Alternative Natural Rubber Cross-Linking Utilizing a Disulfide-Containing Bismaleimide

This study explores a disulfide-selective cross-linking strategy for natural rubber (NR) to formulate elastomeric materials with engineering-relevant mechanical properties. A disulfide-containing bismaleimide (BIS) cross-linker was synthesized from cystamine and maleic anhydride and compounded with NR. Three formulations were prepared: control (no inhibitor), CuCl₂-based, and copper(II) methacrylate (CuMA) based compounds, with BIS concentrations ranging from 3.55 to 8.88 phr. Rheological and mechanical testing revealed that CuCl₂ formulations suffered from molecular degradation, poor thermal stability, and mechanical brittleness due to oxidative reactions in the absence of antioxidants. In contrast, CuMA-based compounds exhibited intermediate molecular weights prior to curing, stable thermal behavior, and improved mechanical properties, including enhanced torque and tensile strength, indicating effective cross-linking and partial recyclability. The control formulations also performed reasonably well but did not match the mechanical strength of conventional sulfur-vulcanized NR. The results demonstrate that metal coordination, particularly with CuMA, can modulate disulfide metathesis kinetics and offer a pathway to thermally triggered network rearrangement. Overall, CuMA emerges as a promising candidate for developing high-performance, recyclable rubber materials, while CuCl₂ requires further stabilization strategies. This work establishes a baseline for future recyclability studies and advances the design of dynamic covalent networks in elastomers.