is as follows. First, a natural rubber sample is soaked in dehydrated toluene, light-shielded in the dark and left for 1 week, and then the toluene solution is centrifuged at 1.3 × 10 ⁵ rpm for 30 minutes to obtain an insoluble gel content and a toluene soluble content. Isolate. Methanol is added to the insoluble gel and solidified, and then dried, and the gel content is determined from the ratio between the mass of the gel and the original mass of the sample.

The modified natural rubber is preferably substantially free of phospholipids. “Substantially free of phospholipid” 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.

The modified natural rubber can be obtained, for example, by the production method described in JP 2010-138359 A. Among them, the process (A) of preparing a saponified natural rubber latex by saponifying the natural rubber latex, A production method comprising a step (B) of subjecting the agglomerated rubber obtained by agglomerating the saponified natural rubber latex to an alkali treatment and a step (C) of washing until the phosphorus content contained in the rubber is 200 ppm or less. What is prepared is preferred. By this manufacturing method, phosphorus content, nitrogen content, etc. can be reduced effectively. Moreover, by using the modified natural rubber obtained by the production method, processability, fuel efficiency and bending crack growth resistance can be remarkably improved, and these performances can be obtained at a high level. Further, when the agglomeration with acid is performed, the remaining acid is neutralized with an alkali treatment, whereby deterioration of the rubber by the acid can be prevented.

In the production method described above, the saponification treatment can be performed by adding an alkali and, if necessary, a surfactant to natural rubber latex and allowing to stand at a predetermined temperature for a certain time. In addition, you may perform stirring etc. as needed. According to the above production method, the phosphorus content and nitrogen content of natural rubber can be suppressed.

As the natural rubber latex, conventionally known latexes such as raw latex, purified latex, and high ammonia latex can be used. Examples of the alkali used for the saponification treatment include sodium hydroxide, potassium hydroxide, calcium hydroxide, and amine compounds. From the viewpoint of good saponification and the stability of natural rubber latex, sodium hydroxide, Potassium hydroxide is preferred. As the surfactant, known anionic surfactants, nonionic surfactants, and amphoteric surfactants can be used. Of these, anionic surfactants are preferred, and sulfonic acid anions are preferred. A surfactant is more preferable.

In the saponification treatment, the amount of alkali added may be appropriately set, but is preferably 0.1 to 10 parts by mass, more preferably 0.3 to 5 parts by mass with respect to 100 parts by mass of the solid content of the natural rubber latex. It is. Moreover, the addition amount of the surfactant is preferably 0.01 to 6.0 parts by mass, more preferably 0.1 to 3.5 parts by mass with respect to 100 parts by mass of the solid content of the natural rubber latex. The temperature and time of the saponification treatment may be appropriately set, and is usually about 20 to 70 ° C. (preferably 30 to 70 ° C.) and about 1 to 72 hours (preferably 3 to 48 hours).

After completion of the saponification reaction, the agglomerated rubber obtained by agglomerating the saponified natural rubber latex obtained by the reaction is crushed as necessary, and then the obtained agglomerated rubber or crushed rubber is brought into contact with an alkali. Perform alkali treatment. By the alkali treatment, the nitrogen content in the rubber can be efficiently reduced, and the effects of the present invention are further exhibited. Examples of the aggregation method include a method of adding an acid such as formic acid. The alkali treatment method is not particularly limited as long as it is a method in which rubber and alkali are brought into contact with each other, and examples thereof include a method in which agglomerated rubber and crushed rubber are immersed in alkali. Examples of the alkali that can be used for the alkali treatment include alkali metal carbonates such as potassium carbonate, sodium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, lithium carbonate, lithium hydrogen carbonate, and ammonia in addition to the alkali in the saponification treatment described above. Water etc. are mentioned. Of these, alkali metal carbonates are preferred, sodium carbonate and potassium carbonate are more preferred, and sodium carbonate is even more preferred from the viewpoint of excellent effects of the present invention.

When the alkali treatment is performed by the above immersion, the treatment can be performed by immersing rubber (crushed rubber) in an alkaline aqueous solution having a concentration of preferably 0.1 to 5 mass%, more preferably 0.2 to 3 mass%. Thereby, the amount of nitrogen in the rubber can be further reduced.

When the alkali treatment is performed by the immersion, the temperature of the alkali treatment can be set as appropriate, but is usually preferably 20 to 70 ° C. The alkali treatment time depends on the treatment temperature, but is preferably 1 to 20 hours, more preferably 2 to 12 hours, considering sufficient treatment and productivity.

