Synergistic anti-reversion effects of bis-citraconimides and HTS in natural rubber vulcanizates: A comparative study

This study investigates the synergistic effects of novel citraconimide-based additives, N,N'-p-phenylene-biscitraconimide (BCmB) and bis(2-citraconimide phenyl) disulfide (BCoPD), combined with hexamethylene-1,6-bis(thiosulfate) disodium salt (HTS) in natural rubber (NR)/copper-plated steel wire composites typically utilized in radial tire applications. Both additives were designed and synthesized based on the structural insights from the commercial anti-reversion agent 1,3-bis(citraconimidomethyl)benzene (BCDB). The curing characteristics, crosslinking network structure, mechanical properties, thermal oxidative stability, heat build-up, dynamic mechanical properties, and adhesion strength of NR compounds with these additives were comprehensively analyzed. Quantum chemical calculations and kinetic analyses elucidated the reaction mechanisms and reactivity patterns during vulcanization and over-vulcanization processes. Results indicate that BCmB and BCoPD significantly enhance the overall crosslink density, adhesion strength, and initial mechanical performance compared to BCDB. BCoPD, in particular, provides superior adhesion to copper-plated steel wire through both enhanced crosslinking density and direct interfacial bonding facilitated by disulfide linkages. Although BCDB demonstrated superior long-term anti-reversion performance due to its persistent reactive sites enabling Diels–Alder reactions, BCoPD exhibited the best overall adhesion and aging resistance. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) confirmed cohesive failure within the rubber matrix, indicative of strong interfacial adhesion. This research presents novel citraconimide-based additives as promising materials for achieving improved tire performance, durability, and sustainability.