Eugenol-Based Siloxane-Containing Recyclable Epoxy Resins with Tunable Dynamic Properties, Reprocessability, and Degradability

Developing recyclable and biobased thermosetting polymers is of great significance for environmental protection and resource recycling. In this work, three liquid eugenol-based siloxane-containing epoxides (ESI1–3) with different rigid side groups and functionalities were synthesized and cured by anhydride or amine hardeners. The epoxy resins cured by the anhydride hardener (ESI-M) show quite different dynamic properties, reprocessability, and degradability compared to those cured by the amine hardener (ESI-H). Bifunctional epoxides (ESI1 and ESI2) cured with anhydrides exhibit significantly faster stress relaxation at lower temperatures compared to those cured with amines. All the cured resins can be reprocessed by hot pressing due to their dynamic structures. Both the ESI1-M and ESI2-M resins almost fully restore their mechanical properties after reprocessing. However, the ESI1-H and ESI2-H resins exhibit poor reprocessability even when pressed at high temperature. All cured ESI resins degrade rapidly in a tetrabutylammonium fluoride trihydrate/tetrahydrofuran solution, but they exhibit distinctly different degradation behaviors in acidic solutions. Notably, the degraded ESI-H resins in HCl solution can be recycled to prepare new epoxy resins, whose tensile strengths are comparable to those of the original resins. Finally, recyclable carbon fiber reinforced composites and electromagnetic wave absorbing coatings were fabricated based on the ESI2-M resin. Carbon fibers and magnetite powders can be quickly and nondestructively recycled through degrading the ESI2-M resin.