Ionic adaptive network: A sustainable route to replace synthetic rubbers with natural polymers for high-temperature applications

Natural rubber (NR) is a biopolymer consisting of cis-1,4-isoprene units extracted from the sap of rubber trees, mainly Hevea Brasiliensis. This rubber is widely used in the automotive and other industries due to its performance and elasticity. However, synthetic rubber has largely replaced natural rubber in many applications because of the poor heat resistance of natural rubber. On the other hand, non-recyclable sulfur-based synthetic rubber composites pose a major environmental issue from the viewpoint of sustainability. In this report, a flexible (non-directional) crosslinking network based on ionic bonds in modified natural rubber (epoxy-modified NR) using dicarboxylic acid and dimethylimidazole (DMI) is presented, which eliminates the disadvantages of natural rubber and outperforms synthetic rubber without compromising its mechanical performance. Accelerated aging, temperature scanning stress relaxation, compression set, and temperature-dependent FT-IR analyses confirm the high thermal stability of ionically crosslinked natural elastomer. The ionic crosslinked rubber shows a significant improvement in initial degradation temperature (196 °C) compared to thermally stable synthetic elastomers, such as NBR (acrylonitrile butadiene rubber), CR (polychloroprene rubber), and peroxide-cured EPDM (ethylene propylene diene monomer). Unlike sulfur-cured elastomers, the ionically crosslinked natural rubber exhibits superior cut growth resistance and self-repairing capabilities, as demonstrated by X-ray microtomography. These findings, along with the natural origin of the developed crosslinked elastomers, can reduce environmental damage and the carbon footprint associated with sulfur-cured and petroleum-based synthetic rubber products.