JP5860660B2 - Silica / natural rubber composite and production method thereof, rubber composition and pneumatic tire - Google Patents

Description

The present invention relates to a silica / natural rubber composite, a method for producing the same, a rubber composition containing the composite, and a pneumatic tire using the same.

In order to reduce the fuel consumption of tires, it has been studied to add silica as a filler to obtain good low heat build-up, but silica has a silanol group on the surface and exhibits hydrophilicity. It is not easy to disperse uniformly in the rubber because it has a low affinity with rubber exhibiting good properties and also has strong self-aggregation.

As a method for dispersing silica in rubber, a method is known in which silica slurry is added to natural rubber latex and stirred, and then acid is added to solidify natural rubber. According to this method, although the dispersibility of silica is improved as compared with the method of mixing natural rubber and silica in a dry state, the dispersibility is not yet sufficient and the loss of silica is also large.

Also, Patent Document 1 discloses a method of producing a composite by mixing rubber latex with fine particle silica produced from water glass in a liquid state, but there is room for improvement in terms of silica dispersibility and loss. There is.

JP 2009-51955 A

An object of the present invention is to solve the above problems and provide a natural rubber / silica composite in which silica is finely dispersed and a method for producing the same. Another object of the present invention is to provide a rubber composition and a pneumatic tire using the composite.

In the method of solidifying by adjusting the pH of the mixture of natural rubber latex and silica slurry to the acidic side, the rubber becomes a large block, and silica is not sufficiently taken into the rubber, so that the silica and the rubber are separated. As a result, since silica becomes a large aggregate, a composite in which silica is uniformly finely dispersed cannot be obtained. In addition, due to the separation, the packing amount of silica is lowered, and silica loss occurs.

On the other hand, it is conceivable to promote silica uptake by adjusting the pH at the time of solidification to near neutrality and slowing the solidification rate of rubber, but generally it takes a long time for solidification (solidification) and is practical. There is a problem.

The present inventor examined the preparation of a natural rubber / silica composite in which silica is finely dispersed, and by mixing natural rubber latex and fine particle silica dispersion in the presence of a surfactant, the silica and rubber are mixed. The present invention has been completed by finding that it is possible to solidify in an enhanced interaction state and to improve the dispersibility of silica. Thereby, separation of silica and rubber, aggregation of silica is suppressed, and a composite in which silica is finely dispersed can be obtained. In addition, loss of silica and natural rubber can be suppressed.

That is, the present invention relates to a silica / natural rubber composite obtained from a compounded latex prepared by mixing a natural rubber latex and a fine particle silica dispersion having an average particle size of 1 μm or less in the presence of a surfactant.

The surfactant is preferably a nonionic or cationic surfactant.

It is preferable that the surfactant is a nonionic surfactant having a polyoxyethylene group and a hydrocarbon group.

It is preferable that the surfactant is a cationic surfactant having a quaternary ammonium group and a hydrocarbon group.

The present invention also includes Step 1 for preparing a blended latex by mixing a natural rubber latex and a fine particle silica dispersion having an average particle size of 1 μm or less in the presence of a surfactant, and the blended latex obtained in Step 1 above. It is related with the manufacturing method of the said silica and natural rubber composite_body | complex including the process 2 which adjusts pH to 5-7 and solidifies.

The present invention also relates to a rubber composition containing the silica-natural rubber composite.

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

According to the present invention, since the compounded latex prepared by mixing the natural rubber latex and the fine particle silica dispersion liquid having a specific particle size in the presence of the surfactant is used, the silica is finely dispersed in the rubber component. A silica-natural rubber composite can be obtained. In addition, loss of silica and natural rubber during production can be suppressed.

<Silica-natural rubber composite>
The silica / natural rubber composite of the present invention is obtained from a compounded latex prepared by mixing a natural rubber latex and a fine particle silica dispersion having an average particle size of 1 μm or less in the presence of a surfactant.

The composite is prepared, for example, by mixing natural rubber latex with a fine particle silica dispersion having an average particle size of 1 μm or less in the presence of a surfactant to prepare a compounded latex, and the compound obtained in the above step 1 It is obtained by a production method including step 2 in which the pH of the latex is adjusted to 5 to 7 and coagulated.

