Investigating the Crucial Role of 1,2-Dimethyl Imidazole in Developing Ferric-Mediated Recyclable XNBR Composites

To resolve the trade-off between permanent cross-linking and recycling, the integration of dynamic metal-ligand coordination bonds into elastomer systems is a burning topic of current research. This approach could provide recyclable elastomeric materials that are otherwise not possible with conventionally cross-linked elastomers. Therefore, proper utilization of such a dynamic bond could significantly contribute to sustainability as well as promote the principles of a circular economy. In this work, we utilized a heterocyclic imidazole base (1,2-dimethyl imidazole, DMI) to achieve controlled cross-linking of carboxylated nitrile butadiene rubber (XNBR) via ferric-carboxylate interaction. This was investigated and confirmed by X-ray photoelectron and infrared spectroscopy. This is further supported by swelling and rheological studies. The resulting composites show multistep recyclability without any further deterioration of mechanical performance. Even after the third recycling, the DMI containing composites XNBR-DMI₁-Fe_1.5 and XNBR-DMI₄-Fe_1.5 demonstrate recycling efficiencies as high as 84 % and 90 %, respectively. This is well-supported in the creep study, where the deformation recovery for those composites at 130 °C was found to be 53.4 % and 65.2 %, respectively. The development of such an excellent recyclable and ecofriendly elastomer material becomes possible via the dynamic coordination network of the ferric ion complex throughout the XNBR matrix. Furthermore, the formation of clusters, as evidenced by the small-angle X-ray scattering study, is believed to enhance the mechanical properties. A plausible mechanism is proposed that shows the critical role of DMI in the cross-linking process.