CARBON BLACK FILLED RUBBER COMPOSITIONS

Abstract: A carbon-containing rubber composition containing a rubber composition containing carbon black, obtained by coagulating, dehydrating, and drying a rubber latex mixture containing a latex of at least one starting rubber A having an average glass transition temperature TgA of -120°C to -15°C, carbon black, and part of a softening agent, mixed in an internal mixer with a starting rubber B and part of the softening agent, with a ratio F A /F B of the concentration F A of carbon black in the copolymer in the rubber latex mixture with the concentration F B of carbon black in the copolymer after kneading by the hermetic mixture being 1.2 to 3.0 and a rubber composition containing carbon black comprising a starting rubber A comprising a polymer having functional groups represented by formula (I) C = N + bonded with a polymer chain directly or through another atomic group or a terminal modified diene-based polymer first brought into contact with carbon black, then blended with a starting rubber B comprising a diene based stock rubber.

Description: TECHNICAL FIELD The present invention relates to a rubber composition containing carbon black, more specifically relates to a carbon black-containing rubber composition suitable for use for the tread of a pneumatic tire. BACKGROUND ART Proposals have been made to improve the balance of the tanδ of tire tread rubber for reducing the fuel consumption of automobiles. More specifically, combinations of ingredients, separate mixing, and use of terminal modified rubber have been proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 59-159839 describes a rubber composition obtained by wet mixing SBR and BR with different styrene contents, while Japanese Unexamined Patent Publication (Kokai) No. 2-129241 describes a process of production of a rubber composition comprising mixing a terminal modified/coupled rubber in a solution with carbon black, then mixing in a diene based rubber. On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 58-152031 discloses a master batch comprising SBR and BR, while Japanese Unexamined Patent Publication (Kokai) No. 57-00430 proposes separately mixing a high molecular weight rubber and low molecular weight rubber to produce a rubber composition. Further, Japanese Examined Patent Publication (Kokoku) No. 5-1298 describes a rubber composition which improves the impact resilience and tensile strength of a vulcanite comprising a conjugated diene-based polymer having an aromatic tertiary amino group at a terminal portion of the conjugated diene-based polymer. On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 55-10434 describes, when producing a rubber composition for a tire tread by blending carbon black into a starting rubber comprising an amorphous 1,2-polybutadiene rubber and natural rubber and/or polyisoprene rubber (further possibly including a partially conjugated diene-based rubber), first mixing at least 25% by weight of the starting rubber component containing 1,2-polybutadiene and carbon black in a specific ratio, then adding and mixing in the remaining starting rubber to improve the fuel economy and safety of automobiles. Further, Japanese Unexamined Patent Publication (Kokai) No. 2-129241 proposes blending a terminal modified conjugated diene-based polymer with carbon black in an organic solvent to improve the tensile strength and abrasion resistance of the vulcanite. Further, Japanese Unexamined Patent Publication (Kokai) No. 8-269243 discloses blending a high glass transition point SBR and a low glass transition point SBR master batch carbon black-containing. However, the above separate mixing method of ingredients, use of terminal modified rubber, and other arts have problems, for example, the low effect in systems containing high amounts of oil and carbon black and improvements are still sought. Further, in the case of separate mixing, the concentration of the carbon black at the time of the initial mixing is increased and deterioration of the processability occurs such as scorching at the time of mixing. The amount of