Catalyst-Free and Green Crosslinking of Epoxy-Functionalized EPDM With Dicarboxylic Acids: Insights into Curing Mechanism, Recyclability and Thermal Stability

The conventional curing methods pose a significant environmental threat due to the use of toxic vulcanizing agents, the release of irritating volatile organic compounds, and the difficulties in recycling end-of-life rubber products. This work demonstrates a catalyst-free, facile, and eco-friendly crosslinking strategy based on the reaction between epoxy-functionalized ethylene-propylene-diene monomer (EEPDM) and dicarboxylic acids. The EEPDM was synthesized via a reusable reaction-controlled phase-transfer catalyst. Then, EEPDM could be effectively crosslinked by dicarboxylic acids without additional additives. The increased acidity of dicarboxylic acids could be conducive to improving the crosslinking rate. Moreover, the curing mechanism and the nature of the crosslinks were investigated, which is crucial for the recyclability of rubbers. The dicarboxylic acid-cured EEPDM exhibited undesirable side reactions, the extent of which showed a positive correlation with the acidity of carboxylic groups. This phenomenon could be primarily attributed to acid-catalyzed self-polymerization via epoxide ring-opening mechanisms, which would be detrimental to the reprocessability. Due to the formation of thermally stable ether linkages, the crosslinked EEPDM exhibited enhanced thermal-oxidative aging behavior. Hence, we envision that this catalyst-free, high-efficiency crosslinking strategy may offer a promising bridge between sustainable modification and high-performance for epoxy-functionalized rubbers, holding potential application in eco-friendly, low-odor automotive sealing strips.