Synergistic borate ester and siloxane-borate bonds enable room-temperature closed-loop recycling of high-strength glass fiber-epoxy composites

The cross-linked structure of traditional thermosetting resins imparts excellent mechanical properties to glass fiber-reinforced composites (GFRPs), but it also significantly limits their recyclability. To address this challenge, aniline was used to chain-extend bisphenol-A epoxy (E51), and phenylboronic acid (PBA) was introduced to crosslink and cure the resulting chain-extended product (EA). By varying the PBA content, a series of room-temperature degradable resins (EAPs) containing dynamic borate ester (B-O-C) bonds were synthesized, and glass fiber-reinforced composites (GF/EAPs) were fabricated. In the GF/EAPs, the introduced borohydroxy groups not only participated in borate ester crosslinking but also formed silicon-oxygen-boron (Si–O-B) bonds with silicon hydroxyl (Si–OH) groups on the glass fiber surface. This significantly enhanced the interfacial bonding strength and improved the composite’s mechanical properties. The GF/EAP15 composite, benefiting from the synergistic effects of the B-O-C and Si–O-B bonds, exhibited a flexural strength of 1138 MPa and an interlaminar shear strength of 62 MPa. Furthermore, it enabled closed-loop recycling of GFRPs via the dissociation of the B-O-C bonds in a methanol solution. After recycling, the flexural and interlaminar shear strengths of the GF/REAP15 composites were restored to 80.1% and 88.5% of their original values, respectively. This work presents a novel approach for the closed-loop recycling of high-performance, degradable GFRPs.