blending of the carbon black which can be separately mixed is also limited. Further, with use in generally used master batches, the desired effects are not obtained, therefore it becomes essential to modify the master batches. EP-0 051 450 discloses a process for preparing a composition comprising carbon black and two or more curable polymers prepared by (i) forming a mixture of at least 60 % of carbon black with 33-60 % of one or more curable polymers and (ii) adding the remainder of the curable polymers and carbon black. The vulcanizates prepared according to the cited process have lower heat build-up, lower power loss and associated reduced rolling resistance, lower elongation at break, lower hardness, improved skid resistance, higher rebound and higher resistance to flex crack growth. DE-A-4034064 discloses a mixing method for a tire tread rubber comprising natural rubber (NR) and styrene-butadiene rubber (SBR), wherein the carbon black is mixed with SBR in the form of a latex and after coagulation the resultant rubber composition is mixed with natural rubber. DISCLOSURE OF THE INVENTION Accordingly, an object of the present invention is to provide a rubber composition which can be suitably used as a tire tread superior in tanδ balance of the vulcanized rubber while maintaining the abrasion resistance. In accordance with the present invention, there is provided a rubber composition comprising: (i) a carbon black-containing rubber composition, which is obtainable by coagulating, dehydrating and drying a rubber latex mixture containing 50 to 90 parts by weight, in terms of solid components, of latex of at least one starting rubber A having an average glass transition temperature TgA of -120°C to -15°C, 40 to 100 parts by weight of carbon black, and optionally 0 to 70 parts by weight of a softening agent and (ii) (a) a starting rubber B selected from the group consisting of polybutadiene, styrene-butadiene copolymer and styrene-isoprene-butadiene copolymer and having an average glass transition temperature TgB of the starting rubber B, which fulfills the relationship of TgA + 10(°C) ≤ TgB in an amount such that the total amount of the starting rubbers A and B in the rubber composition is 100 parts by weight and optionally (b) a softening agent in an amount such that the total amount of the softening agent in the rubber composition is 0 to 80 parts by weight, the component (i) being mixed with the component (ii) in an internal mixer and the ratio F A /F B being 1.2 to 3.0, wherein: F A is the weight ratio of the carbon black to the starting rubber A in the component (i) and F B is the weight ratio of the carbon black to the total amount of the starting rubbers A and B in the rubber composition. BEST MODE FOR CARRYING OUT THE INVENTION The constitution, action, and effects of the first embodiment of the present invention will be explained in detail below. The present inventors found that tanδ of a rubber composition has almost no relation with the rubber absorption phase near the carbon black or other filler and is due to only the matrix phase and succeeded in improving the balance of tanδ by separating as much as possible the carbon black surface and matrix phase. Further, the present inventors found that by evenly distributing carbon black in high Tg and low Tg starting rubber so as to cause it to concentrate in the low Tg starting rubber, the temperature gradient of the tanδ at 60°C to 0°C became better and that it was possible to use this to improve the balance of tanδ. Therefore, in the present invention, from the viewpoint of the interaction of the carbon black and rubber, the rubber matrix phase far from the carbon in the compound is made to be not restrained by the carbon so as to control the tanδ. Specifically, by mixing a predetermined master batch comprising a latex of a starting rubber of a group A with an average glass transition temperature TgA of -120°C to -15°C, preferably -120°C to -20°C, and carbon black and a starting rubber group B of an average glass transition temperature TgB of at least TgA+10°C, optionally a softening agent, and other