Ellagic acid-based non-isocyanate polyurethane water-soluble sizing agents for enhancing the interfacial properties of carbon fiber/epoxy composites

The preparation of environmentally friendly and efficient waterborne polyurethane sizing agents for carbon fiber (CF) is difficult, especially to avoid the use of isocyanates. Herein, ellagic acid and CO 2 were used to prepare a five-membered cyclic carbonate compound (EARBO). The polymerization between EARBO and diamines enabled the preparation of highly efficient and green aqueous non-isocyanate polyurethane (WNIPU) sizing agents. WNIPU sizing agent exhibits excellent heat resistance (T 5% :244.5 °C). The hydroxyl group and tertiary amine effectively enhance the polarity and wettability of CF (CF surface energy is as high as 55.42 mJ/m 2 ) and show excellent compatibility with epoxy resin. Carbon fiber/epoxy composites treated with this sizing agent showed exceptional interfacial properties, achieving interfacial shear strength, ply shear strength, flexural strength, and tensile strength of 116.84 MPa, 71.58 MPa, 1352.79 MPa, and 1719.27 MPa, respectively—representing 112.05 %, 42.05 %, 86.68 %, and 87.11 % improvements over traditional composites. The WNIPU sizing agent’s efficiency and environmental advantages indicate it as a promising alternative to commercial polyurethanes.