Phosphorus content can be reduced by performing a washing treatment after the alkali treatment. Examples of the washing treatment include a method of diluting a rubber component with water and washing, followed by a centrifugal separation treatment, and a method of leaving the rubber to stand and discharging the aqueous phase and taking out the rubber component. When centrifuging, it is first diluted with water so that the rubber content of the natural rubber latex is 5 to 40% by mass, preferably 10 to 30% by mass. Then, it may be centrifuged at 5000 to 10000 rpm for 1 to 60 minutes, and washing may be repeated until a desired phosphorus content is obtained. In addition, when the rubber is allowed to stand still, the addition of water and stirring may be repeated until the desired phosphorus content is obtained. The modified natural rubber in the present invention is obtained by drying after completion of the washing treatment.

The content of the modified natural rubber in 100% by mass of the rubber component contained in the rubber composition of the present invention is 5% by mass or more, preferably 10% by mass or more, more preferably 30% by mass or more, and further preferably 40% by mass. % Or more. If it is less than 5% by mass, the processability, fuel efficiency and resistance to flex crack growth may not be sufficiently improved. Moreover, this content becomes like this. Preferably it is 90 mass% or less, More preferably, it is 80 mass% or less. When it exceeds 90 mass%, there exists a possibility that the durability of a tire may deteriorate.

In addition to the modified natural rubber, examples of rubber components that can be used in the present invention include natural rubber (NR) (non-modified), isoprene rubber (IR), butadiene rubber (BR), and styrene butadiene rubber (SBR). A diene rubber is mentioned. Among these, SBR is preferable because good workability and flex crack growth resistance can be obtained.

The content of SBR in 100% by mass of the rubber component is preferably 5% by mass or more, more preferably 20% by mass or more. If the amount is less than 5% by mass, sufficient flex crack growth resistance may not be obtained. The content is preferably 95% by mass or less, more preferably 60% by mass or less. If it exceeds 95% by mass, there is a possibility that the effect of improving the workability, fuel efficiency and bending crack growth resistance cannot be obtained sufficiently.

The total content of the modified natural rubber and SBR in 100% by mass of the rubber component is preferably 80% by mass or more, more preferably 90% by mass or more, and further preferably 100% by mass. When the total content is within the above range, excellent processability, low fuel consumption and bending crack growth resistance can be obtained.

Examples of carbon black include GPF, FEF, HAF, ISAF, and SAF, but are not particularly limited. By blending carbon black, a reinforcing effect can be obtained and the effects of the present invention can be obtained well.

The nitrogen adsorption specific surface area (N ₂ SA) of carbon black is preferably 30 m ² / g or more, and more preferably 60 m ² / g or more. If it is less than 30 m < ² > / g, there exists a possibility that sufficient reinforcement may not be obtained. The _N 2 SA is preferably from 130m ² / g or less, more preferably 120 m ² / g, more preferably not more than ^90m 2 / g. When it exceeds 130 m ² / g, it becomes difficult to disperse carbon black, and the fuel efficiency tends to deteriorate.
Incidentally, _N 2 SA of carbon black, JIS K 6217-2: determined by 2001.

The dibutyl phthalate oil absorption (DBP) of carbon black is preferably 50 ml / 100 g or more, more preferably 90 ml / 100 g or more. The oil absorption is preferably 150 ml / 100 g or less, and more preferably 120 ml / 100 g or less. Within the above range, excellent rubber strength can be obtained, and the effects of the present invention can be obtained well.
The DBP of carbon black is measured according to JIS K6217-4: 2001.

The content of carbon black is preferably 20 parts by mass or more, more preferably 35 parts by mass or more, and still more preferably 45 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 is preferably 90 parts by mass or less, more preferably 80 parts by mass or less, and still more preferably 70 parts by mass or less. If it exceeds 90 parts by mass, the fuel efficiency tends to deteriorate.

As white filler, those commonly used in the rubber industry, for example, mica such as silica, calcium carbonate, sericite, aluminum hydroxide, magnesium oxide, magnesium hydroxide, clay, talc, alumina, titanium oxide Etc. can be used. Of these, silica is preferable from the viewpoint of fuel efficiency and rubber strength.

The silica is not particularly limited, and for example, dry method silica (anhydrous silica), wet method silica (hydrous silica), and the like can be used. Wet silica (hydrous silica) is preferred because of the large number of silanol groups.