rubber additives in an internal mixer, it is possible to obtain the desired rubber composition. As the carbon black used in the present invention, in the case of a tire tread, one with a nitrogen specific area (N 2 SA) (measured based on ASTM D 3037) of preferably 50 to 200 m 2 /g, more preferably 80 to 160 m 2 /g, and a DBP oil absorption (measured based on JIS K 6221) of preferably 60 to 140 ml/100 g, more preferably 100 to 130 ml/100 g is used. Note that the carbon black used may be one which is given an organic or inorganic surface treatment or has a small amount of silica or other metallic oxide deposited on its surface to increase the bonding with rubber. The amount of blending of the carbon black blended in the rubber composition of the present invention, that is, the rubber composition containing carbon black, is 40 to 100 parts by weight, based upon 100 parts by weight of the final rubber polymer, preferably 50 to 100 parts by weight. If the amount of the carbon black blended is too small, the reinforcing effect falls and the desired physical properties are not obtained, while if too great the production of the carbon black master batch becomes difficult. The carbon black master batch according to the present invention, that is, the rubber composition containing carbon black, is not particularly limited so long as the glass transition temperature TgA is satisfied, but a conjugated diene-based polymer is particularly preferable. As such a conjugated diene-based polymer, butadiene, isoprene, pentadiene, chloroprene, and other conjugated dienes alone or in polymers or copolymers of two or more, copolymers of at least one conjugated diene and at least one monomer copolymerizable with the same, for example, an aromatic vinyl compound such as styrene, vinyl toluene, and α-methyl styrene, an unsaturated nitrile compound such as acrylonitrile and methacrylonitrile, unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, and anhydrous maleic acid, unsaturated carboxylic acid esters such as methyl acrylate, ethyl acrylate, butyl acrylate, and methoxyethyl acrylate, etc. may be mentioned. A particularly preferable polymer is a block copolymer having a polymer block comprised mainly of a conjugated diene and a polymer block comprised mainly of an aromatic vinyl compound. The boundary between the blocks in the block copolymer may be clear or not clear. Further, the ratios of the blocks is 0 to 100% by weight of conjugated diene and 0 to 100% by weight of aromatic vinyl diene, preferably 20 to 80% by weight of conjugated diene and 20 to 80% by weight of aromatic vinyl diene. The amounts of the rubber polymers blended differ according to the type of the starting rubber to be added, the contents in the polymer, molecular weight of the polymer, etc., but is 50 to 90 parts by weight, preferably 50 to 70 parts by weight per 100 parts by weight of the rubber copolymer. If the amount of the rubber polymer is too small, it is difficult to produce the carbon master batch, while if too large, the desired effects cannot be obtained. The method of blending the latex containing carbon black containing the starting rubber A is not particularly limited, but in general it is sufficient to mix and stir the rubber latex in which carbon black and the softening agent have been uniformly dispersed, then follow a usual method to coagulate, dehydrate, and dry the same. The softening agent used here may be an aromatic process oil, a paraffin based oil, or other oil generally used for rubber formulations blended in an amount of not more than 70 parts by weight, preferably 0 to 55 parts by weight. On the other hand, as the stock rubber B, there is no particular problem so long as the above glass transition temperature is satisfied, but preferably an emulsion polymerized or solution polymerized diene-based starting rubber (polybutadiene, styrene-butadiene copolymer, or styrene-isoprene-butadiene copolymer, ), etc. may be mentioned. The amounts of blending of these stock rubbers B are amounts giving 100 parts by weight of the rubber as a whole, that is, 50 to 10 parts by weight. If