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.