(Process 1)
In step 1, natural rubber latex is used.
As natural rubber latex, conventionally known natural rubber latex can be used. For example, in addition to field latex obtained from natural rubber tree, ammonia-treated latex (for example, high ammonia type natural rubber latex) is used. Can do. The natural rubber latex preferably has a rubber solid content of 10 to 70% by mass.

In step 1, a fine particle silica dispersion having an average particle size of 1 μm or less is used. That is, not a silica powder but a dispersion (slurry) in which silica having a specific particle diameter is dispersed in water is used. The fine particle silica contained in the dispersion is not particularly limited, and examples thereof include dry method silica (anhydrous silicic acid), wet method silica (hydrous silicic acid), and the wet method silica is used because there are many silanol groups. preferable. The fine particle silica may be used alone or in combination of two or more.

In the dispersion, the average particle diameter of the fine particle silica is preferably 0.1 μm or less, more preferably 0.05 μm or less. When it exceeds 1 μm, the reinforcing performance of silica tends to deteriorate. The average particle diameter is preferably 0.005 μm or more, more preferably 0.01 μm or more. If it is less than 0.005 μm, the fine-particle silica tends to strongly aggregate.

In this specification, transmission electron microscope (TEM) observation is used as a method for measuring the average particle size of the fine particle silica. Specifically, the microparticles are photographed with a transmission electron microscope. If the shape of the microparticles is spherical, the diameter of the sphere is the particle diameter, and if it is needle-shaped or rod-shaped, the minor diameter is the particle diameter. In this case, the average particle diameter from the center is defined as the particle diameter, and the average value of the particle diameters of 100 fine particles is defined as the average particle diameter.

In the dispersion, the nitrogen adsorption specific surface area (N ₂ SA) of the fine particle silica by the BET method is preferably 40 m ² / g or more, and more preferably 80 m ² / g or more. If it is less than 40 m ² / g, the reinforcing property of silica tends to deteriorate. The N ₂ SA is preferably 220 m ² / g or less, and more preferably 200 m ² / g or less. When it exceeds 220 m ² / g, the fine particle silica tends to agglomerate strongly.
In addition, the nitrogen adsorption specific surface area by the BET method of a silica can be measured by the method based on ASTM D3037-81.

The fine particle silica dispersion can be produced by a known method and is not particularly limited. For example, the fine particle silica dispersion can be prepared by dispersing fine particle silica using a high-pressure homogenizer, an ultrasonic homogenizer, a colloid mill, or the like. Here, the content of the fine particle silica in the dispersion is not particularly limited, but is preferably 1 to 10% by mass from the viewpoint of uniform dispersibility in the dispersion (100% by mass).

The mixing step of step 1 is performed in the presence of a surfactant. As the surfactant, nonionic and cationic surfactants are preferable and nonionic surfactants are more preferable because the dispersibility of silica can be enhanced.

The nonionic surfactant is not particularly limited, and polyoxyalkylene alkyl ethers such as polyoxyethylene alkyl ethers, polyoxyethylene polypropylene alkyl ethers, polyoxyethylene polybutylene alkyl ethers; polyoxyethylenes such as polyoxyethylene alkenyl ethers Conventionally known ones such as alkylene alkenyl ethers; polyoxyethylene alkyl phenyl ethers; higher fatty acid alkanolamides can be used, and the polyoxyethylene group can increase the hydrogen bonding force with the silica surface. A nonionic surfactant having a hydrocarbon group as a group or a hydrophobic group can be preferably used.

As such a nonionic surfactant, the compounds represented by the following formulas (I) to (III) can be preferably used from the viewpoint that the effects of the present invention can be satisfactorily obtained. The compounds represented are particularly suitable.

R ¹ —O— (EO) _x —H (I)
(In the formula (I), R ¹ represents an alkyl group having 3 to 50 carbon atoms or an alkenyl group having 3 to 50 carbon atoms. EO represents an oxyethylene group. The average added mole number x is 3 to 100. )

The number of carbon atoms of R ¹ is preferably 5-30, more preferably 8-20.
Said x becomes like this. Preferably it is 5-50, More preferably, it is 8-30.

^{_{R 2 -O- (EO) y (}} AO) z -H (II)
(In the formula (II), R ² represents an alkyl group having 3 to 50 carbon atoms or an alkenyl group having 3 to 50 carbon atoms. EO represents an oxyethylene group, AO represents an oxypropylene group or an oxybutylene group. The number of moles y is 3 to 100, and the average number of added moles z is 3 to 100.)

