Hyperbranched Bio-Based Sizing Agents for Strengthening the Interfacial Adhesion and Wettability of Carbon Fibre/Epoxy Composites

Abstract

Hyperbranched bio-based waterborne polyurethane (CWPU) sizing agents prepared by replacing traditional petroleum-based petrochemical products with natural renewable bio-extracts are attracting the attentions in the carbon fibre (CF) reinforced epoxy resin (EP) composites industries. However, CWPU sizing agents prepared using only castor oil (CO), diisocyanate, and carboxylate hydrophilic chain extenders suffer from poor thermal stability, insufficient mechanical strength, and weak adhesion, thus limiting their applications. 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 agents and as interphases of the CF/EP composites, a compound cross-linker (IETT) with functional tri-acrylate branched and tri-isocyanate chain-endings was synthesised and used to prepare hyperbranched CWPU with CO-acrylate-isocyanate interpenetrating cross-linking networks. CWPU sizing agents revealed favourable thermodynamic performance achieving a T5% decomposition temperature and toughness of 271.8 °C and 36.2 MJ/m3. CWPU sizing agents imparted excellent wettability to CF through oxygen-containing polar groups and cross-linking structures, resulting in an increase in fibre surface energy to 70.9 mN/m. Stable and robust interphases were constructed in the CF/EP composites by CWPU through "polar similarity compatibility" and multiple physico-chemical reactions. The flexural strength, interlaminar and interfacial shear strength of CWPU-CF/EP were increased by 54.2%, 33.2% and 36.3%, respectively, compared with the untreated CF/EP composites. The research contributes to the development and industrial production of high-performance, environmentally friendly bio-based polyurethane sizing agents.

Keywords: Waterborne polyurethane, Carbon fibre, Polymer-matrix composites, Interfacial adhesion, Interlaminar wettability