JP5650798B2 - Rubber composition for breaker topping and pneumatic tire - Google Patents

Description

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

In recent years, due to soaring fuel costs and the introduction of environmental regulations, the demand for lower fuel consumption of vehicles has become stronger, and excellent fuel efficiency is also required for rubber for breaker topping, which is an internal member. Since natural rubber is frequently used for the rubber for topping, it is necessary to promote the reduction of fuel consumption of natural rubber in order to reduce the fuel consumption of the entire tire.

For example, Patent Document 1 discloses a method of adding a surfactant to natural rubber latex and performing a cleaning process as fuel efficiency reduction by modifying natural rubber. However, although the protein and gel content can be reduced to some extent by this method, it is not a sufficient level and further reduction of tan δ is desired. Also, tire rubbers are required to have performance such as heat aging resistance, but the method of Patent Document 1 is insufficient in heat resistance, and an improvement in both low fuel consumption and heat aging resistance is desired. Yes.

In addition, natural rubber has a high Mooney viscosity compared to other synthetic rubbers and has poor processability, and is usually used after mastication with a masticant added to reduce Mooney viscosity. The nature is bad. Furthermore, the molecular chain of natural rubber is cleaved by mastication, which causes a problem that the characteristics (good wear resistance, low fuel consumption, rubber strength, etc.) of the high molecular weight polymer inherent to natural rubber are lost.

On the other hand, in recent years, since the wear resistance of tires has been improved and the period of use in the market has been prolonged, there is a concern about durability performance due to internal damage of the tires. A typical example of internal damage is breaker edge damage (hereinafter also referred to as BEL (BRAKER EDGE LOOSENS)), which is caused by a decrease in breaking strength of the breaker topping rubber, and has been conventionally known. Even if it is dealt with by increasing the amount of filler, the heat generation performance of rubber deteriorates and the fuel efficiency decreases.

As described above, in a rubber composition for a tire breaker topping, it is difficult to improve the fuel efficiency, breaking strength and heat aging resistance in a well-balanced manner while obtaining excellent processability, and improvements are required.

Japanese Patent No. 3294901

The present invention solves the above-mentioned problems, obtains excellent processability, and improves the fuel economy, breaking strength and heat aging resistance in a well-balanced manner, and the air produced using the rubber composition An object is to provide a tire entering.

The present invention relates to a rubber composition for breaker topping, which comprises a modified natural rubber that is highly purified and has a pH adjusted to 2 to 7, and carbon black.

The modified natural rubber is preferably obtained by removing the non-rubber component of the natural rubber and then treating with an acidic compound, and the pH is 2-7.

The modified natural rubber is obtained by washing saponified natural rubber latex and further treating with an acidic compound, and preferably has a pH of 2-7.

The modified natural rubber is preferably obtained by washing deproteinized natural rubber latex and further treating with an acidic compound, and having a pH of 2-7.

The phosphorus content of the modified natural rubber is preferably 200 ppm or less.

The nitrogen content of the modified natural rubber is preferably 0.15% by mass or less.

The pH is measured by cutting the modified natural rubber into 2 mm squares of each side and immersing in distilled water, extracting at 90 ° C. for 15 minutes while irradiating with microwaves, and measuring the immersion water using a pH meter. It is preferable that it is a value obtained.

The modified natural rubber has a Mooney viscosity ML (1 + 4) of 130 ° C. measured in accordance with JIS K 6300: 2001-1 and a heat aging index represented by the following formula of 75 to 120%. Is preferred.

Preferably it contains zinc oxide.

It is preferable that organic acid cobalt is included.

It is preferable to contain a heat resistant anti-aging agent.

The modified natural rubber is preferably produced without going through a mastication step.

The present invention also relates to a pneumatic tire produced using the rubber composition for breaker topping.

According to the present invention, since it is a rubber composition for breaker topping comprising a modified natural rubber that is highly purified and has a pH adjusted to 2 to 7, and carbon black, while obtaining excellent processability, Low fuel consumption, breaking strength and heat aging resistance can be improved in a well-balanced manner.

The rubber composition for a breaker topping according to the present invention includes a modified natural rubber that is highly purified and has a pH adjusted to 2 to 7, and carbon black.

The modified natural rubber is highly purified and has a pH adjusted to 2-7.
Non-rubber components such as proteins and phospholipids are removed to achieve high purity, and the modified natural rubber controls the pH of the rubber to an appropriate value. Strength is improved. Also, the removal of non-rubber components and the rubber becoming basic or strongly acidic facilitates the deterioration of the rubber, but the adjustment of the pH of the rubber to a predetermined range suppresses the decrease in molecular weight during storage. Therefore, good heat aging resistance can be obtained. As a result, it is possible to prevent deterioration of the physical properties of rubber in the kneading step and to improve the dispersibility of the filler, and to remarkably improve the performance balance of fuel efficiency, breaking strength and heat aging resistance while obtaining excellent processability.

Here, high purification means removing impurities such as phospholipids and proteins other than natural polyisoprenoid components. Natural rubber has a structure in which the isoprenoid component is covered with the impurity component. By removing the component, the structure of the isoprenoid component changes, and the interaction with the compounding agent changes, resulting in energy. It is presumed that loss can be reduced, durability can be improved, and a better rubber composition can be obtained.

The modified natural rubber having a high purity and a pH adjusted to 2 to 7 may be a modified natural rubber having a high purity by reducing the amount of non-rubber components and having a rubber pH of 2 to 7. Specifically, (1) modified natural rubber obtained by removing non-rubber components of natural rubber and then treating with an acidic compound, and having a pH of 2 to 7, (2) Ken Natural rubber latex is washed and further treated with an acidic compound, modified natural rubber having a pH of 2 to 7, and (3) deproteinized natural rubber latex is washed and further treated with an acidic compound. And modified natural rubber having a pH of 2 to 7.

As described above, the modified natural rubber can be prepared by a method of washing a saponified natural rubber latex or a deproteinized natural rubber latex with distilled water or the like and further treating with an acidic compound. It is important to lower the pH value by shifting to the acidic side by treatment with an acidic compound. Usually, the pH of distilled water is not 7.00 and is about 5 to 6, but in this case, it is important to reduce the pH value to the acidic side from 5 to 6 by treatment with an acidic compound. Become. Specifically, it is preferable to lower the pH value by 0.2 to 2 by treatment with an acidic compound, rather than the pH value of water used for washing.

