Interfacial property enhancement of carbon fiber/epoxy composites via constructing a gradual modulus interphase with a waterborne bio-based spiro diacetal epoxy emulsion

In this article, to explore the utilization of bio-based epoxy resin in the preparation of sizing agents for carbon fibers (CFs), a bio-based epoxy emulsion (S-P-S) derived from spiro diacetal epoxy (SDE) resin was prepared and compared to the traditional diglycidyl ether of bisphenol A (DGEBA) based emulsion (D-P-D) and a mixed emulsion of S-P-S and D-P-D (S-P-S&D-P-D). The CF sized with S-P-S (CF-(S-P-S)) showed improved surface oxygen content (22.86%) and roughness (40.9 nm), enhancing wettability and interfacial bonding between the fiber and the matrix. Nanoindentation test and AFM results showed the enhanced interfacial stiffness and a gradual modulus interphase with a thickness of 381 nm in CF-(S-P-S) reinforced epoxy resin composite (CF-(S-P-S)/EP). Therefore, the resulting CF-(S-P-S)/EP composite possesses the best mechanical properties, with the transverse fiber bundle tensile (TFBT) strength of 27.3 ± 1.45 MPa and the interfacial shear strength (IFSS) of 85.6 ± 8.26 MPa. These results highlight the potential of the SDE-based emulsion (S-P-S) to enhance the interfacial properties and mechanical performance of CF/EP composite, offering a sustainable alternative to conventional petroleum-based epoxy resins. Highlights Emulsion (S-P-S) was prepared using spiro diacetal epoxy (SDE) for carbon fiber sizing. Properties of S-P-S sized CF (CF-(S-P-S)) and its composite were evaluated. Spiro diacetal structure created a gradual modulus interphase in CF-(S-P-S)/EP. The interfacial properties of the CF-(S-P-S)/EP composite were greatly improved. Bio-based SDE resin showed promise for optimizing carbon fiber composite interphase.