The nitrogen adsorption specific surface area (N ₂ SA) of silica is preferably 90 m ² / g or more, and more preferably 100 m ² / g or more. If it is less than 90 m < ² > / g, there exists a tendency for sufficient reinforcement property not to be acquired. Further, the _N 2 SA is preferably 250 meters ² / g or less, more preferably 220 m ² / g, more preferably not more than 130m ² / g. When it exceeds 250 m < ² > / g, the dispersibility of a silica will fall and there exists a tendency for workability to deteriorate.
In addition, the nitrogen adsorption specific surface area of a silica is a value measured by BET method according to ASTM D3037-93.

The content of the white filler is preferably 5 parts by mass or more, more preferably 7 parts by mass or more with respect to 100 parts by mass of the rubber component. The content is preferably 100 parts by mass or less, more preferably 90 parts by mass or less, and still more preferably 30 parts by mass or less. When the content is within the above range, good processability, low fuel consumption and resistance to flex crack growth can be obtained.

In the present invention, when silica is used as the white filler, it is preferable to use a silane coupling agent. Examples of the silane coupling agent include sulfide, mercapto, vinyl, amino, glycidoxy, nitro, and chloro silane coupling agents. Among them, bis (3-triethoxysilylpropyl) tetrasulfide, bis (2-triethoxysilylethyl) tetrasulfide, bis (3-triethoxysilylpropyl) disulfide, bis (2-triethoxysilylethyl) disulfide, etc. Sulfide systems are preferred, with bis (3-triethoxysilylpropyl) disulfide being particularly preferred.

When the silane coupling agent is contained, the content of the silane coupling agent is preferably 2 parts by mass or more, more preferably 6 parts by mass or more, and preferably 15 parts by mass or less with respect to 100 parts by mass of silica. Less than the mass part is more preferable. When the content is within the above range, good processability, low fuel consumption and resistance to flex crack growth can be obtained.

The total content of carbon black and white filler is preferably 30 parts by mass or more, more preferably 45 parts by mass or more, and still more preferably 50 parts by mass or more with respect to 100 parts by mass of the rubber component. The total content is preferably 120 parts by mass or less, more preferably 100 parts by mass or less, and still more preferably 70 parts by mass or less. When the total content is within the above range, good processability, low fuel consumption and resistance to flex crack growth can be obtained.

In the present invention, it is preferable to use sulfur. The sulfur content is preferably 0.2 parts by mass or more, more preferably 0.4 parts by mass or more, and still more preferably 2.5 parts by mass or more with respect to 100 parts by mass of the rubber component. The content is preferably 6 parts by mass or less, more preferably 4 parts by mass or less. When the content is within the above range, a good crosslinking density can be obtained, and excellent fuel economy and flex crack growth resistance can be obtained. Also, good adhesion to the cord can be obtained.

In the present invention, it is preferable to use an antioxidant. As the anti-aging agent, for example, 2,2,4-trimethyl-1,2-dihydroquinoline polymer, 6-ethoxy-1,2-dihydro-2,2,4 can be used from the viewpoint of excellent resistance to flex crack growth. -Amine and ketones such as trimethylquinoline, N- (1,3-dimethylbutyl) -N'-phenyl-p-phenylenediamine (6PPD), N-phenyl-N'-isopropyl-p-phenylenediamine (IPPD) P-phenylenediamine derivatives such as N, N′-di-2-naphthyl-p-phenylenediamine are preferred. Of these, p-phenylenediamine derivatives are preferable, and 6PPD is more preferable.

The content of the anti-aging agent is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, preferably 6 parts by mass or less, more preferably 4 parts by mass or less, with respect to 100 parts by mass of the rubber component. It is. When the content is in the above range, the flex crack growth resistance can be obtained satisfactorily.

In the present invention, it is preferable to use oil. Thereby, the effect of this invention is acquired more suitably. As the oil, for example, paraffinic oil, aroma oil, naphthenic oil, or the like can be used.

When the rubber composition contains oil, the oil content is preferably 4 parts by mass or more, and more preferably 7 parts by mass or more with respect to 100 parts by mass of the rubber component. If it is less than 4 parts by mass, the effects of the present invention may not be sufficiently obtained.
The oil content is preferably 15 parts by mass or less, more preferably 12 parts by mass or less. If it exceeds 15 parts by mass, the fuel efficiency may be deteriorated.

