Sulphonated hyperbranched wholly bio-based waterborne polyurethane sizing agents for strengthening the wettability and mechanical properties of carbon fibres

The surface of carbon fibres (CF) is covered with numerous carbon-containing groups that appear chemically inert, which results in poor wettability and infiltration of the fibres. Additionally, industrial wet-weaving create surface grooves and defects on CF, reducing mechanical performance and product lifespan. To enhance CF's application value and performance, eco-friendly water-soluble resins are needed for sizing. Wholly bio-based waterborne polyurethane (CWPU) prepared by replacing petroleum-based materials with natural renewable bio-extracts and crop-derivatives are attracting the attentions in the CF industries. However, CWPU made with bio-based reagents including castor oil (CO), l-lysine diisocyanate (L-LDI), and gallic acid (GA) exhibits low thermal stability, mechanical strength, and adhesion. In addition to the use of GA as a former chain-extender and cross-linker, the study introduced diaminosulphonate segments with excellent hydrophilicity and polarity into the hyperbranched side-chains of CWPU by the post chain-extension method. SWPU revealed favourable thermal stability and mechanical properties achieving a T5 % decomposition temperature of 293.8 °C and a maximum tensile strength of 41.3 MPa. The "polar similarity compatibility" effects of the reactive groups in SWPU together with the synergism of SO₃Na strengthened hydrogen bonding and CO-GA hyperbranched cross-linking networks significantly improved the surface energy (70.9 mN/m), roughness (58.1 nm) and tensile strength (5.8 GPa) of the filaments. The morphological testing of SWPU-CF proved the effectiveness of the sizing coatings in repairing defects on fibre surfaces. As a high-performance and eco-friendly sizing agent, SWPU could contribute to the green and sustainable development of CF used in the new energy vehicle and high-speed railway industries.