Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-03-09
2001-11-06
Sanders, Kriellion A. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S424000, C524S505000, C524S526000
Reexamination Certificate
active
06313213
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rubber composition for a tire tread. More specifically, it relates to a rubber composition for a tire tread obtained by blending an A-B type block copolymer composed of (A) a poly(styrene-butadiene) block or a polybutadiene block, and (B) a polyisoprene block, as a compatibilizing agent, into a rubber composed of natural rubber (NR) and/or polyisoprene rubber (IR) and a high cis and high linearity polybutadiene rubber (BR) so as to improve the abrasion resistance and chipping resistance thereof, without impairing the processability thereof.
2. Description of the Related Art
In the cap treads of large sized tires for trucks, buses, etc., it has been known in the past to blend in butadiene so as to improve the abrasion resistance. Further, for the butadiene rubber, it has been known that blending of butadiene rubber having a large cis component or butadiene rubber having a large linearity (or melt viscosity) has a large effect in the improvement of the abrasion resistance. However, if a plurality of polymers are blended and used as tire tread rubber compositions, etc., a problem of phase separated interfaces arises when the polymers are incompatible to each other. In most cases, this has a detrimental effect on the tensile strength, tear strength, abrasion resistance, etc. With respect to the problem of decrease in the breaking characteristics due to the above-mentioned incompatibility of the polymer blend, the incorporation of a small amount of a block copolymer composed of polybutadiene and polyisoprene into a blend of natural rubber/polybutadiene rubber is described in
J. Apply. Polym. Sci.,
49 (1983) and
RCT.
66 (1993). However, these references only consider the above specific block copolymers in a limited fashion and do not describe any common studies or opinions as to the industrially important polymers containing SBR blocks having a large degree of freedom.
The present inventors previously provided a rubber composition for a tire tread having improved abrasion resistance, heat buildup, and wet grip by blending a high cis polybutadiene rubber and medium vinyl polybutadiene rubber into natural rubber and/or polyisoprene (Japanese Unexamined Patent Publication (Kokai) No. 1-90232) and also provided a rubber composition having improved breaking characteristics by incorporating not more than 20 parts by weight of a specific A-B block copolymer, as a compatibilizing agent, into a blend of natural rubber and/or polyisoprene and high cis polybutadiene rubber (Japanese Unexamined Patent Publication (Kokai) No. 8-193145). However, even in these rubber compositions for treads, there are still problems in the processability at the time of mixing such as kneading performance of mixing and the chipping resistance was not satisfactory either.
SUMMARY OF THE INVENTION
Accordingly, the objects of the present invention are to eliminate the above-mentioned problems in the prior art and to provide a rubber composition for a tire tread having improved processability, and also improved abrasion resistance and chipping resistance.
In accordance with the present invention, there is provided a rubber composition for a tire tread comprising:
(1) (i) 60 to 90 parts by weight of at least one rubber selected from the group consisting of natural rubber and polyisoprene rubber, (ii) 10 to 35 parts by weight of a high cis polybutadiene rubber having a melt viscosity of 90 to 150 cps and a cis content of at least 90% by weight, and (iii) 0.5 to 5 parts by weight of an A-B type block copolymer composed of (A) a poly(styrene-butadiene) block or polybutadiene block having a styrene content (St) of 0 to 35% by weight and a butadiene content (Bd) of 65 to 100% by weight, having a 1,2-vinyl bond content (Vn) of 5 to 80 mol %, and satisfying the relationship of Vn≦2St+30 and (B) a polyisoprene block having a cis content of at least 70% by weight, in a ratio by weight (A)/(B) of 20 to 80/80 to 20, and
(2) 40 to 60 parts by weight, based upon 100 parts by weight of the total rubber component, of carbon black having a nitrogen specific surface area (N
2
SA) of at least 125 m
2
/g.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the present invention, 10 to 35 parts by weight of the above specific polybutadiene (BR) is blended into natural rubber (NR) and/or polyisoprene rubber (IR) so as to intend to improve the abrasion resistance and a high cis high linearity polybutadiene (BR) is selected and blended to further improve the abrasion resistance and improve the extrudability. Further, 0.5 to 5.0 parts by weight of the above specific A-B block copolymer is blended to improve the mixability (or kneading performance) by fine dispersion of the above specific polybutadiene (BR) phase and improve the chipping resistance.
The NR, IR, and high cis high linearity BR used in the rubber composition according to the present invention have to be blended in amounts suited for use for the rubber composition for a tire tread of the present invention. The NR and/or IR can be blended in an amount of 60 to 90 parts by weight, preferably 70 to 80 parts by weight, based upon the total weight of the rubber content, and the ratio of NR/IR can be made to 100/0 to 50/50 and the high cis high linearity BR can be blended in an amount of 10 to 35 parts by weight, preferably 15 to 30 parts by weight, based upon the total weight of the rubber content. Note that the melt viscosity of the high cis high linearity BR used in the present invention is at least 90 cps, preferably 105 to 125 cps, and the cis content is 90 to 100% by weight, preferably 95 to 100% by weight. If the melt viscosity is less than 90 cps, the abrasion resistance and the extrudability are decreased, and therefore, this is not preferred. Further, if the cis content is less than 90% by weight, the desired abrasion resistance is not obtained, and therefore, this is also not preferred.
Next, according to the present invention, when a predetermined amount of a specific A-B type block copolymer is incorporated into the above-mentioned blend, it is possible to achieve the object of improvements in the mixability (or kneading performance) and chipping resistance. If the amount of the A-B type block copolymer is too small, the desired effect of improvement cannot be obtained, while conversely if too large, the viscoelastic properties of the block copolymer per se are manifested, and therefore, the viscoelastic properties such as the abrasion resistance, extrudability, naturally expected from an NR and/or IR/high cis high linearity BR blend are lost, and therefore, this is not preferred. From this viewpoint, the A-B type block copolymer used in the present invention comprises 0.5 to 5 parts by weight, preferably 1 to 3 parts by weight, more preferably 1 to 2.5 parts by weight, of the A-B type block copolymer, based upon 100 parts by weight of the total polymer component including A-B type block copolymer.
The A-B type block copolymer used in the present invention is composed of (A) a styrene (St) and butadiene (Bd) copolymer block or a polybutadiene (Bd) polymer and (B) a polyisoprene block. The styrene content (St), 1,2-vinyl bond content (Vn), and cis isoprene content of the block components (A) and (B) and the weight ratio (A)/(B) of the (A) component and (B) component are as follows:
Block (A)
Poly(styrene-butadiene) or polybutadiene having a styrene content of 0 to 35% by weight, preferably 1 to 35% by weight, more preferably 5 to 30% by weight, and the balance of butadiene content, a 1,2-vinyl bond content (Vn) of the butadiene portion of 5 to 80 mol %, preferably 10 to 70 mol %, and Vn≦2St+30, wherein St is a styrene content.
Block (B)
Polyisoprene having a cis content of at least 70% by weight, preferably at least 75% by weight
(A)/(B)=20 to 80/80 to 20 (parts by weight), preferably 30 to 70/70 to 30 (parts by weight)
If the styrene content of the block component (A) is too large, the component becomes incompatible with the high cis and high linearity BR,
Kawazura Tetsuji
Kikuchi Yasushi
Nakamura Toru
Arent Fox Kintner & Plotkin & Kahn, PLLC.
Sanders Kriellion A.
The Yokohama Rubber Co. Ltd.
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