Recyclable and Self-Healing Natural Rubber Vitrimers from Anhydride-Epoxy Exchangeable Covalent Bonds

Abstract

Dynamic covalent networks (DCNs) contain exchangeable covalent bonds that can undergo dynamic structural changes under external stimuli. Employment of DCNs in elastomers instead of static cross-links provides a pathway for designing reprocessable and recyclable rubbers. Vitrimers are examples of DCNs that utilize associative covalent bond-exchanging chemistry to keep the total number of cross-links constant, making them recyclable, reprocessable, and self-healing. This study primarily investigated the design of a natural rubber (NR) vitrimer via anhydride-epoxy dynamic cross-linking using a scalable process and benign reagents, such as maleic anhydride (MA) or bisphenol A diglycidyl ether (DGEBA), which can be reprocessed and self-heal with heat stimuli. Reactive melt mixing was employed to synthesize the vitrimers, and the reaction success was confirmed by using various chemical analysis approaches. The rubber vitrimer possesses a high activation energy (139.7 kJ/mol) and low freezing topology temperature (65 °C), demonstrating a robust exchange network. The NR vitrimers could undergo multiple rounds of reprocessing, unlike peroxide- or sulfur-cured NR, due to their robust dynamical cross-linking networks generated by the adaptable covalent bonds. Moreover, the NR vitrimers exhibited unprecedented self-healing capabilities to maintain their original mechanical characteristics. The recyclability of NR is a significant achievement in reducing post-consumer rubber waste and virgin material utilization. The self-healing functionality is also appealing in applications that require on-site assembly or repair as well as to help increase the lifespan of the elastomers.