The modified natural rubber has a pH of 2-7, preferably 3-6, more preferably 4-6. By adjusting within the above range, deterioration in heat aging resistance can be prevented, and the performance balance of fuel efficiency, breaking strength and heat aging resistance can be remarkably improved while obtaining excellent processability. The pH of the modified natural rubber is cut into a size of 2 mm square or less on each side, soaked in distilled water, extracted at 90 ° C. for 15 minutes while irradiating with microwaves, and the soaked water was measured using a pH meter Specifically, it is measured by the method described in the examples described later. Here, regarding extraction, even if it is extracted for 1 hour with an ultrasonic cleaner or the like, the water-soluble component cannot be completely extracted from the inside of the rubber, so the internal pH cannot be accurately determined. The present inventor has found that the substance of rubber can be known by extracting.

The modified natural rubber is purified by various methods such as (1) to (3). For example, the phosphorus content in the modified natural rubber is preferably 200 ppm or less, more preferably It is 150 ppm or less. If it exceeds 200 ppm, the Mooney viscosity may increase during storage and processability may deteriorate, or tan δ may increase and fuel economy may not be improved. The phosphorus content can be measured by a conventional method such as ICP emission analysis. Phosphorus is considered to be derived from phospholipids contained in natural rubber.

When the modified natural rubber contains an artificial anti-aging agent, the nitrogen content after being immersed in acetone at room temperature (25 ° C.) for 48 hours is preferably 0.15% by mass or less, More preferably, it is 0.1 mass% or less. If it exceeds 0.15% by mass, the Mooney viscosity may increase during storage, resulting in poor processability, and the effect of improving fuel economy may not be sufficiently obtained. Highly purified natural rubber may be deteriorated by long-term storage because the natural anti-aging component, which is said to be inherent to natural rubber, has been removed. Therefore, an artificial anti-aging agent may be added. The nitrogen content is a measured value after removing an artificial anti-aging agent in rubber by extraction with acetone. The nitrogen content can be measured by a conventional method such as Kjeldahl method or trace nitrogen meter. Nitrogen is derived from proteins and amino acids.

The modified natural rubber preferably has a Mooney viscosity ML (1 + 4) 130 ° C. measured in accordance with JIS K 6300: 2001-1 of 75 ° C. or less, more preferably 40 to 75, still more preferably 45 to 75. Particularly preferred is 50 to 70, most preferred 55 to 65. By being 75 or less, the kneading normally required before rubber kneading becomes unnecessary. Therefore, the modified natural rubber produced without the mastication step can be suitably used as a compounding material for the rubber composition. On the other hand, when it exceeds 75, it is necessary to masticate before use, and there is a tendency for exclusive use of equipment, loss of electricity and thermal energy, and the like.

The modified natural rubber is preferably a rubber having a heat aging index represented by the following formula of 75 to 120% with respect to the Mooney viscosity ML (1 + 4) 130 ° C.

The heat aging index represented by the above formula is more preferably 80 to 115%, still more preferably 85 to 110%. Various methods have been reported for evaluating the heat aging resistance of rubber. By using the method of evaluating the rate of change before and after heat treatment at 80 ° C. for 18 hours for Mooney viscosity ML (1 + 4) at 130 ° C., tire production is possible. It is possible to accurately evaluate heat aging resistance at times and when using tires. Here, if it is in the said range, the outstanding heat aging property and workability will be obtained, and the performance balance of fuel-consumption property, breaking strength, and heat aging property can be improved remarkably.

The modified natural rubber having a high purity such as (1) to (3) and having a pH adjusted to 2 to 7 is (Production method 1) Step 1-1 of saponifying natural rubber latex; A production method comprising a step 1-2 for washing the saponified natural rubber latex and a step 1-3 for treating with the acidic compound; (Production method 2) a step 2-1 for deproteinizing the natural rubber latex; It can be prepared by a production method including a step 2-2 for washing rubber latex and a step 2-3 for treating with an acidic compound.

[Production method 1]
(Step 1-1)
In step 1-1, natural rubber latex is saponified. Thereby, saponified natural rubber latex in which phospholipids and proteins in the rubber are decomposed and non-rubber components are reduced is prepared.

Natural rubber latex is collected as sap of natural rubber trees such as Hevea, and contains rubber, water, proteins, lipids, inorganic salts, etc., and the gel content in rubber is based on the complex presence of various impurities. It is considered a thing. In the present invention, as natural rubber latex, raw latex (field latex) produced by tapping Hevea tree, or concentrated latex concentrated by centrifugal separation or creaming (refined latex, high ammonia to which ammonia is added by a conventional method) Latex, zinc oxide, TMTD, and LATZ latex stabilized with ammonia can be used.

As a method for the saponification treatment, for example, the method described in JP 2010-138359 A and JP 2010-174169 A can be suitably performed, and specifically, the following method can be used.

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 period of time. Stirring may be performed as necessary.

The alkali used for the saponification treatment is preferably sodium hydroxide or potassium hydroxide, but is not limited thereto. The surfactant is not particularly limited, and examples include known anionic surfactants such as polyoxyethylene alkyl ether sulfate salts, nonionic surfactants, and amphoteric surfactants. Anionic surfactants such as polyoxyethylene alkyl ether sulfates are preferred because they can be saponified. In the saponification treatment, the addition amount of alkali and surfactant, the temperature and time of the saponification treatment may be appropriately set.

(Step 1-2)
In step 1-2, the saponified natural rubber latex obtained in step 1-1 is washed. By the washing, non-rubber components such as proteins are removed.

Step 1-2 includes, for example, aggregating the saponified natural rubber latex obtained in Step 1-1 to produce an agglomerated rubber, and then treating the obtained agglomerated rubber with a basic compound and further washing. Can be implemented. Specifically, after the agglomerated rubber is produced, the non-rubber component can be removed by diluting with water and transferring the water-soluble component to the aqueous layer and removing the water, and further by treating with a basic compound after agglomeration. Non-rubber components confined in the rubber during agglomeration can be redissolved. Thereby, non-rubber components such as protein strongly adhered to the agglomerated rubber can be removed.