necessary, it may be mixed with the above rubber composition containing carbon black toqether with an additional softening agent or other general use rubber additives in a Banbury mixer or other hermetic mixer to obtain the desired rubber composition. According to the present invention, preferably the average weight-average molecular weight Mw(A) of the stock rubber A is 100,000 to 1,200,000, the average weight average molecular weight Mw(B) of the starting rubber B is at least 200,000, and the relationship of 0.1 ≤ Mw(A)/Mw(B) ≤ 6.0 stands. According to the present invention, the ratio F A /F B of the concentration F A of carbon black in the copolymer in the rubber latex mixture with the concentration F B of carbon black in the copolymer after kneading by the hermetic mixture is 1.2 to 3.0. According to the present invention, the tanδ (0°C) of the rubber composition used for a cap tread is at least 0.4 and the tanδ (60°C) is at least 0.1. INDUSTRIAL APPLICABILITY As explained above, according to the present invention, it is possible to improve the scorching at the time of mixture and further to improve the balance of the tanδ by using a master batch. It is possible to obtain the effect of improvement equal to or better than that of separate mixture no matter what the concentration of the carbon without having to consider scorching at the time of mixing due to the rise in the concentration of the carbon in conventional separate mixing and it is possible to use the invention for any formulation. EXAMPLES The present invention will now be explained in further detail by Examples. Examples I-1 to I-8, Standard Examples I-1 to I-8, and Comparative Examples I-1 to I-8 The rubber compositions of the formulations shown in Tables I-1 to I-4 were prepared and evaluated for physical properties. The ingredients used for the formulations of the Standard Examples, Examples, and Comparative Examples were as follows: Master Batches I-1 to I-3 Starting rubber latex: a styrene-butadiene copolymer rubber latex having a styrene content of 25%, a vinyl content of 16%, a glass transition temperature of -51°C, and a weight average molecular weight of 620,000. Carbon black: ISAF grade carbon black having an N 2 SA (m 2 /g) of 112 and a DBP oil absorption (ml/100 g) of 112. Softening agent: aromatic process oil Starting rubber latex (parts by weight) Carbon black (parts by weight) Softening agent (parts by weight) Master batch I-1 50 75 50 Master batch I-2 50 60 35 Master batch I-3 50 50 25 Master Batches I-4 to I-5 Starting rubber latex: a styrene-butadiene copolymer rubber latex having a styrene content of 25%, a vinyl content of 16%, a glass transition temperature of -51°C, and a weight average molecular weight of 620,000. Carbon black: HAF grade carbon black having an N 2 SA (m 2 /g) of 92 and a DBP oil absorption (ml/100 g) of 117. Softening agent: aromatic process oil Starting rubber latex (parts by weight) Carbon black (parts by weight) Softening agent (parts by weight) Master batch I-4 70 85 50 Master batch I-5 70 65 30 Master Batches I-6 to I-7 Starting rubber latex: a polyisoprene rubber latex having a glass transition temperature of -63°C and a weight average molecular weight of 1,200,000 and a cis-polybutadiene rubber latex having a glass transition temperature of -106°C and a weight average molecular weight of 650,000 Carbon black: HAF grade carbon black having an N 2 SA (m 2 /g) of 84 and a DBP oil absorption (ml/100 g) of 130. Softening agent: aromatic process oil Starting rubber latex (parts by weight) Carbon black (parts by weight) Softening agent (parts by weight) Master batch I-6 50 50 10 Master batch I-7 Isoprene: 50 50 10 Butadiene: 20 Master Batch I-8 Starting rubber latex: a styrene-butadiene copolymer rubber latex having a styrene content of 36%, a vinyl content of 16%, a glass transition temperature of -36°C, and a weight average molecular weight of 820,000. Carbon black: HAF grade carbon black having an N 2 SA (m 2 /g) of 153 and a DBP oil absorption (ml/100 g) of 127. Softening agent: aromatic process oil Starting rubber latex (parts by weight) Carbon