Covalent adaptable network offers “sustainable” closed-loop circularity in epoxy vitrimers

Abstract

Traditional Epoxies take hours to cure before their properties can be realized and this restricts its use in applications where quick turn-around time is imperative. Herein, we developed fast curing Epoxies (which cure within 2 h) using Vitrimer chemistries. A unique dynamic curing agent (bio-based Schiff motif) consisting of an aldehyde (here, cinnamaldehyde) and an amine (here, mussel inspired dopamine) is used to facilitate imine exchange during the curing process and maintains the network integrity- a key requirement to achieve excellent structural properties in epoxy based components. By incorporating this covalent adaptable network, various epoxies were designed which can cure within 2 h. The resulting epoxy vitrimers exhibited high tensile strength (68 MPa), exceptionally high Tg (128°C), fast stress relaxation (a relaxation time of 15.2 s at 180°C) and an activation energy of 105 kJ/mol. This associative covalent adaptable network (imine exchange) also rendered the epoxies re-processable and re-usable and offers closed-loop circularity in plastics. The resulting epoxy also exhibited self-healing property triggered by the exchangeable bonds. The self-healing behavior of this epoxy was manifested from the surface conductivity measurements wherein the conducting pathway (facilitated by spraying conducting particles on the vitrimer epoxy substrate) was restored post self-healing in substrates containing epoxy vitrimers. In addition, the epoxies cured using this Schiff based motif dissolves in a specific solvent and can be recovered and re-purposed. The excellent mechanical properties achieved by retaining the network integrity together with the self-healing properties offers myriad applications for the designed epoxy cured using bio-based Schiff motif. Highlights: Developed fast curing Epoxies that cures in 2 h using Vitrimer chemistries. Fully bio-based hardener was used to realize the network topology. The dynamic exchange reaction results in re-processability, reusable, recyclable and self-healable that is not possible in the case of traditional epoxy. It exhibited high tensile strength, exceptional Tg, fast stress relaxation and an activation energy of 105 KJ/mol.