Examples of the aggregating method include a method of adjusting the pH by adding an acid such as formic acid, acetic acid or sulfuric acid, and further adding a polymer flocculant as necessary. Thereby, not a large aggregate but a granular rubber having a diameter of about 20 mm is formed from a diameter of several mm to 1 mm or less, and proteins and the like are sufficiently removed by the basic compound treatment. The pH is preferably adjusted in the range of 3.0 to 5.0, more preferably 3.5 to 4.5.

Examples of polymer flocculants include cationic polymer flocculants such as methyl chloride quaternary salt polymer of dimethylaminoethyl (meth) acrylate, anionic polymer flocculants such as acrylate polymer, and acrylamide polymer. Nonionic polymer flocculants such as, and amphoteric polymer flocculants such as dimethylaminoethyl (meth) acrylate methyl chloride quaternary salt-acrylate copolymer. The addition amount of the polymer flocculant can be selected as appropriate.

Next, the resulting agglomerated rubber is treated with a basic compound. Here, although it does not specifically limit as a basic compound, A basic inorganic compound is suitable from the point of removal performance, such as protein.

Examples of basic inorganic compounds include metal hydroxides such as alkali metal hydroxides and alkaline earth metal hydroxides; metal carbonates such as alkali metal carbonates and alkaline earth metal carbonates; alkali metal hydrogen carbonates and the like Metal phosphates such as alkali metal phosphates; metal acetates such as alkali metal acetates; metal hydrides such as alkali metal hydrides; ammonia and the like.

Examples of the alkali metal hydroxide include lithium hydroxide, sodium hydroxide, potassium hydroxide and the like. Examples of the alkaline earth metal hydroxide include magnesium hydroxide, calcium hydroxide, and barium hydroxide. Examples of the alkali metal carbonate include lithium carbonate, sodium carbonate, and potassium carbonate. Examples of the alkaline earth metal carbonate include magnesium carbonate, calcium carbonate, barium carbonate and the like. Examples of the alkali metal hydrogen carbonate include lithium hydrogen carbonate, sodium hydrogen carbonate, and potassium hydrogen carbonate. Examples of the alkali metal phosphate include sodium phosphate and sodium hydrogen phosphate. Examples of the alkali metal acetate include sodium acetate and potassium acetate. Examples of the alkali metal hydride include sodium hydride and potassium hydride.

Of these, metal hydroxides, metal carbonates, metal hydrogen carbonates, metal phosphates, and ammonia are preferable, alkali metal carbonates, alkali metal hydrogen carbonates, and ammonia are more preferable, and sodium carbonate and sodium hydrogen carbonate are more preferable. preferable. The said basic compound may be used independently and may use 2 or more types together.

The method for treating the agglomerated rubber with the basic compound is not particularly limited as long as the agglomerated rubber is brought into contact with the basic compound. For example, the method of immersing the agglomerated rubber in an aqueous solution of the basic compound, And a method of spraying an aqueous solution of the active compound. An aqueous solution of a basic compound can be prepared by diluting and dissolving each basic compound with water.

The content of the basic compound in 100% by mass of the aqueous solution is preferably 0.1% by mass or more, more preferably 0.3% by mass or more. If it is less than 0.1% by mass, the protein may not be sufficiently removed. The content is preferably 10% by mass or less, more preferably 5% by mass or less. If it exceeds 10% by mass, a large amount of basic compound is required, but the amount of proteolysis does not increase, and the efficiency tends to be poor.

Although the said processing temperature should just be selected suitably, Preferably it is 10-50 degreeC, More preferably, it is 15-35 degreeC. Moreover, processing time is 1 minute or more normally, Preferably it is 10 minutes or more, More preferably, it is 30 minutes or more. If it is less than 1 minute, the effects of the present invention may not be obtained satisfactorily. The upper limit is not limited, but is preferably 48 hours or less, more preferably 24 hours or less, and still more preferably 16 hours or less from the viewpoint of productivity.

After the basic compound treatment, a washing treatment is performed. By the washing treatment, non-rubber components such as proteins can be removed. Examples of the washing method include a method of diluting a rubber component with water and washing and then centrifuging, and a method of allowing the rubber component to float by standing and discharging only the aqueous phase and taking out the rubber component. The number of washings can be any number of times that can reduce non-rubber components such as proteins to a desired amount, but a method of repeating a washing cycle of adding 1000 mL of water to 300 g of dry rubber and stirring and then dehydrating Then, 3 times (3 cycles) or more is preferable, 5 times (5 cycles) or more is more preferable, and 7 times (7 cycles) or more is more preferable.

The washing treatment is preferably carried out until the phosphorus content in the rubber is 200 ppm or less and / or the nitrogen content is 0.15 mass% or less. By sufficiently removing phospholipids and proteins by washing treatment, fuel economy, breaking strength and processability are improved.

(Step 1-3)
In Step 1-3, the washed rubber obtained in Step 1-2 is treated with an acidic compound. Although heat aging resistance tends to decrease due to the treatment of a basic compound or the like, further treatment with an acidic compound prevents such problems and provides good heat aging resistance.

The acidic compound is not particularly limited, and inorganic acids such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, polyphosphoric acid, metaphosphoric acid, boric acid, boronic acid, sulfanilic acid, sulfamic acid; formic acid, acetic acid, glycolic acid, oxalic acid, propion Acid, malonic acid, succinic acid, adipic acid, maleic acid, malic acid, tartaric acid, citric acid, benzoic acid, phthalic acid, isophthalic acid, glutaric acid, gluconic acid, lactic acid, aspartic acid, glutamic acid, salicylic acid, methanesulfonic acid, Itaconic acid, benzenesulfonic acid, toluenesulfonic acid, naphthalenedisulfonic acid, trifluoromethanesulfonic acid, styrenesulfonic acid, trifluoroacetic acid, barbituric acid, acrylic acid, methacrylic acid, cinnamic acid, 4-hydroxybenzoic acid, aminobenzoic acid , Naphthalene disulfonic acid, hydroxybenze Examples include organic acids such as sulfonic acid, toluenesulfinic acid, benzenesulfinic acid, α-resorcinic acid, β-resorcinic acid, γ-resorcinic acid, gallic acid, phloroglicin, sulfosalicylic acid, ascorbic acid, erythorbic acid, and bisphenolic acid. It is done. Of these, acetic acid, sulfuric acid, formic acid and the like are preferable. The said acidic compound may be used independently and may use 2 or more types together.