The carbon number of R ² is preferably 5 to 30, more preferably 8 to 20.
Said y becomes like this. Preferably it is 5-50, More preferably, it is 8-30.
Said z becomes like this. Preferably it is 5-50, More preferably, it is 8-30.
The arrangement of EO and AO may be block or random.

（式（ＩＩＩ）において、Ｒ^３〜Ｒ^５は、同一若しくは異なって、炭素数１〜３０のアルキル基、炭素数１〜３０のアルケニル基又は炭素数１〜３０のアルコキシ基を表す。Ｒ^６は炭素数１〜３０のアルキレン基を表す。ＥＯはオキシエチレン基を表す。平均付加モル数ａは０〜５０、平均付加モル数ｂは０〜５０、平均付加モル数ｃは１〜５０である。）

(In Formula (III), R < ³ > -R < ⁵ > is the same or different and represents a C1-C30 alkyl group, a C1-C30 alkenyl group, or a C1-C30 alkoxy group. R < ⁶ >. Represents an alkylene group having 1 to 30 carbon atoms, EO represents an oxyethylene group, the average added mole number a is 0 to 50, the average added mole number b is 0 to 50, and the average added mole number c is 1 to 50. is there.)

The number of carbon atoms of R ³ and R ⁴ is preferably 1 to 25, and the number of carbon atoms of R ⁵ is preferably 1 to 25.
The number of carbon atoms in R ⁶ is preferably 3-8.

The a and b are preferably 0 to 30, more preferably 10 to 30, and the c is preferably 1 to 30, and more preferably 10 to 30.

As the cationic surfactant, a quaternary ammonium salt type, that is, a surfactant having a quaternary ammonium group and a hydrocarbon group is preferable. Specific examples include compounds represented by the following formula (IV).
[R ⁷ R ⁸ R ⁹ R ¹⁰ N] ⁺ X ⁻ (IV)
(Wherein R ⁷ and R ⁸ are the same or different and each represents an alkyl group or alkenyl group having 1 to 22 carbon atoms, and at least one of R ⁷ and R ⁸ has 4 or more carbon atoms.) ⁹ and R ¹⁰ each represent an alkyl group having 1 to 3 carbon atoms, and X represents a monovalent anion.)

In the above formula (IV), it is preferable that one of R ⁷ and R ⁸ is a methyl group and the other is an alkyl group having 6 to 18 carbon atoms. R ⁹ and R ¹⁰ are preferably a methyl group.
Examples of X include halogen ions such as chloride ions and bromide ions.

Specific examples of the compound represented by the formula (IV) include, for example, hexyltrimethylammonium chloride, octyltrimethylammonium chloride, decyltrimethylammonium chloride, dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, hexadecyltrimethylammonium chloride, Examples include stearyltrimethylammonium chloride and the corresponding alkyltrimethylammonium salts such as bromide. Among these, hexadecyltrimethylammonium bromide is preferable from the viewpoint of improving the dispersibility of silica.

In the mixing step of Step 1, natural rubber latex and fine particle silica dispersion are mixed by a known method in the presence of a surfactant, and then mixed sufficiently until the mixed latex becomes a uniform solution. Latex (mixed solution) can be prepared.

In the mixing step, it is preferable to mix the fine particle silica dispersion so that the fine particle silica is 5 to 150 parts by mass (in terms of SiO ₂ ) with respect to 100 parts by mass (solid content) of natural rubber. When the amount is less than 5 parts by mass, the amount of fine particle silica is small, and the effects of the present invention tend not to be obtained sufficiently. When it exceeds 150 parts by mass, the uniform dispersibility of silica tends to be lowered. The content is more preferably 30 parts by mass or more. The content is more preferably 100 parts by mass or less, still more preferably 70 parts by mass or less.

In the mixing step, the amount of the surfactant added is preferably 1 to 20 parts by mass with respect to 100 parts by mass (solid content) of natural rubber. If the amount is less than 1 part by mass, the blending amount of the surfactant is small, and the effects of the present invention tend not to be sufficiently obtained. When it exceeds 20 parts by mass, the uniform dispersibility of silica tends to be lowered. The content is more preferably 3 parts by mass or more. Further, the content is more preferably 10 parts by mass or less, and still more preferably 6 parts by mass or less.

