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

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.