The method of treating the agglomerated rubber with an acid is not particularly limited as long as the agglomerated rubber is brought into contact with the acidic compound. For example, the method of immersing the agglomerated rubber in an aqueous solution of the acidic compound, The method of spraying etc. are mentioned. An aqueous solution of an acidic compound can be prepared by diluting and dissolving each acidic compound with water.

The content of the acidic compound in 100% by mass of the aqueous solution is not particularly limited, but the lower limit is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and the upper limit is preferably 15% by mass or less. More preferably, it is 10 mass% or less, More preferably, it is 5 mass% or less. When the content is within the above range, good heat aging resistance can be obtained.

Although the said processing temperature should just be selected suitably, Preferably it is 10-50 degreeC, More preferably, it is 15-35 degreeC. The treatment time is usually preferably 3 seconds or more, more preferably 10 seconds or more, and further preferably 30 seconds or more. If it is less than 3 seconds, it cannot be sufficiently neutralized and the effects of the present invention may not be obtained satisfactorily. Although there is no restriction | limiting in an upper limit, From the point of productivity, Preferably it is 24 hours or less, More preferably, it is 10 hours or less, More preferably, it is 5 hours or less.

In the treatment such as immersion of the acidic compound in an aqueous solution, the pH is preferably adjusted to 6 or less.
By such neutralization, excellent heat aging resistance is obtained. The upper limit of the pH is more preferably 5 or less, still more preferably 4.5 or less. The lower limit is not particularly limited, and although it depends on the immersion time, it is preferably 1 or more, more preferably 2 or more, because if the acid is too strong, the rubber deteriorates or the wastewater treatment becomes troublesome. The immersion treatment can be performed by leaving the agglomerated rubber in an acidic compound aqueous solution.

After the treatment, the compound used for the treatment of the acidic compound may be removed, and then the agglomerated rubber after the treatment may be appropriately washed. Examples of the cleaning treatment include the same methods as described above. For example, the non-rubber component may be further reduced by repeating the cleaning and adjusted to a desired content. Further, the agglomerated rubber after the treatment with the acidic compound may be squeezed with a roll type squeezer or the like to form a sheet. By adding a step of squeezing the agglomerated rubber, the pH of the agglomerated rubber surface and inside can be made uniform, and a rubber having a desired performance can be obtained. If necessary, the modified natural rubber can be obtained by carrying out a washing and squeezing step, then cutting through a creper and drying. In addition, drying is not specifically limited, For example, it can implement using normal dryers, such as a trolley-type dryer used in order to dry TSR, a vacuum dryer, an air dryer, and a drum dryer.

[Production method 2]
(Step 2-1)
In step 2-1, the natural rubber latex is deproteinized. Thereby, a deproteinized natural rubber latex from which non-rubber components such as proteins are removed can be prepared. Examples of the natural rubber latex used in Step 2-1 include those described above.

As a method for deproteinization, a known method capable of removing a protein can be used without particular limitation, and examples thereof include a method of degrading a protein by adding a proteolytic enzyme to natural rubber latex.

The proteolytic enzyme used for the deproteinization treatment is not particularly limited, and any of those derived from bacteria, those derived from filamentous fungi, and those derived from yeast may be used. Specifically, protease, peptidase, cellulase, pectinase, lipase, esterase, amylase and the like can be used alone or in combination.

The amount of proteolytic enzyme added is preferably 0.005 parts by mass or more, more preferably 0.01 parts by mass or more, still more preferably 0.05 parts by mass or more, with respect to 100 parts by mass of the solid content in the natural rubber latex. It is. If it is less than the lower limit, the protein degradation reaction may be insufficient.

In the deproteinization treatment, a surfactant may be added together with the proteolytic enzyme. Examples of the surfactant include anionic, cationic, nonionic, and amphoteric surfactants.

(Process 2-2)
In step 2-2, the deproteinized natural rubber latex obtained in step 2-1 is washed. By the washing, non-rubber components such as proteins are removed.

Step 2-2 can be performed, for example, by aggregating the deproteinized natural rubber latex obtained in Step 2-1 to produce an aggregated rubber, and then washing the obtained aggregated rubber. Thereby, non-rubber components such as protein strongly adhered to the agglomerated rubber can be removed.

The aggregation method can be carried out in the same manner as in Step 1-2. Furthermore, you may process with a basic compound as mentioned above as needed. After the production of the agglomerated rubber, a cleaning process is performed. The washing treatment can be performed by the same method as in Step 1-2, and thereby non-rubber components such as proteins can be removed. The cleaning treatment is preferably performed until the phosphorus content in the rubber is 200 ppm or less and / or the nitrogen content is 0.15 mass% or less for the same reason as described above.

(Step 2-3)
In step 2-3, the washed rubber obtained in step 2-2 is treated with an acidic compound. When the acid amount is small in acid aggregation as well as in the treatment with a basic compound, the heat aging resistance is reduced due to the fact that the finally obtained rubber becomes alkaline to neutral when extracted with water. Tend. In general, an enzyme having an optimum pH in the alkaline region is used as a proteolytic enzyme because it can be suitably deproteinized, and the enzyme reaction is performed under alkaline conditions in accordance with the optimum pH. There are many cases. Thereafter, it is coagulated under acidity at the time of agglomeration, but if the rubber is washed only with water, the pH will be higher than that of the extract by the extraction described later, and in this case, particularly the heat aging resistance is greatly reduced. On the other hand, such a problem can be prevented and good heat aging resistance can be obtained by treatment with an acidic compound after solidification, if necessary, after solidification.

Examples of the acidic compound include the same ones as in Step 1-3. The method of treating the agglomerated rubber with an acid is not particularly limited as long as the agglomerated rubber is brought into contact with the acidic compound. For example, the method of immersing the agglomerated rubber in an aqueous solution of the acidic compound, Examples include a method of spraying an aqueous solution. An aqueous solution of an acidic compound can be prepared by diluting and dissolving each acidic compound with water.

The content of the acidic compound in 100% by mass of the aqueous solution is not particularly limited, but the lower limit is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and the upper limit is preferably 15% by mass or less. More preferably, it is 10 mass% or less, More preferably, it is 5 mass% or less. When the content is within the above range, good heat aging resistance can be obtained.