(Process 2)
In step 2, the pH of the compounded latex obtained in step 1 is adjusted to 5-7 (preferably 6-7) and coagulated. If the pH is less than 5, the latex rapidly solidifies, so that the silica and the rubber are separated, and the silica dispersion tends to deteriorate. On the other hand, if the pH exceeds 7, the latex coagulation reaction tends to be slow and impractical.

As a method of adjusting the pH to 5 to 7 and coagulating the compounded latex, an acid is usually used, and the acid is coagulated by adding it to the compounded latex. Examples of the acid for coagulation include sulfuric acid, hydrochloric acid, formic acid, acetic acid and the like.

The obtained coagulated material (agglomerated rubber and agglomerate containing silica) is filtered and dried by a known method, and further dried, and then kneaded with a biaxial roll, a banbury, etc., so that the fine particle silica is uniform in the natural rubber matrix. A complex dispersed in can be obtained.
The silica / natural rubber composite of the present invention may contain other components as long as the effects of the present invention are not impaired.

<Rubber composition>
The rubber composition of the present invention contains the above silica / natural rubber composite. The silica / natural rubber composite can be used as a masterbatch. Since the silica / natural rubber composite has the silica uniformly dispersed in the rubber, the silica can be evenly dispersed in the rubber composition mixed with other components. Therefore, effective reinforcement can be expected.

In addition to the silica / natural rubber composite, the rubber composition of the present invention includes rubber components other than natural rubber generally used in the tire industry, fillers such as carbon black, silane coupling agents, and zinc oxide. Various materials such as stearic acid, anti-aging agent, sulfur, and vulcanization accelerator can be appropriately blended. Further, the rubber composition may separately contain a natural rubber component, silica and the like in addition to the composite.

In the rubber composition, the total silica content is preferably 5 to 150 parts by mass with respect to 100 parts by mass (solid content) of the total rubber. The content of other compounding agents such as carbon black can also be set as appropriate.

The rubber composition can be produced by a method of kneading the components using a rubber kneader such as an open roll, a Banbury mixer, a closed kneader, and then vulcanizing. The resulting rubber composition has the required performance of the tire such as low fuel consumption, wear resistance, breaking strength and elongation at break. Therefore, the said rubber composition can be used conveniently for each member of a tire.

<Pneumatic tire>
The rubber composition of the present invention can be suitably used for a pneumatic tire. The pneumatic tire is manufactured by a normal method using the rubber composition. That is, a rubber composition containing various additives as necessary is extruded in accordance with the shape of each member of the tire at an unvulcanized stage and molded by a normal method on a tire molding machine. After forming an unvulcanized tire by heating, the tire can be manufactured by heating and pressing in a vulcanizer.

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

界面活性剤（５）：へキサデシルトリメチルアンモニウムブロミド（和光純薬工業（株）製、下記式で表される界面活性剤）

１０％硫酸：和光純薬工業（株）製
シランカップリング剤：ＥＶＯＮＩＫ−ＤＥＧＵＳＳＡ社製のＳｉ６９（ビス（３−トリエトキシシリルプロピル）テトラスルフィド）
亜鉛華：三井金属鉱業（株）製の亜鉛華２種
ステアリン酸：日油（株）製の椿
老化防止剤：大内新興化学工業（株）製のノクラック６Ｃ（Ｎ−（１，３−ジメチルブチル）−Ｎ’−フェニル−ｐ−フェニレンジアミン）
硫黄：鶴見化学工業（株）製の粉末硫黄
加硫促進剤ＮＳ：大内新興化学工業（株）製のノクセラ−ＮＳ（Ｎ−ｔｅｒｔ−ブチル−２−ベンゾチアゾリルスルフェンアミド） Hereinafter, various chemicals used in Examples and Comparative Examples will be described together.
Natural rubber latex: high ammonia type (rubber solid content concentration 60 mass%)
Wet silica: Tokuyama USG manufactured by Tokuyama Corporation (average particle size: 18 nm, N ₂ SA: 170 m ² / g)
Surfactant (1): Huntsman Corp. of _{teric 16A29 (CH 3 (C 2} H 4) 16 (OC 2 H 4) 29 -OH)
Surfactant (2): PD-430 (R- (OC ₄ H ₈ ) _p (OC ₂ H ₄ ) _q —OH: R = long chain alkyl group) manufactured by Kao Corporation
Surfactant (3): manufactured by Kao Corp. of _{Emulgen420 (C 8 H 17 CH =} CHC 8 H 16 (OC 2 H 4) r -OH)
Surfactant (4): Si363 (surfactant represented by the following formula) manufactured by Evonik Degussa

Surfactant (5): Hexadecyltrimethylammonium bromide (surfactant represented by the following formula, manufactured by Wako Pure Chemical Industries, Ltd.)