black (parts by weight) Softening agent (parts by weight) Master batch I-8 60 95 52 Master Batch I-9 Starting rubber latex: a styrene-butadiene copolymer rubber latex having a styrene content of 25%, a vinyl content of 16%, a glass transition temperature of -51°C, and a weight average molecular weight of 620,000. Carbon black: ISAF grade carbon black having an N 2 SA (m 2 /g) of 112 and a DBP oil absorption (ml/100 g) of 112. Softening agent: aromatic process oil Starting rubber latex (parts by weight) Carbon black (parts by weight) Softening agent (parts by weight) Master batch I-9 100 60 34 (Note: The amounts of starting rubber latex are amounts converted into solid contents.) Other Ingredients SBR I-1: a 50.0 phr oil extended emulsion polymerized styrene-butadiene copolymer rubber having a styrene content of 36%, a vinyl content of 16%, a glass transition temperature of -36°C, and a weight average molecular weight of 720,000. SBR I-2: a 37.5 phr oil extended emulsion polymerized styrene-butadiene copolymer rubber having a styrene content of 25%, a vinyl content of 16%, a glass transition temperature of -51°C, and a weight average molecular weight of 620,000. SBR I-3: a 50.0 phr oil extended emulsion polymerized styrene-butadiene copolymer rubber having a styrene content of 35%, a vinyl content of 14%, a glass transition temperature of -36°C, and a weight average molecular weight of 820,000. SBR I-4: a 20.0 phr oil extended emulsion polymerized styrene-butadiene copolymer rubber having a styrene content of 48%, a vinyl content of 13%, a glass transition temperature of -21°C, and a weight average molecular weight of 1,280,000. IRI-1: isoprene having a glass transition temperature of -63°C and a weight average molecular weight of 870,000. cis-BRI-1: cis type butadiene copolymer rubber having a glass transition temperature of -106°C and a weight average molecular weight of 650,000. Carbon black I-1: ISAF grade carbon black having an N 2 SA (m 2 /g) of 112 and a DBP oil absorption (ml/100 g) of 112. Carbon black I-2: HAF grade carbon black having an N 2 SA (m 2 /g) of 92 and a DBP oil absorption (ml/100 g) of 117. Carbon black I-3: HAF grade carbon black having an N 2 SA (m 2 /g) of 84 and a DBP oil absorption (ml/100 g) of 130. Carbon black I-4: SAF grade carbon black having an N 2 SA (m 2 /g) of 153 and a DBP oil absorption (ml/100 g) of 127. Zinc oxide: Zinc White No. 3 Stearic acid: Industrial grade stearic acid Antioxidant 6C: N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine Softening agent: aromatic process oil Wax: Industrial grade paraffin wax Powdered sulfur: 5% oil treated powdered sulfur Vulcanization accelerator NS: N-tert-butyl-2-benzothiazoyl sulfenamide Vulcanization accelerator CZ: N-cyclohexyl-2-benzothiazyl sulfenamide Vulcanization accelerator DPG: Diphenyl guanidine Preparation of Sample As a first step, the ingredients shown in Table I-1 were mixed in a 1.8 liter hermetic mixer for 3 to 5 minutes and the mixture discharged after reaching 165±5°C. Next, as the final step, a vulcanization accelerator and sulfur were mixed by an 8-inch open roll to obtain a rubber composition. In the case of two-step mixing as a Comparative Example, the first step was to mix the ingredients in a 1.8 liter internal mixer for 3 to 4 minutes and discharge the mixture after reaching 150±5°C, the second step was to mix the mixture with the remaining ingredients in the 1.8 liter internal mixer for 3 to 5 minutes and discharge it after reaching 165±5°C, then the final step was to mix the vulcanization accelerator and sulfur by an open roll to obtain a rubber composition. In the case of one-step mixing, the ingredients other than the vulcanization accelerator and sulfur were mixed in a 1.8 liter internal mixer for 3 to 4 minutes and discharged after reaching 165±5°C. The vulcanization accelerator and sulfur were then mixed in this by an 8-inch open roll to obtain the rubber composition. The sample composition was vulcanized by pressing in a 15 x 15 x 0.2 cm mold at 160°C for 20 minutes to prepare the desired test piece which was then evaluated as to