Mechanically Robust and Flame‐Retardant High‐Fiber‐Loading Bamboo Fiber/Waterborne Epoxy Composites via a Reactive Ternary Deep Eutectic Solvent

ABSTRACT High‐fiber‐loading bamboo fiber (BF) composites are attractive for lightweight and renewable structural applications, yet their practical use is limited by the intrinsic flammability of lignocellulosic fibers, insufficient resin impregnation, and weak interfacial bonding at high fiber contents. Herein, BF was functionalized with ammonium phosphate groups (PBF) using a reactive ternary deep eutectic solvent (TDES) composed of phytic acid, urea, and sulfamic acid, and then mixed with a waterborne epoxy resin (WEP) to fabricate PBF/WEP composites (PBECs) by hot‐press molding at approximately 95 wt% fiber loading. The PBECs exhibited enhanced mechanical performance, attributed to restricted interfacial sliding and fiber pull‐out. Specifically, the tensile strength increased from 180.22 MPa (BEC) to 200.18 MPa (PBEC‐120), whereas the elongation at break decreased due to the suppression of plastic deformation. Compared with BEC, the TDES‐treated composites (PBECs) exhibited markedly improved flame resistance, with the limiting oxygen index (LOI) increasing from 22.3% to 44.5%. These improvements are attributed to an N–P synergistic flame‐retardant effect in the gas and condensed phases and to strengthened PBF‐WEP interfacial interactions. This work demonstrates a TDES‐enabled, solvent‐minimized route to mechanically robust and flame‐retardant bamboo fiber/WEP composites for fire‐safe structural applications.