Fundamental understanding of phase incompatibility in HSBR / NR blends and optimization of blend ratio

Designing hydrogenated styrene–butadiene rubber (HSBR)/natural rubber (NR) elastomer blends is intrinsically challenging because HSBR and NR differ markedly in molecular structure, polarity and cure kinetics, which often leads to phase incompatibility. Nevertheless, if such blends can successfully be developed, synergistic improvement in properties and performances may be achieved, which are highly desirable for high‐end applications, such as tyre tread materials. In this work, an attempt has been made to develop silica‐filled HSBR/NR blends with systematically varying compositions and to interpret phase incompatibility by corelating phase microstructures with mechanical, dynamic mechanical and fatigue properties. The compound solely based on HSBR exhibited superior overall performance. However, the introduction of NR resulted in faster curing. Even a small replacement of HSBR with NR (10 phr) caused a significant reduction in crosslink density, tensile strength and storage modulus, accompanied by an increase in abrasion loss. These results indicate the inherent incompatibility between the HSBR and NR phases. The origin of such incompatibility was critically investigated by studying the phase morphology of the developed blends. Transmission electron microscopy (TEM) analysis revealed that the selective migration of silica towards the HSBR phase as well as sulfur towards the NR phase led to cure mismatch and consequent deterioration in mechanical and abrasion resistance properties. The difference in cure kinetics also attributed to phase incompatibility. Among the investigated formulations, the 80 phr HSBR/20 phr NR (H 80 N 20 ) blend showed an optimal balance of processing characteristics and performance properties, indicating that, upon appropriate compatibilization, it could have strong potential for tyre tread application.