What is necessary is just to select the said process temperature and process time suitably, and should just employ | adopt the temperature similar to the said process 1-3. In the treatment such as immersion of the acidic compound in an aqueous solution, the pH is preferably adjusted to the same value as in Step 1-3.

After the treatment, the compound used for the treatment of the acidic compound may be removed, and then the agglomerated rubber after the treatment may be appropriately washed. Examples of the cleaning treatment include the same methods as described above. For example, the non-rubber component may be further reduced by repeating the cleaning and adjusted to a desired content. The modified natural rubber is obtained by drying after the washing treatment. In addition, drying is not specifically limited, The above-mentioned method etc. are employable.

In the rubber composition of the present invention, the content of the modified natural rubber in 100% by mass of the rubber component is 5% by mass or more, preferably 10% by mass or more, more preferably 20% by mass or more, and further preferably 50% by mass. % Or more, particularly preferably 80% by mass or more, and the upper limit is not particularly limited, and may be 100% by mass. If it is less than 5% by mass, excellent fuel economy and rubber strength may not be obtained.

Examples of rubber components that can be used in addition to the modified natural rubber include natural rubber (non-modified) (NR), epoxidized natural rubber (ENR), isoprene rubber (IR), butadiene rubber (BR), and styrene butadiene rubber (SBR). ), Styrene isoprene butadiene rubber (SIBR), ethylene propylene diene rubber (EPDM), chloroprene rubber (CR), acrylonitrile butadiene rubber (NBR), and the like. Among these, NR is preferable because good fracture characteristics (breaking strength) can be obtained. As NR, those generally used in the tire industry such as SIR20, RSS # 3, TSR20 and the like can be used. Among them, it is particularly preferable to use RSS # 3 from the viewpoint of rubber strength.

The rubber composition of the present invention contains carbon black. Thereby, the outstanding reinforcement effect is acquired.

Carbon black is not particularly limited, and examples thereof include GPF, FEF, HAF, ISAF, and SAF. These carbon blacks may be used alone or in combination of two or more.

The nitrogen adsorption specific surface area (N ₂ SA) of carbon black is preferably 20 m ² / g or more, more preferably 30 m ² / g or more. The N ₂ SA is preferably 150 m ² / g or less, more preferably 100 m ² / g or less, still more preferably 70 m ² / g or less, and particularly preferably 50 m ² / g or less. If it is less than 30 m ² / g, there is a tendency that a sufficient reinforcing effect cannot be obtained. If it exceeds 150 m ² / g, the viscosity at the time of unvulcanization becomes very high, the workability deteriorates, and the fuel efficiency is low. There is a tendency 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 15 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 80 parts by mass or less, and still more preferably 70 parts by mass or less. When the amount is less than 10 parts by mass, sufficient reinforcing properties tend not to be obtained. When the amount exceeds 100 parts by mass, heat generation increases and fuel efficiency tends to deteriorate.

The rubber composition of the present invention may contain other fillers of carbon black as fillers. As fillers, those generally 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 low fuel consumption.

The silica is not particularly limited, and examples thereof include dry process silica (anhydrous silicic acid), wet process silica (hydrous silicic acid), and the like, but wet process silica is preferable because of its large number of silanol groups.

The rubber composition of the present invention preferably contains an organic acid cobalt. Since the organic acid cobalt plays a role of cross-linking the cord (steel cord) and the rubber, the adhesion between the cord and the rubber can be improved by blending this component.

Examples of the organic acid cobalt include cobalt stearate, cobalt naphthenate, cobalt neodecanoate, and boron 3 neodecanoate cobalt. Of these, cobalt stearate and cobalt naphthenate are preferable.

The content of the organic acid cobalt is preferably 0.05 parts by mass or more, more preferably 0.08 parts by mass or more in terms of the amount of cobalt metal with respect to 100 parts by mass of the rubber component. If it is less than 0.05 parts by mass, the adhesion between the steel cord plating layer and the rubber may be insufficient. Moreover, this content becomes like this. Preferably it is 0.5 mass part or less, More preferably, it is 0.3 mass part or less, More preferably, it is 0.2 mass part or less. If the amount exceeds 0.5 parts by mass, the oxidative degradation of the rubber becomes significant, and the fracture characteristics tend to deteriorate.

The rubber composition of the present invention preferably contains zinc oxide. By blending zinc oxide, the adhesion between the steel cord plating layer and the rubber is improved. It also serves as a rubber vulcanization aid. There is also an effect of suppressing scorch.

The content of zinc oxide is preferably 2 parts by mass or more, more preferably 5 parts by mass or more, and further preferably 6 parts by mass or more with respect to 100 parts by mass of the rubber component. If it is less than 2 parts by mass, the adhesion between the steel cord plating layer and the rubber may be insufficient, or the rubber may be insufficiently vulcanized. The content is preferably 25 parts by mass or less, more preferably 20 parts by mass or less, and still more preferably 15 parts by mass or less. If it exceeds 25 parts by mass, the rubber strength may be reduced.

In the rubber composition of the present invention, sulfur can be suitably used as a vulcanizing agent. Examples of sulfur include powdered sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, and highly dispersible sulfur.

The sulfur content is preferably 2 parts by mass or more, more preferably 3 parts by mass or more, and still more preferably 4 parts by mass or more with respect to 100 parts by mass of the rubber component. If it is less than 2 parts by mass, a sufficient crosslinking density cannot be obtained, and the adhesion performance may be deteriorated. Moreover, this content becomes like this. Preferably it is 10 mass parts or less, More preferably, it is 8 mass parts or less, More preferably, it is 6 mass parts or less. If it exceeds 10 parts by mass, the heat aging resistance may deteriorate.

In addition to the above materials, the rubber composition of the present invention appropriately includes various materials generally used in the tire industry such as plasticizers such as oil, stearic acid, various anti-aging agents, and vulcanization accelerators. It may be blended.

Examples of the vulcanization accelerator include sulfenamide, thiazole, thiuram, thiourea, guanidine, dithiocarbamic acid, aldehyde-amine or aldehyde-ammonia, imidazoline, or xanthate vulcanization. Accelerators are mentioned. Of these, sulfenamide-based vulcanization accelerators are preferred because of their excellent scorch properties.

Examples of the sulfenamide vulcanization accelerator include N-tert-butyl-2-benzothiazolylsulfenamide (TBBS), N-cyclohexyl-2-benzothiazolylsulfenamide (CBS), N, N And '-dicyclohexyl-2-benzothiazolylsulfenamide (DZ).

