Hyperbranched bio-based waterborne sizing coating for enhancing the wettability of carbon fibre and interfacial adhesion of fibre/epoxy composites

Novel castor oil-based waterborne polyurethane (CWPU) sizing coatings prepared by replacing traditional petroleum-based petrochemical products with natural renewable bio-extracts are attracting the attentions in the carbon fibre (CF) reinforced epoxy (EP) composites industries. However, CWPU coatings prepared using only castor oil (CO), diisocyanate, and carboxylate hydrophilic chain extender suffer from poor thermal stability, insufficient mechanical strength and weak adhesion. For enhancing the thermo-mechanical properties of CWPU coatings, as well as the surface wettability and interfacial adhesion to the substrates when serving as fibre sizing coatings and as interphases of the CF/EP composites, a compound cross-linker with tri-acrylate branched and tri-isocyanate chain-endings was synthesized and used to prepare hyperbranched CWPU with CO-acrylate-isocyanate interpenetrating cross-linking networks. CWPU coatings revealed favourable thermodynamic performance achieving a T5% decomposition temperature and toughness of 271.8 °C and 36.2 MJ/m³. CWPU coatings imparted excellent wettability to CF through oxygen-containing polar groups and synergy between covalent/hydrogen bonding, resulting in an increase in fibre surface energy to 61.0 mN/m. Stable and robust interphases were constructed in the CF/EP composites by CWPU coatings through "polar similarity compatibility" and multiple physico-chemical reactions. The flexural modulus, interlaminar shear strength, and interfacial shear strength of CWPU-CF/EP were increased by 54.8 %, 36.6 %, and 58.9 %, respectively, compared with those of the unsized CF/EP composites. The research contributes to the development and industrial production of high-performance, eco-friendly bio-based water soluble organic coatings.