In addition to the above-described components, the rubber composition of the present invention can be appropriately mixed with a compounding agent generally used for producing a rubber composition, for example, stearic acid, zinc oxide, a vulcanization accelerator, and the like.

The rubber composition of the present invention can be produced by a general method. That is, it can be produced by a method of kneading the above components with a Banbury mixer, kneader, open roll or the like and then vulcanizing. Here, when producing a rubber composition containing natural rubber, a natural rubber mastication step is usually performed before a kneading step of each component such as a rubber component and a filler. In the present invention, since the modified natural rubber is used, the kneading step can be carried out satisfactorily without performing the kneading step, and a desired rubber composition can be produced.

The rubber composition of the present invention is used as a case topping rubber composition (carcass cord covering rubber composition).

The pneumatic tire of the present invention can be produced by a usual method using the rubber composition. That is, if necessary, a rubber composition containing various additives is kneaded at an unvulcanized stage, and then the carcass cord is covered with the obtained kneaded product, and the carcass (casing (casing) is formed on the tire molding machine. ) Is formed into a shape, and a sidewall is formed using this. Then, it bonds together with another tire member, and forms an unvulcanized tire. This unvulcanized tire can be heated and pressurized in a vulcanizer to produce a tire (such as a radial tire).

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.
Natural rubber latex: Field latex surfactant obtained from Muhibah Lateks, Inc .: Emal-E27C (polyoxyethylene lauryl ether sodium sulfate) manufactured by Kao Corporation
NaOH: NaOH manufactured by Wako Pure Chemical Industries, Ltd.

(Production of saponified natural rubber)
Production Example 1
After adjusting solid content concentration (DRC) of natural rubber latex to 30% (w / v), 25 g of 10% Emal-E27C aqueous solution and 50 g of 40% NaOH aqueous solution are added to 1000 g of natural rubber latex (wet state), A saponification reaction was carried out at room temperature for 48 hours to obtain a saponified natural rubber latex. 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 for aggregation.
The agglomerated rubber is pulverized, dipped in a 1% aqueous sodium carbonate solution at room temperature for 5 hours, pulled up, washed repeatedly with 1000 ml of water, and then dried at 90 ° C. for 4 hours to obtain a solid rubber (saponified natural rubber A). Obtained.

Production Example 2
A solid rubber (saponified natural rubber B) was obtained in the same manner as in Production Example 1 except that the amount of the 40% NaOH aqueous solution was changed to 25 g.

The solid content (saponified natural rubber A, B) and TSR obtained in Production Examples 1 and 2 were measured for nitrogen content, phosphorus content, and gel content by the methods described 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 the sample 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 of the sample 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.

(Measurement of gel content)
A raw rubber sample 70.00 mg cut to 1 mm × 1 mm was weighed, 35 mL of toluene was added thereto, and the mixture was allowed to stand in a cool dark place for 1 week. Subsequently, centrifugation was performed to precipitate a gel component insoluble in toluene, the soluble component of the supernatant was removed, and only the gel component was solidified with methanol, and then dried and the mass was measured. The gel content (%) was determined by the following formula.
Gel content (mass%) = [mass mg after drying / mg of initial sample] × 100

As shown in Table 1, the saponified natural rubbers A and B had a reduced nitrogen content, phosphorus content, and gel content as compared with TSR.
In ³¹ P-NMR measurement, the saponified natural rubbers A and B did not have a peak due to phospholipid at -3 ppm to 1 ppm.

Hereinafter, various chemicals used in Examples and Comparative Examples will be described together.
Saponified natural rubber A: Production Example 1
Saponified natural rubber B: Production Example 2
Natural rubber: TSR20
Synthetic rubber: Nipol 1502 (E-SBR, styrene content 23.5% by mass) manufactured by Nippon Zeon Co., Ltd.
Carbon black: Dia Black HAN manufactured by Mitsubishi Chemical Corporation (N ₂ SA: 80 m ² / g, DBP oil supply amount: 103 ml / 100 g)
Silica: Siloa Zeosl 115Gr (N ₂ SA: 110 m ² / g) manufactured by Rhodia Japan Co., Ltd.
Silane coupling agent: Si266 (bis (3-triethoxysilylpropyl) disulfide) manufactured by Degussa
Process oil: Diana process NH-70S manufactured by Idemitsu Kosan Co., Ltd.
Anti-aging agent: Vulcanox 4020 (6PPD) manufactured by Bayer
Stearic acid: Zinc oxide manufactured by NOF Corporation: Zinc oxide No. 3 20% manufactured by Mitsui Mining & Smelting Co., Ltd. Oil-treated insoluble sulfur: Insoluble sulfur manufactured by Shikoku Chemicals Co., Ltd. Mucron OT-20 (20% Oil treatment)
Vulcanization accelerator: Noxeller NS (N-tert-butyl-2-benzothiazolylsulfenamide) manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.

