Fully Bio-Based High-Performance Thermosets with Closed-Loop Recyclability

Abstract

Thermosets are important commodity polymeric materials, but they are rarely biorenewable and recyclable. Although some previously reported bio-based aromatic thermosets with a high aromatic content have good thermal/mechanical properties, the mechanical properties of fully bio-based vitrimers are relatively poor owing to low aromatic contents. To address this important issue, vanillin-based dialdehyde and trialdehyde containing high aromatic content were synthesized, and renewable diamines containing short aliphatic chains were carefully screened. Then, fully bio-based thermosets were prepared via the Schiff base reaction between vanillin-based aldehydes and diamines. Attributed to the high aromatic content (59.2–61.3 wt %), the mechanical performances of these fully bio-based thermosets were significantly improved, demonstrating comparable properties to traditional thermosets and higher than any previously reported fully bio-based thermosets [high mechanical properties (σ = 58.0 MPa; E′ = 3.07 GPa)]. In addition, it could be completely degraded under mild acidic conditions. This significantly expands the end-of-life options such as recovery of monomers. More importantly, the fully bio-based thermosets demonstrated excellent closed-loop recyclability without changing their chemical structures and mechanical properties after repolymerization via commonly used approaches, such as thermomechanical recycling and chemical recycling. Even after three hot-pressing cycles, the recovery ratio of the tensile strength was higher than 84%, which was even better than the results of many reprocessable commodity thermoplastics. Therefore, these fully bio-based thermosets are expected to be excellent alternatives to traditional thermosets in the future.

A fully bio-based Schiff base vitrimer with self-healing ability at room temperature

“…The low aromatic content and the long aliphatic chain length in the crosslinking network weakened the tensile strength. 36 However, the elongation at break was greatly enhanced, and the high toughness and the elastic nature broadened the application area of the vitrimer synthesized in the present work.…”

Closed-loop chemical recycling of thermosetting polymers and their applications: a review

“…Such a closed-loop recycling method based on an imine exchange reaction was also a research direction and interest of researchers in recent years. [60][61][62] The exchange reaction of imine bonds can be carried out at room temperature, unlike other dynamic bonds that require high temperature, which provides the potential for the closed-loop chemical recycling of thermosets based on dynamic imine bonds. In 2020, Guo and co-workers 63 prepared a cross-linked network based on imine bonds through the reaction between aldehyde-terminated polybutadiene and tris(2-aminoethyl) amines.…”

Design of inherent fire retarding and degradable bio-based epoxy vitrimer with excellent self-healing and mechanical reprocessability

“…It can be observed that although the tensile strength, modulus and elongation at break of A 7 P 3 -D230 decreased gradually, the tensile strength still retained 86.1% and 66.8% after one and two reprocessing steps in comparison with the original A 7 P 3 -D230. Two main reasons can justify this result: (1) during the reprocessing process, only C = N, as the dynamic covalent bond, can be repaired, while other covalent bonds such as C − C, P-O and P = O cannot be repaired after break [35,51]; (2) epoxy vitrimer was probably degraded during the hot pressing [52,53].…”

“…The decomposition mechanism and rearrangement mechanism of the crosslinking network of thermosets are the uniqueness of vitrimer materials [54]. Due to the reversible reaction properties of Schiff base bonds, epoxy vitrimer can be degraded via the exchange reaction between amino and imine groups under acidic conditions [26,52]. To prove this hypothesis, A 7 P 3 -D230 was immersed in a mixed solution of 1M HCl/THF (v:v = 2:8) and stirred continuously at 60 ºC for 60 min, as shown in Fig.…”