10% sulfuric acid: silane coupling agent manufactured by Wako Pure Chemical Industries, Ltd .: Si69 (bis (3-triethoxysilylpropyl) tetrasulfide) manufactured by EVONIK-DEGUSSA
Zinc Hana: Mitsui Mining & Mining Co., Ltd. Zinc Hana 2 types Stearic Acid: NOF Co., Ltd. Anti-aging Agent: Nouchi 6C (N- (1,3- Dimethylbutyl) -N′-phenyl-p-phenylenediamine)
Sulfur: Powder sulfur vulcanization accelerator manufactured by Tsurumi Chemical Co., Ltd. NS: Noxera-NS (N-tert-butyl-2-benzothiazolylsulfenamide) manufactured by Ouchi Shinsei Chemical Co., Ltd.

<Examples 1-5>
(Preparation of silica dispersion)
85.5 g of pure water was added to 4.5 g of wet silica to prepare a 5% silica suspension, which was stirred and sonicated for 10 minutes to obtain a silica dispersion.

(Preparation of silica / natural rubber composite)
Silica dispersion was added to 16.7 g of natural rubber latex, 0.45 g of the surfactant shown in Table 1 was further added, and the mixture was mixed and stirred for 1 hour. After stirring, sulfuric acid was added to adjust the pH to 5 to 7 to obtain a coagulated product. The obtained coagulated product was filtered and dried to obtain a silica / natural rubber composite.

<Comparative Example 1>
In Comparative Example 1, a silica / natural rubber composite was obtained in the same manner as in the Examples except that the surfactant was not added.

The following evaluation was performed using the obtained silica-natural rubber composite. The results are shown in Table 1.

(Filtration operation)
Regarding filtration operability in the preparation of the composite, it was evaluated whether aggregates were generated and filtration operation was possible.

(The state of the filtrate (silica / natural rubber loss))
Regarding the loss of silica and natural rubber, the state of the filtrate after filtration was observed and evaluated according to the following criteria.
Transparent or translucent: There is almost no loss.
Cloudiness: There are many losses.

(Sample after drying (natural rubber / silica composite))
The silica dispersibility in the sample after drying was visually evaluated according to the following criteria.
Translucent: Good dispersibility of silica.
Opaque: The dispersibility of silica is poor.

In the examples using the surfactant, aggregates were formed and the filterability was good, whereas in Comparative Example 1, the solid content could not be recovered even after filtration. Moreover, the state of the filtrate of the Example was transparent or translucent, and there was almost no loss of silica and natural rubber, but in the Comparative Example, it became cloudy and had a lot of loss.

About the sample after drying, in Examples 1-4 using a nonionic surfactant, it turned out that a sample is translucent and the silica is disperse | distributing uniformly. In Example 5 using a cationic surfactant, the sample became opaque, and the dispersibility of silica was inferior to those of other Examples.

<Examples 6 to 9, Comparative Example 2>
According to the formulation shown in Table 2, chemicals other than sulfur and a vulcanization accelerator were kneaded using a 1.7 L Banbury mixer. Next, using a roll, sulfur and a vulcanization accelerator were added to the obtained kneaded product and kneaded to obtain an unvulcanized rubber composition. The obtained unvulcanized rubber composition was press vulcanized at 150 ° C. for 30 minutes to obtain a vulcanized product.
The obtained vulcanizates were evaluated as follows, and the results are shown in Table 2.