its vulcanized properties. The test methods of the vulcanized properties of the compositions obtained in the Examples were as follows: 1) 300% deformation stress and elongation at break: measured according to JIS K 6251 (dumbbell no. 3 shape) 2) tanδ: measured by Rheograph Solid viscoelasticity apparatus made by Toyo Seiki Seisakusho at 20 Hz, initial elongation of 10%, and dynamic stress of 2% (sample width of 5 mm, measured at temperatures of 0°C and 60°C) 3) Abrasion resistance: Measured by Lanborne abrasion tester with abrasion loss indicated as index by following method: Abrasion resistance (index) = ((loss at reference test piece)/(loss at test piece)) x 100 The reference test pieces were Standard Example I-1 in Table I-1 to Table I-2, Standard Example I-6 in Table I-3, and Standard Example I-8 in Table I-4. Standard Ex. I-8 Ex. I-8 Comp. Ex. I-7 (First step) SBR I-3 90.00 - 90.00 SBR I-4 48.00 48.00 8.00 Master batch I-8 - 195.00 - Carbon black I-4 95.00 - 95.00 Zinc oxide 3.00 3.00 3.00 Stearic acid 2.00 2.00 2.00 Antioxidant 6C 3.00 3.00 3.00 Softening agent 22.00 - 22.00 (Second step) SBR I-4 - - 40.00 (Final step) Powdered sulfur 1.90 1.90 1.90 Vulcanization accelerator CZ 1.90 1.90 1.90 Vulcanization accelerator DPG 0.10 0.10 0.10 - Vulcanized properties - 300% deformation stress(MPa) 9.3 11.9 10.8 Breakage strength(MPa) 20.1 19.9 19.1 tanδ (0°C) 0.811 0.987 0.962 tanδ (60°C) 0.512 0.442 0.467 tanδ gradient (0°C/60°C) 1.58 2.23 2.06 Abrasion resistance index 100 115 105 F A /F B - 1.67 1.5 F A - 1.58 1.43 F B 0.25 0.95 0.95 Mw(A)/Mw(B) 0.64 0.64 0.68

Claims: A rubber composition comprising: (i) a carbon black-containing rubber composition, which is obtainable by coagulating, dehydrating and drying a rubber latex mixture containing 50 to 90 parts by weight, in terms of solid components, of latex of at least one starting rubber A having an average glass transition temperature TgA of -120°C to -15°C, 40 to 100 parts by weight of carbon black, and optionally 0 to 70 parts by weight of a softening agent and (ii) (a) a starting rubber B selected from the group consisting of polybutadiene, styrene-butadiene copolymer and styrene-isoprene-butadiene copolymer and having an average glass transition temperature TgB of the starting rubber B, which fulfills the relationship of TgA + 10(°C) ≤ TgB in an amount such that the total amount of the starting rubbers A and B in the rubber composition is 100 parts by weight and optionally (b) a softening agent in an amount such that the total amount of the softening agent in the rubber composition is 0 to 80 parts by weight, the component (i) being mixed with the component (ii) in an internal mixer and the ratio F A /F B being 1.2 to 3.0, wherein: F A is the weight ratio of the carbon black to the starting rubber A in the component (i) and F B is the weight ratio of the carbon black to the total amount of the starting rubbers A and B in the rubber composition. A rubber composition as claimed in claim 1, wherein the weight average molecular weight Mw(A) of the starting rubber A is 100,000 to 1,200,000, the weight average molecular weight Mw(B) of the starting rubber B is at least 200,000, and 0.1 ≤ Mw(A)/Mw(B) ≤ 6.0. A rubber composition as claimed in claim 1 or 2, wherein the nitrogen specific surface area (N 2 SA) of the carbon black is 50 to 200 m 2 /g and the DBP oil absorption is 60 to 140 ml/100g. A rubber composition as claimed in any one of claims 1 to 3, wherein the starting rubber B is a styrene-butadiene copolymer rubber (SBR) obtainable by solution polymerization and/or emulsion polymerization. A pneumatic tire comprising a cap tread portion using the rubber composition according to any one of claims 1 to 4. A pneumatic tire as claimed in claim 5, wherein the tan δ (0°C) of the rubber composition is 0.4 more and the tan δ (60°C) is 0.1 or more.

Inventors: KAIDO HIROYUKI, KAWAZURA TETSUJI, YATSUYANAGI FUMITO, KAIDO, HIROYUKI, KAWAZURA, TETSUJI, YATSUYANAGI, FUMITO
Applicants: YOKOHAMA RUBBER CO LTD, THE YOKOHAMA RUBBER CO., LTD.
Application year: 1997
Main classification: C08L21
IPC codes: C08K3, B60C1