Examples and Comparative Examples According to the formulation shown in Tables 2-3, materials other than sulfur and a vulcanization accelerator were kneaded using a 1.7 L Banbury mixer to obtain a kneaded product. Next, sulfur and a vulcanization accelerator were added to the obtained kneaded product, and kneaded using an open roll to obtain an unvulcanized rubber composition.
The obtained unvulcanized rubber composition was press vulcanized with a 2 mm thick mold at 170 ° C. for 15 minutes to obtain a vulcanized rubber composition (vulcanized rubber sheet).

In Comparative Examples 1 to 4, 0.4 parts by mass of a peptizer was added to 100 parts by mass of natural rubber, and the mixture was kneaded in advance. On the other hand, practicing was not performed in Examples 1-8.

The following evaluation was performed about the obtained unvulcanized rubber composition and vulcanized rubber composition (vulcanized rubber sheet). The results are shown in Tables 2-3.

(Mooney viscosity)
The unvulcanized rubber composition was measured at 130 ° C. according to the Mooney viscosity measurement method based on JIS K6300, and indicated by the following formula (Mooney viscosity index). The larger the index, the lower the Mooney viscosity (ML _{1 + 4} ), which indicates superior workability.
(Mooney viscosity index) = (ML _{1 + 4} of Comparative Example 2 or ₄ ) / (ML _{1 + 4 of} each formulation) × 100

(Low heat generation)
Loss tangent (tan δ) of each compound (vulcanized rubber sheet) using a viscoelastic spectrometer VES (manufactured by Iwamoto Seisakusho Co., Ltd.) under conditions of a temperature of 70 ° C., an initial strain of 10%, a dynamic strain of 2%, and a frequency of 10 Hz. ) Was measured and expressed as an index according to the following formula (low exothermic index). The larger the index, the smaller the tan δ and the better the fuel efficiency.
(Low exothermic index) = (tan δ of Comparative Example 2 or 4) / (tan δ of each formulation) × 100

(Flexible crack growth test)
Using a vulcanized rubber sheet, a sample was prepared based on JIS-K-6260 "Vulcanized rubber and thermoplastic rubber-Dematcher bending crack test method", a bending crack growth test was performed, and 70% elongation was repeated 1 million times. After bending the rubber sheet, the length of the generated crack was measured.
The reciprocal of the measured value (crack length) of Comparative Example 2 or 4 was taken as 100, and displayed as an index. The larger the index is, the more the crack growth is suppressed and the resistance to flex crack growth is excellent.

As shown in Tables 2 and 3, in the examples using modified natural rubber (saponified natural rubbers A and B) having a phosphorus content of 200 ppm or less in both the carbon black compounding and the silica compounding, processability and fuel efficiency are reduced. Properties and flex crack growth resistance were improved in a well-balanced manner. From these results, it became clear that the pneumatic tire excellent in these performance balances can be provided by using the casing produced using the rubber composition of the Example.

Claims (4)

Saponifying natural rubber latex to prepare saponified natural rubber latex (A), alkali-treating the agglomerated rubber obtained by agglomerating the saponified natural rubber latex (B), contained in rubber Washing until the phosphorus content is 200 ppm or less, modified natural rubber having a phosphorus content of 200 ppm or less obtained through the steps (A) to (C), and carbon black and / or white filling And a step (D) of obtaining a rubber composition for case topping in which the content of the modified natural rubber in 100% by mass of the rubber component is 5% by mass or more , and the step (D). Including a step (E) of producing a carcass using a rubber composition for case topping,
The alkali treatment is a method for manufacturing a pneumatic tire having a carcass using an alkali metal carbonate . The method for producing a pneumatic tire according to claim 1, wherein the nitrogen content of the modified natural rubber is 0.3 mass% or less, and the gel content measured as a toluene insoluble content is 20 mass% or less. The method of manufacturing a pneumatic tire according to claim 1 or 2, wherein the white filler is silica. The method for producing a pneumatic tire according to any one of claims 1 to 3, wherein in the rubber composition for case topping, the content of the white filler is 5 to 30 parts by mass with respect to 100 parts by mass of the rubber component.