Examples of the anti-aging agent include amine / ketones, amines, phenols, imidazoles, and thioureas. These anti-aging agents may be used alone or in combination of two or more. Among them, amine-based anti-aging agents may be used because of their excellent fracture characteristics and heat resistance. preferable.

Examples of amine-based antioxidants include diphenylamine-based and p-phenylenediamine-based amine derivatives. Examples of the diphenylamine derivative include p- (p-toluenesulfonylamide) -diphenylamine, octylated diphenylamine, and the like. Examples of p-phenylenediamine derivatives include N- (1,3-dimethylbutyl) -N′-phenyl-p-phenylenediamine (6PPD), N-phenyl-N′-isopropyl-p-phenylenediamine (IPPD). ), N, N′-di-2-naphthyl-p-phenylenediamine, and the like.

The content of the anti-aging agent is preferably 1 part by mass or more, more preferably 1.3 parts by mass or more with respect to 100 parts by mass of the rubber component. If it is less than 1 part by mass, the fracture characteristics may not be improved. Moreover, this content becomes like this. Preferably it is 6 mass parts or less, More preferably, it is 4 mass parts or less, More preferably, it is 2 mass parts or less. When the amount exceeds 6 parts by mass, the anti-aging agent may be deposited on the surface and the rubber physical properties may be deteriorated. Since the rubber composition of the present invention is excellent in heat aging resistance, it can have sufficient durability even when the anti-aging agent is 2 parts by mass or less.

As the oil, for example, process oil, vegetable oil and fat, a mixture thereof and the like can be used. Examples of the process oil include paraffinic process oil, naphthenic process oil, and aromatic process oil.

The oil content is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more with respect to 100 parts by mass of the rubber component. Moreover, this content becomes like this. Preferably it is 10 mass parts or less, More preferably, it is 8 mass parts or less, More preferably, it is 6 mass parts or less. If it is out of the above range, there is a possibility that wet heat resistance and durability cannot be improved sufficiently.

As a method for producing the rubber composition of the present invention, known methods can be used. For example, the above components are kneaded using a rubber kneader such as an open roll or a Banbury mixer, and then vulcanized. Can be manufactured.

The rubber composition of the present invention is used as a rubber composition for a breaker (steel cord) topping. Examples of the steel cord include a single-stranded steel cord having a 1 × n configuration, a layer-twisted steel cord having a k + m configuration, and the like (n is an integer of 1 to 27, k is an integer of 1 to 10, and m is an integer of 1 to 3). Such).

The rubber composition for breaker topping of the present invention is used for a breaker disposed inside a tread and radially outside of a carcass. Specifically, it is used for the breaker shown in FIG. 3 of JP-A-2003-94918, FIG. 1 of JP-A-2004-67027, and FIGS. 1-4 of JP-A-4-356205.

The pneumatic tire of the present invention is produced by a usual method using the rubber composition. That is, a steel cord is coated with the above rubber composition and formed into a breaker shape, and then bonded to another tire member to form an unvulcanized tire and vulcanize to form a pneumatic tire (radial tire or the like). Can be manufactured.

The pneumatic tire of the present invention is preferably used as a tire for passenger cars, a tire for trucks and buses, a tire for motorcycles, a tire for competition, and the like, and particularly preferably used as a tire for passenger cars.

The present invention will be specifically described based on examples, but the present invention is not limited to these examples.
The various chemicals used in the examples are described below.
Field Latex: Field Latex Emar E-27C (surfactant) obtained from Muhiba Latex Co., Ltd .: Emar E-27C (Polyoxyethylene lauryl ether sodium sulfate, 27% by mass of active ingredient) manufactured by Kao Corporation
NaOH: NaOH manufactured by Wako Pure Chemical Industries, Ltd.
Wingstay L (anti-aging agent): WINGSTAY L (compound obtained by butylating the condensate of ρ-cresol and dicyclopentadiene) manufactured by ELIOKEM
Emulvin W (surfactant): Emalvin W (aromatic polyglycol ether) manufactured by LANXESS
Tamol NN9104 (surfactant): Tamol NN9104 manufactured by BASF (Naphthalenesulfonic acid / formaldehyde sodium salt)
Van gel B (surfactant): Van gel B (magnesium aluminum silicate hydrate) manufactured by Vanderbilt
NR: RSS # 3 grade carbon black: N550 manufactured by Cabot Japan K.K. (N ₂ SA: 42 m ² / g, average particle size: 48 nm, DBP oil absorption: 113 ml / 100 g)
Oil: Diana Process Oil PS323 manufactured by Idemitsu Kosan Co., Ltd.
Organic acid cobalt: cost-F (cobalt stearate, cobalt content: 9.5% by mass) manufactured by Dainippon Ink & Chemicals, Inc.
Zinc oxide: 2 types of zinc oxide manufactured by Mitsui Mining & Smelting Co., Ltd. Stearic acid: Beads stearic acid anti-aging agent manufactured by NOF Corporation: NOCRACK 6C (N-phenyl-) manufactured by Ouchi Shinsei Chemical Co., Ltd. N ′-(1,3-dimethylbutyl) -p-phenylenediamine) (6PPD)
Sulfur: Powder sulfur vulcanization accelerator manufactured by Tsurumi Chemical Industry Co., Ltd .: Noxeller DZ manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.

<Examples and Comparative Examples>
(Preparation of anti-aging agent dispersion)
462.5 g of water was mixed with 12.5 g of Emulvin W, 12.5 g of Tamol NN9104, 12.5 g of Van gel B, and 500 g of Wingstay L (total of 1000 g) for 16 hours by a ball mill to prepare an antioxidant dispersion.

