Decoding Fiber-Matrix Interphase Formation through Waterborne Polyurethane Sizing Strategies, Interfacial Engineering, and Performance Frontiers in Carbon Fiber Composites

Carbon fiber (CF) is renowned for its exceptional mechanical properties; however, its inherent surface inertness and susceptibility to processing damage frequently compromise interfacial adhesion with polymer matrices. Sizing treatment serves as an effective and industrially scalable strategy to overcome this limitation. This review systematically examines the protective and thermodynamic roles of sizing agents, the four fundamental interfacial mechanisms of chemical bonding, mechanical interlocking, physical adsorption, and interphase formation, alongside the specific characterization techniques employed to analyze them. Conventional sizing formulations, encompassing epoxy, acrylic, polyester, polyimide, polyether, and vinyl resin systems, are evaluated with a primary focus on matrix compatibility and inherent performance trade-offs. The discussion subsequently highlights waterborne polyurethane (WPU) as a highly sustainable and structurally versatile sizing platform. We critically assess recent advancements in WPU modification achieved through chemical grafting, supramolecular engineering, bio-based feedstocks, and metal oxide hybridization. While these targeted modifications significantly enhance fiber wettability and interfacial shear strength, they can concurrently introduce challenges related to brittleness, moisture sensitivity, or manufacturing complexity. Drawing from these comparative insights, we outline rational design principles for engineering robust composite interphases. Finally, we explore emerging research directions, emphasizing self-healing functionalities, adaptability to rapid manufacturing processes, and long-term reliability in energy-critical applications such as wind turbine blades and hydrogen storage vessels. Life cycle considerations, particularly the integration of bio-based components and the recyclability of composite are also addressed. Ultimately, this review aims to provide a clear roadmap for the development of next-generation CF sizing agents through advanced interfacial engineering.