From Levulinic Acid to Imines: Creating Biobased, Recyclable, Cross-linked Rubbers through Covalent Adaptive Networks.

Dynamic covalent bonds provide a powerful tool to design recyclable rubber-based materials. Among possible strategies, imine chemistry offers a valuable approach for achieving an adaptive network. In this work, for the first time, the use of biobased ketones and amines as cross-linkers in a rubber network is used. Specifically, epoxidized natural rubber (ENR) was cross-linked with glycerol trilevulinate (GT) and hexamethylene diamine (HMDA) to produce a fully biobased and recyclable hybrid network based on imine and hydrogen bonds. Comprehensive characterization confirmed the formation of a hybrid adaptive network, while mechanical tests demonstrated that the optimal formulation (5 phr GT and 5 phr HMDA) achieved the best performance after recycling, showing a significant increase in tensile strength while maintaining stable strain at break. The material's ability to reconstruct its network upon reprocessing was supported by cross-link density measurements via swelling and mechanical analyses, while dielectric investigations further confirmed the presence of dynamic interactions. The observed recyclability was thus attributed to the synergistic effect of covalent and noncovalent bonds, which reorganized effectively to preserve network integrity and mechanical performance. This work demonstrates the potential production of a biobased, recyclable and adaptable rubber network with excellent mechanical properties, highlighting how levulinic acid derivatives represent an optimal system for the development of sustainable rubber materials.