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.

High-Performance Recyclable and Malleable Epoxy Resin with Vanillin-Based Hyperbranched Epoxy Resin Containing Dual Dynamic Bonds

“…4,5 It is of great importance to develop technologies for recycling epoxy resins at the end of use. 6−8 Many researchers have incorporated covalent adaptable networks (CANs) in synthesizing recyclable epoxy resins, 9−12 including imine exchange, 13,14 transesterification, 15−17 disulfide exchange, 18−20 boronic ester exchange, 21,22 siloxane exchange, 23 and DA/retro-DA reaction. 24 However, due to the introduction of CANs, the dynamic networks were susceptible to creep when used in engineering and structural fields, 25 and creeping of these materials usually occurred around the topology freezing temperature (T v ), which was usually much lower than the T g .…”

Epoxy Thermosets Designed for Chemical Recycling

“…[93] The Hu group prepared a liquid imine-containing hardener mixture of h3 to harden DGEBA and yield the dual dynamic epoxy polymer p(DGEBA-h3). [94] Tensile strength, Young's modulus, flexural stress, and impact strength of this material reached values of 80.3 MPa, 1.0 GPa, 134.1 MPa, and 24.2 kJ/ mol 2 , respectively. The thermosetting polymer was dissolved in a 1 M HCl/THF mixture after 2.5 h to yield oligomeric structures containing aldehyde and amino functional groups.…”

“…The Hu group prepared a liquid imine‐containing hardener mixture of h3 to harden DGEBA and yield the dual dynamic epoxy polymer p(DGEBA‐h3) [94] . Tensile strength, Young's modulus, flexural stress, and impact strength of this material reached values of 80.3 MPa, 1.0 GPa, 134.1 MPa, and 24.2 kJ/mol 2 , respectively.…”

“…These oligomers were upcycled into a toughening agent for a brittle epoxy thermoset. By adding 14 wt.% of this oligomeric mixture into such an epoxy thermoset, the impact strength was enhanced by 94% compared with the pristine network [94] . The same research group developed another hardener based on imidazole ( h4 ) [95] .…”

“…By adding 14 wt.% of this oligomeric mixture into such an epoxy thermoset, the impact strength was enhanced by 94% compared with the pristine network. [94] The same research group developed another hardener based on imidazole (h4). [95] This monomer was synthesized by the condensation reaction between 4-imidazole formaldehyde and 3-aminobenzylamine.…”