Closed-Loop Recycling of Carbon Fiber-Reinforced Composites Enabled by a Dual-Dynamic Cross-linked Epoxy Network

Abstract

The development of covalent adaptable networks has addressed the issue that conventional epoxy thermosets are difficult to degrade and reprocess, but the greater challenge lies in how to overcome the trade-off between excellent degradability and mechanical properties such as stiffness and extensibility. Herein, a new concept of a dual-dynamic cross-linked network composed of dynamic covalent and non-covalent bonds (imine bonds and hydrogen bonds) was proposed for closed-loop recovery of epoxy thermosets/composites. Among them, the additional non-covalent crosslinks were constructed through the designed imine-containing hardener as the medium. The cooperative effects of hydrogen-bond interactions and molecular interlocking of rigid–flexible molecular chains enhanced greatly the stiffness and ductility, and the recoverable energy dissipation enabled the prepared epoxy thermoset to have excellent fracture toughness. Its tensile strength and impact strength reached ∼80.3 MPa and ∼24.2 kJ/m2, respectively. Importantly, driven by the pH-sensitive feature of the dual-dynamic network, the resultant thermoset was completely degraded within 3 h. Along the closed-loop sustainable route, the recoverable oligomer from the thermoset/matrix resin showed good potential in toughening brittle materials with a 55–94% increase in impact strength. Further, the recovered CF cloths that kept their usage value were re-prepared into new high-performance composites, realizing the conversion of waste to high-value applications.