(Production Example 1)
After adjusting the solid content concentration (DRC) of the field latex to 30% (w / v), 25 g of 10% Emar E-27C aqueous solution and 60 g of 25% NaOH aqueous solution were added to 1000 g of the latex and saponified for 24 hours at room temperature. Reaction was performed to obtain a saponified natural rubber latex. Next, 6 g of the anti-aging dispersion was added and stirred for 2 hours, and then further diluted with water to a rubber concentration of 15% (w / v). Next, formic acid was added with slow stirring to adjust the pH to 4.0, and then a cationic polymer flocculant was added and stirred for 2 minutes for aggregation. The diameter of the aggregate (aggregated rubber) thus obtained was about 0.5 to 5 mm. The obtained agglomerates were taken out and immersed in 1000 ml of a 2% by weight aqueous sodium carbonate solution at room temperature for 4 hours, and then the rubber was taken out. To this, 2000 ml of water was added and stirred for 2 minutes to remove water as much as possible 7 times. Thereafter, 500 ml of water was added, 2% by mass formic acid was added until pH 4 was reached, and the mixture was allowed to stand for 15 minutes. Further, after removing the water as much as possible, repeating the work of adding water again and stirring for 2 minutes three times, squeezing the water with a water squeezing roll to form a sheet, and then drying at 90 ° C. for 4 hours to solid rubber ( A high-purity natural rubber A) was obtained.

(Comparative Production Example 1)
After adjusting the solid content concentration (DRC) of the field latex to 30% (w / v), 25 g of 10% Emar E-27C aqueous solution and 60 g of 25% NaOH aqueous solution were added to 1000 g of the latex and saponified for 24 hours at room temperature. Reaction was performed to obtain a saponified natural rubber latex. Next, 6 g of the anti-aging dispersion was added and stirred for 2 hours, and then further diluted with water to a rubber concentration of 15% (w / v). Next, formic acid was added with slow stirring to adjust the pH to 4.0, and then a cationic polymer flocculant was added and stirred for 2 minutes for aggregation. The diameter of the aggregate (aggregated rubber) thus obtained was about 3 to 15 mm. The obtained agglomerates were taken out and immersed in 1000 ml of a 2% by weight aqueous sodium carbonate solution at room temperature for 4 hours, and then the rubber was taken out. To this, 1000 ml of water was added and stirred for 2 minutes to remove water as much as possible. Thereafter, 500 ml of water was added, 2% by mass formic acid was added until pH 4 was reached, and the mixture was stirred for 15 minutes. Further, the operation of removing water as much as possible, adding water again and stirring for 2 minutes was repeated three times, and then dried at 90 ° C. for 4 hours to obtain a solid rubber (high purity natural rubber B).

(Comparative Production Example 2)
The same procedure was followed, except that it was treated with an aqueous sodium carbonate solution in Production Example 1 and washed with water seven times, and then the sheet was formed by squeezing water with a water squeezing roll without acid treatment with 2% by mass formic acid. A solid rubber (high purity natural rubber C) was obtained.

(Production Example 2)
Commercially available high-ammonia latex (manufactured by Mujiba Latex of Malaysia, solid rubber content 62.0%) is diluted with 0.12% sodium naphthenate aqueous solution to make the solid rubber content 10%, and dihydrogen phosphate. Sodium was added to adjust the pH to 9.2. And with respect to 10g of rubber | gum, after adding proteolytic enzyme (Alcalase 2.0M) in the ratio of 0.87g and adjusting pH again to 9.2, it maintained at 37 degreeC for 24 hours.
Next, a 1% aqueous solution of a nonionic surfactant (trade name Emulgen 810 manufactured by Kao Corporation) was added to the latex that had been subjected to the enzyme treatment to adjust the rubber concentration to 8%, and 11,000 r. p. m. The solution was centrifuged at a rotation speed of 30 minutes. Next, the cream-like fraction produced by centrifugation was dispersed in a 1% aqueous solution of Emulgen 810 to adjust the rubber concentration to 8%. p. m. The solution was centrifuged at a rotation speed of 30 minutes. After repeating this operation twice, the obtained creamy fraction was dispersed in distilled water to prepare a deproteinized rubber latex having a solid rubber content of 60%.
To this latex, 2% by mass formic acid was added until the pH reached 4, and a cationic polymer flocculant was further added to obtain rubber particles of 0.5 to 5 mm. The water was removed as much as possible, 50 g of water was added to 10 g of rubber, and 2% by mass formic acid was added until pH 3 was reached. After 30 minutes, the rubber was pulled up, formed into a sheet with a creper, and then dried at 90 ° C. for 4 hours to obtain a solid rubber (high purity natural rubber D).

(Comparative Production Example 3)
Commercially available high-ammonia latex (manufactured by Mujiba Latex of Malaysia, solid rubber content 62.0%) is diluted with 0.12% sodium naphthenate aqueous solution to make the solid rubber content 10%, and dihydrogen phosphate. Sodium was added to adjust the pH to 9.2. And with respect to 10g of rubber | gum, after adding proteolytic enzyme (Alcalase 2.0M) in the ratio of 0.87g and adjusting pH again to 9.2, it maintained at 37 degreeC for 24 hours.
Next, a 1% aqueous solution of a nonionic surfactant (trade name Emulgen 810 manufactured by Kao Corporation) was added to the latex that had been subjected to the enzyme treatment to adjust the rubber concentration to 8%, and 11,000 r. p. m. The solution was centrifuged at a rotation speed of 30 minutes. Next, the cream-like fraction produced by centrifugation was dispersed in a 1% aqueous solution of Emulgen 810 to adjust the rubber concentration to 8%. p. m. The solution was centrifuged at a rotation speed of 30 minutes. After repeating this operation once more, the resulting creamy fraction was dispersed in distilled water to prepare a deproteinized rubber latex having a solid rubber content of 60%.
50% by mass formic acid was added to the latex until the rubber solidified, and the solidified rubber was taken out. The rubber was formed into a sheet while being washed with water with a creper and then dried at 90 ° C. for 4 hours to obtain a solid rubber (high-purity natural rubber E).

(Comparative Production Example 4)
The rubber solidified in Comparative Production Example 3 was taken out, immersed in an aqueous 0.5% by mass sodium carbonate solution for 1 hour, then formed into a sheet while being washed with water with a creper, and then dried at 90 ° C. for 4 hours. The solid rubber (high-purity natural rubber F) was obtained by the procedure described above.

The solid rubber obtained above was evaluated as follows, and the results are shown in Table 1.

<Measurement of pH of rubber>
5 g of the obtained rubber was cut into a total of 3 mm or less (about 1-2 × about 1-2 × about 1-2 (mm)) and placed in a 100 ml beaker, and 50 ml of distilled water at room temperature was added. The temperature was raised to 90 ° C. for 1 minute, and then irradiation with microwaves (300 W) was performed for 13 minutes (total 15 minutes) while maintaining the temperature at 90 ° C. Next, the immersion water was cooled with an ice bath to 25 ° C., and then the pH of the immersion water was measured using a pH meter.