(Rolling resistance)
Using a viscoelastic spectrometer VES (manufactured by Iwamoto Seisakusho Co., Ltd.), tan δ of each compound (vulcanized product) was measured under the conditions of a temperature of 50 ° C., an initial strain of 10%, a dynamic strain of 2%, and a frequency of 10 Hz. The tan δ of the rubber test piece (reference test piece) of Comparative Example 2 was set to 100, and the index was expressed by the following formula (rolling resistance index). The larger the index, the better the rolling resistance characteristics (low fuel consumption).
(Rolling resistance index) = (tan δ of standard test piece) / (tan δ of each formulation) × 100

(Abrasion test)
Using a Lambourn abrasion tester, the Lambourn abrasion amount was measured under the conditions of a temperature of 20 ° C., a slip ratio of 20% and a test time of 2 minutes. Further, the volume loss amount was calculated from the measured amount of lamborn wear, the wear index of the rubber test piece (reference test piece) of Comparative Example 2 was taken as 100, and the volume loss amount of each formulation was displayed as an index using the following formula. It shows that it is excellent in abrasion resistance, so that an index | exponent is large.
(Abrasion index) = (Volume loss amount of standard specimen) / (Volume loss amount of each compound) × 100

(Breaking strength / elongation at break)
No. 3 dumbbell-shaped rubber test piece was prepared from the vulcanized product, and a tensile test was conducted according to JIS K6251 “Vulcanized rubber and thermoplastic rubber-Determination of tensile properties”. Breaking strength (TB), at break Elongation (EB) was measured. The TB index and EB index of the rubber test piece (reference test piece) of Comparative Example 2 were set to 100, and the TB and EB of each compound were indicated by an index according to the following formula. It shows that it is excellent in reinforcement property, so that TB index is large, and it is excellent in crack resistance, so that EB index is large.
(TB index) = (TB of each formulation) / (TB of reference specimen) × 100
(EB index) = (EB of each formulation) / (EB of reference specimen) × 100

From Table 2, Examples using silica / natural rubber composites obtained from blended latexes prepared by mixing natural rubber latex and fine particle silica dispersion with an average particle size of 1 μm or less in the presence of a surfactant. In comparison with Comparative Example 2, 6 to 9 were obtained in a well-balanced manner with high fuel economy, wear resistance, breaking strength, and elongation at break required for the tire.
In particular, in Example 9 using the silica / natural rubber composite of Example 4 (silica / natural rubber composite obtained using surfactant (4) (Si363)), the rubber composition for tires was produced. There was no need to add a silane coupling agent separately, which was excellent in terms of cost.

Claims (6)

In the presence of a nonionic surfactant having a polyoxyethylene group and a hydrocarbon group, a natural rubber latex and a fine particle silica dispersion having an average particle diameter of 1 μm or less are added to 100 parts by mass (solid content) of natural rubber. A silica-natural rubber composite obtained from a compounded latex prepared by mixing so that the fine particle silica is 5 to 150 parts by mass (in terms of SiO ₂ ). In the presence of a cationic surfactant having a quaternary ammonium group and a hydrocarbon group, a natural rubber latex and a fine particle silica dispersion having an average particle diameter of 1 μm or less are added to 100 parts by mass (solid content) of natural rubber. 5 to 150 parts by mass of fine-particle silica (SiO ₂ A silica-natural rubber composite obtained from a compounded latex prepared by mixing so that In the presence of a nonionic surfactant having a polyoxyethylene group and a hydrocarbon group, a natural rubber latex and a fine particle silica dispersion having an average particle size of 1 μm or less are finely divided with respect to 100 parts by mass (solid content) of natural rubber. Step 1 for preparing a blended latex by mixing so that silica is 5 to 150 parts by mass (in terms of SiO ₂ ), and a step of adjusting the pH of the blended latex obtained in Step 1 to 5 to 7 and coagulating it method for producing a silica and natural rubber composite according to claim 1 Symbol mounting including 2. In the presence of a cationic surfactant having a quaternary ammonium group and a hydrocarbon group, a natural rubber latex and a fine particle silica dispersion having an average particle size of 1 μm or less are added to 100 parts by mass (solid content) of natural rubber. 5 to 150 parts by mass of fine-particle silica (SiO 2 ₂ The silica / silica according to claim 2, comprising: a step 1 for preparing a blended latex by mixing to obtain a conversion); and a step 2 for adjusting and coagulating the pH of the blended latex obtained in the step 1 to 5 to 7. A method for producing a natural rubber composite. Claim 1 or 2 silica and natural rubber rubber composition comprising a complex according. A pneumatic tire produced using the rubber composition according to claim 5 .