A hydrophilic Lignin-Based carbon fiber sizing agent assembled with CNTs towards strengthening epoxy resin

The sizing agent has attracted growing attention to improve the interfacial compatibility of carbon fiber reinforced epoxy resin composites (CF/EP), but suffered from poor interfacial adhesion and environmental problems. Here, a hydrophilic lignin-based sizing agent (OLBEDKC) with remarkable storage stability and sizing effect on CF was obtained by the modifying oxidized lignin rich in hydrophilic groups and compounding with carbon nanotubes (CNTs). After treatment with 2.5% OLBEDKC, the hairiness of the CF fabric was reduced by 73.5%, and the surface energy was increased by 43.7%. Moreover, the use of OLBEDKC in CF/EP composites can lead to an increase in their interlaminar shear strength (ILSS), interface shear strength (IFSS), flexural strength, and flexural modulus by 58.4%, 64.8%, 87.9%, and 24.6%, respectively, compared to those without sizing treatment. This increase is attributed to the π-π conjugation and hydrogen bonding among the OLBEDKC, CF, and EP. Furthermore, the mechanical interlocking between the sized CF and EP effectively enhances their interfacial strength. In summary, this study presents a promising and eco-friendly hydrophilic sizing agent based on lignin that possesses high storage stability and exhibits an excellent sizing effect for CF reinforced EP composites.