Studies of Friction Mechanism in Silica and Carbon Black Filled SBR: (3) Friction of Carbon Black and Silica Filled SBR Blends

Tire tread compounds usually consist of polymer blends. In this paper the friction for immiscible SBR blends which consist of two different SBRs having different glass transition temperatures of −51 °C and −6 °C are investigated. The effects of the blend ratio of these two SBRs and the added fillers such as carbon black and silica on the friction are also investigated. The friction forces are measured when rubber specimens are rubbed against dry or wet metal gauze. The friction force-sliding speed curves for the SBR blends are related to their storage modulus E′, loss tangent tan δ, contact area A and shear strength s. The adhesion component of the friction for each SPR blend shows a peak at the sliding speed range examined. As the blend ratio of the SBR having higher Tg of −6 °C is raised, the peak value is lowered and the peak shifts to lower sliding speed. The adhesion component of the friction for silica-filled SBR blends is higher than that of carbon black-filled SBR blends. These trends are also shown when rubber specimens are rubbed against the wet metal gauze. On the other hand, the hysteresis component of friction is raised when the blend ratio of the SBRs having higher Tg is increased.

The Effect of the Environment on the Friction and Wear of Blended Rubber

The friction and wear of rubber are properties that are affected by the environment, especially by the oxygen in the air. Natural rubber (NR) is most sensitive to air, butadiene rubber (BR) is scarcely affected by the presence of air, while styrene butadiene rubber (SBR) is intermediate compared to NR and BR. Both the abrasive and fatigue wear rates of NR and BR blends are known to decrease by increasing the BR content. To reduce the wear rates of NR vulcanizates in the air, SBR was blended with NR. In severe rubbing experiments against abrasive cloths, the wear rate of the NR-SBR blends slightly decreased by increasing the SBR content. The NR-SBR blends showed considerable reduction in wear under fatigue wear conditions against metal gauze. Similar experiments were also conducted for SBR-BR blends. Blending with BR showed a substantial reduction in wear, especially in fatigue wear. When NR-BR, NR-SBR, and SBR-BR blends were rubbed against metal gauze in vacuum, the friction and wear of these three blended rubbers was lower in vacuum than in room air. It is concluded that the wear resistance of the blended rubber is affected by its sensitivity to air as well as by the mechanochemical properties of the blends under fatigue wear conditions.