Properties and Mechanism of Silica-Reinforced Natural Organic Acid Green Cross-Linked Epoxidized Styrene Butadiene Rubber Composites Based on Multiple Cross-Linked Structure

Traditional rubber is mainly vulcanized with sulfur; however, harmful gases may be released during the vulcanization process, leading to considerable environmental pollution, which is inconsistent with green development. In this study, natural organic acids, tartaric acid (TA) or citric acid (CA), are used to cross-link epoxidized emulsion styrene butadiene rubber (EESBR). The EESBR and EBR with different epoxidation degrees are prepared through an epoxidation reaction with hydrogen peroxide and formic acid, then the influence of various cross-linking agents, tartaric acid (TA) or citric acid (CA), on the vulcanization characteristics and mechanical properties is investigated. The CA cross-linked EESBR under different epoxidation degrees shows better mechanical properties than that of TA. The cross-linking mechanism study indicated that the cross-linking bonds of the vulcanized EESBR include β-hydroxy ester and ether bonds. When the added amount of silica was 25 phr, the M_H of CA/EESBR/SiO₂ composites reached 5.95 dN·m with optimum curing time (t₉₀) of 35.37 min, cross-link density (V_e) of composites reached 15.77 × 10^–4 mol/cm³, tensile strength reached 7.39 MPa, and a good balance between the wet traction and low rolling resistance of the material can be realized. Meanwhile, SEM analysis results showed good compatibility between silica and rubber matrix.