RUBBER COMPOSITION FOR SIDE TREAD

To provide a rubber composition for a side tread which satisfactorily maintains hardness and tensile strength and improves low heat generation property (tanδ(60°C)) while suppressing lowering of breaking elongation with time of the rubber composition stored in an unvulcanized state.SOLUTION: Silica having a CTAB adsorption specific surface area of 60 m2/g to 100 m2/g, carbon black having a CTAB adsorption specific surface area of 30 m2/g to 100 m2/g, a sulfur-containing silane coupling agent and a guanidine-based vulcanization accelerator are blended with a diene-based rubber containing 35 mass% to 55 mass% of an isoprene-based rubber and 45 mass% to 65 mass% of a butadiene rubber, a blending amount of the silica with respect to 100 pts.mass of the diene-based rubber is set to be 5 pts.mass or more and 50 pts.mass or less, a total of blending amounts of the silica and the carbon black with respect to 100 pts.mass of the diene-based rubber is set to be less than 55 pts.mass, and a ratio Ma/Ms of a blending amount of the guanidine-based vulcanization accelerator Ma to a blending amount Ms of the silica is set to be 0.02 to 0.08.SELECTED DRAWING: Figure 1 【課題】未加硫状態で保管されたゴム組成物の経時的な破断伸びの低下を抑制しながら、硬度および引張強度を良好に維持し、且つ、低発熱性（ｔａｎδ（６０℃））を改善したサイドトレッド用ゴム組成物を提供する。【解決手段】イソプレン系ゴム３５質量％～５５質量％とブタジエンゴム４５質量％～６５質量％とを含むジエン系ゴムに対して、ＣＴＡＢ吸着比表面積が６０ｍ2／ｇ～１００ｍ2／ｇであるシリカ、ＣＴＡＢ吸着比表面積が３０ｍ2／ｇ～１００ｍ2／ｇであるカーボンブラック、硫黄含有シランカップリング剤、およびグアニジン系加硫促進剤を配合し、ジエン系ゴム１００質量部に対するシリカの配合量を５質量部以上５０質量部以下、ジエン系ゴム１００質量部に対するシリカおよびカーボンブラックの配合量の合計を５５質量部未満、シリカの配合量Ｍｓに対する前記グアニジン系加硫促進剤Ｍａの配合量の比Ｍａ／Ｍｓを０．０２～０．０８に設定する。【選択図】図１