Triethanolamine-Mediated Covalent Adaptable Epoxy Network: Excellent Mechanical Properties, Fast Repairing, and Easy Recycling

Abstract

Early epoxy vitrimers in the literature rely on an inequivalent epoxy/anhydride stoichiometry and a large amount of catalyst to achieve a decent transesterification rate within the crosslinked network. This design approach raises a number of concerns such as poor miscibility of the catalyst with other ingredients, poor mechanical properties owing to insufficient crosslinking, the toxicity of the catalyst, etc. In this study, a hydroxyl-amine compound, triethanolamine (TEOA), is incorporated as a catalytic co-curing agent to a typical BPA epoxy–cyclic anhydride curing system to give a TEOA-mediated covalent adaptable network system. The hydroxyl groups and tertiary amine of TEOA catalyze the curing process, and the tertiary amine and the regenerated hydroxyls in the crosslinked network accelerate dynamic transesterification. The resulting epoxy vitrimer exhibits a high glass transition temperature (∼135 °C), excellent tensile strength (∼94 MPa), and fast repairing rate (10 min at 190 °C). Recycling of the TEOA-mediated epoxy vitrimer and reuse of the recyclate are also studied. In an experiment, the vitrimer is ground into powder, and in another, it is degraded in an aqueous solution. The recyclates collected from both experiments are incorporated into the fresh resin, and the new vitrimer materials exhibited similar T gs and moduli to that of original vitrimer samples. This work provides a solution to eliminate the performance gap between conventional epoxy and epoxy vitrimer and offers simple recycling methods of epoxy vitrimer for new epoxy.

Recyclable High-Performance Epoxy-Anhydride Resins with DMP-30 as the Catalyst of Transesterification Reactions

“…The role of tertiary amine groups in DMP-30 is to participate in the formation of carboxylate and alkoxide anions, thereby initiating the alternating copolymerization between anhydride and epoxy groups ( Figure 2 b) [ 53 ]. The anhydride rings were opened by the hydroxyl in DMP-30 to form the carboxylic acid groups, which can further react with the epoxy groups and generate a new hydroxyl ( Figure 2 c) [ 51 ]. When the tertiary amine and nonphenolic hydroxyl coexisted in the curing system, they formed a complex with the anhydride, which further enhanced the reactivity between the epoxy group and the anhydride ( Figure 2 d) [ 57 ].…”

“…As the curing reaction tended to be complete with increasing the content of the curing agent [ 29 , 58 ], a visible hydroxyl peak at 3500 cm −1 appeared in the FTIR spectra when the ratio of epoxide to acyl, r = 1:1, while it disappeared when r = 1:2. Abundant hydroxyl groups are necessary for the TERs [ 29 , 51 ], but the number of hydroxyl groups in DGEBA/GA/DMP-30 is small. Therefore, the polymers are expected to be more stable, and correspondingly, the stress relaxation time is longer ( Figure S2 ).…”

“…All samples were then taken out and dried in the oven at 180 °C for 12 h. The mass changes during the process were recorded in Table 4 . The resins with higher crosslink densities had better solvent resistance, and the slight mass loss was caused by the dissolution of the unreacted monomers in solvents [ 51 ]. The results were consistent with that of FTIR and thermal stability.…”

“…However, these polymers usually had the problems of complex synthesis processes, low glass transition temperatures ( T g ), and low strengths. Hao et al [ 51 ] incorporated triethanolamine (TEOA) as a catalytic co-curing agent into the traditional epoxy-anhydride system to prepare recyclable epoxy vitrimer with high T g and strength. The resulting resins can be easily hydrolyzed using a phosphotungstic acid aqueous solution.…”

Reprocessable, Reworkable, and Mechanochromic Polyhexahydrotriazine Thermoset with Multiple Stimulus Responsiveness

“…through the reversible depolymerization or exchange reactions of their dynamic cross-links [ 3 , 9 , 10 , 11 ]. So far, a number of CANs based on Michael addition [ 12 , 13 , 14 ], Diels-Alder reaction [ 15 , 16 ], disulfide exchange [ 17 , 18 , 19 ], imine metathesis [ 20 , 21 , 22 ], transesterification [ 23 , 24 , 25 ], olefin metathesis [ 26 , 27 ], silyl ether transalkoxylation [ 28 , 29 ], diketoenamine exchange [ 30 , 31 ], and dioxaborolane metathesis [ 32 , 33 ] have been proposed in the literature. Besides recyclability, many other adaptive properties of CANs were also investigated, such as reconfigurability [ 34 , 35 ], shape memory [ 36 , 37 ], and network topological transformation [ 18 , 25 , 38 ].…”