Designing Schiff-Based Hyperbranched Polysiloxane for Simultaneously Enhancing Epoxy Resin with Mechanical Properties, Thermal Stability, and Recyclability

It is indeed challenging to simultaneously enhance the toughness, thermal stability, and recyclability of epoxy resins. This study presents an approach utilizing a hybrid hyperbranched polysiloxane (STHPSi) structure, which incorporates Schiff base structures (comprising two benzene rings bonded to imines), Si-O-Ar (aryl group) segments, and abundant terminal sulfhydryl groups. This structure was employed to fabricate high-quality hybrid epoxy resins (STHPSi/EP). Experimental techniques including universal testing machines, dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were utilized to assess the performance of the resulting materials. The incorporation of the STHPSi structure imparted a rigid-flexible nature to the epoxy resins, leading to remarkable mechanical properties. Notably, STHPSi not only significantly improved the impact strength by 59.8% and flexural strength by 20.6% but also contributed to enhanced thermal properties. With a 6 wt % addition of STHPSi, the thermal decomposition temperature at 5% weight loss (T_d,5%), glass transition temperature (T_g), and char residues of the hybrid resins increased to 351.0 °C, 128.06 °C, and 9.55%, respectively. Furthermore, the STHPSi/EP composites exhibited complete degradation in 1,3-diaminopropane and the degraded substance was successfully reintroduced into the epoxy matrix as a curing agent, facilitating the recycling of waste epoxy resins. The recycled epoxy resins demonstrated excellent mechanical properties, with the impact strength and flexural strength reaching up to 15.3 kJ/m² and 149.22 MPa, respectively, and interesting luminescent characteristics. This study presents an effective approach for the preparation and reutilization of high-performance epoxy resins, addressing the critical challenges in enhancing their properties and promoting sustainable materials development.