<Measurement of nitrogen content>
(Acetone extraction (test piece preparation))
About 0.5 g of a sample obtained by chopping each solid rubber into 1 mm square was prepared. The sample was immersed in 50 g of acetone, and after 48 hours at room temperature (25 ° C.), the rubber was taken out and dried to obtain each test piece (extracted with anti-aging agent).
(Measurement)
The nitrogen content of the obtained test piece was measured by the following method.
The nitrogen content was determined by decomposing and gasifying each of the acetone-extracted test pieces obtained above using a trace nitrogen carbon measuring device “SUMIGRAPH NC95A (manufactured by Sumika Chemical Analysis Center)”. The nitrogen content was quantified by analyzing with a gas chromatograph “GC-8A (manufactured by Shimadzu Corporation)”.

<Measurement of phosphorus content>
The phosphorus content was determined using an ICP emission spectrometer (P-4010, manufactured by Hitachi, Ltd.).

<Measurement of gel content>
About 70 mg of a raw rubber sample cut to 1 mm × 1 mm was accurately 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 (mass%) was determined by the following formula.
Gel content (mass%) = [mass mg after drying / mg of initial sample] × 100

<Heat aging resistance>
The Mooney viscosity ML (1 + 4) 130 ° C. of the solid rubber before and after being treated at 80 ° C. for 18 hours was measured according to JIS K 6300: 2001-1, and the heat aging index was calculated by the above formula.

According to Table 1, the modified natural rubber having a rubber pH in the range of 2 to 7 was excellent in heat aging resistance as compared with the rubber outside the range.

<Preparation of unvulcanized rubber composition and vulcanized rubber composition>
According to the formulation shown in Table 2, using a 1.7 L Banbury mixer, chemicals other than sulfur and a vulcanization accelerator were kneaded to obtain a kneaded product. Next, using an open roll, sulfur and a vulcanization accelerator were kneaded into the obtained kneaded material to obtain an unvulcanized rubber composition.
Next, the obtained unvulcanized rubber composition was pressed with a 2 mm thick mold at 150 ° C. for 30 minutes to obtain a vulcanized rubber composition. The obtained unvulcanized rubber composition and vulcanized rubber composition were evaluated as follows. The results are shown in Table 2.

<Low fuel consumption (rolling resistance index)>
The vulcanized rubber composition was measured using a viscoelastic spectrometer VES (manufactured by Iwamoto Seisakusho Co., Ltd.), and tan δ of each formulation was measured under conditions of a temperature of 70 ° C., an initial strain of 10%, and a dynamic strain of 2%. The tan δ in Example 1 was set to 100, and the index was displayed by the following calculation formula. The larger the index, the better the rolling resistance.
(Rolling resistance index) = (tan δ of Comparative Example 1) / (tan δ of each formulation) × 100

<Processability index: Measurement of Mooney viscosity>
About the obtained unvulcanized rubber composition, it measured at 130 degreeC according to the measuring method of the Mooney viscosity based on JISK6300. The Mooney viscosity (ML _{1 + 4} ) of Comparative Example 1 was set to 100, and indexed by the following formula (Mooney viscosity index). The larger the index, the lower the Mooney viscosity and the better the processability.
(Mooney viscosity index) = (ML _{1 + 4} of Comparative Example 1) / (ML _{1 + 4 of} each formulation) × 100

<Break strength index>
In accordance with JIS K 6251 “Determination of tensile properties of vulcanized rubber and thermoplastic rubber”, a tensile test was conducted using a No. 3 dumbbell, and the elongation at break (EB) of the vulcanized rubber composition and the tensile at break The strength (TB) was measured. In addition, EBxTB of the comparative example 1 was set to 100, and EBxTB of each mixing | blending was each indicated by the index | exponent by the following formula. The larger the index, the better the breaking strength.
(Breaking Strength Index) = (EB × TB of each formulation) / (EB × TB of Comparative Example 1) × 100

From Table 2, it was clarified that in Examples using the high-purity natural rubber A or D, the fuel efficiency, breaking strength and heat aging resistance were remarkably improved while obtaining excellent processability.

Claims (12)

A rubber composition for breaker topping, comprising a highly purified natural rubber having a pH adjusted to 2 to 7 and carbon black ,
The pH is obtained by immersing 5 g of rubber produced by cutting the modified natural rubber into a size of 2 mm square on each side in 50 ml of distilled water and extracting at 90 ° C. for 15 minutes while irradiating with microwaves, A rubber composition for breaker topping, which is a value obtained by measuring immersion water using a pH meter . The rubber composition for a breaker topping according to claim 1, wherein the modified natural rubber is obtained by removing a non-rubber component of natural rubber and then treating with an acidic compound, and having a pH of 2 to 7. The rubber composition for a breaker topping according to claim 1, wherein the modified natural rubber is obtained by washing a saponified natural rubber latex and further treating with an acidic compound, and having a pH of 2-7. The rubber composition for a breaker topping according to claim 1, wherein the modified natural rubber is obtained by washing a deproteinized natural rubber latex and further treating with an acidic compound, and having a pH of 2-7. The rubber composition for breaker topping according to any one of claims 1 to 4, wherein the phosphorus content of the modified natural rubber is 200 ppm or less. The rubber composition for breaker topping according to any one of claims 1 to 5, wherein the nitrogen content of the modified natural rubber is 0.15% by mass or less. The modified natural rubber has a heat aging index represented by the following formula of 75 to 120% with respect to Mooney viscosity ML (1 + 4) 130 ° C. measured according to JIS K 6300: 2001-1. breaker topping rubber composition claim 1-6, wherein.

The rubber composition for break cartping according to any one of claims 1 to 7 , comprising zinc oxide. The rubber composition for break cartping according to any one of claims 1 to 8 , comprising organic acid cobalt. The rubber composition for break cartping according to any one of claims 1 to 9 , comprising a heat-resistant anti-aging agent. The rubber composition for breaker topping according to any one of claims 1 to 10 , wherein the modified natural rubber is produced without going through a mastication step. A pneumatic tire produced using the rubber composition for break cartping according to any one of claims 1 to 11 .