In Situ Synthesis of Organic Polymer–Inorganic Nano ZnO Core–Shell Structured Sizing Agents and Their Effect on Carbon Fiber Interfaces and Composite Properties

Sizing agents are essential to address the increasing demands of enhanced carbon fibers (CFs), where increasing interfacial adhesion and the analysis of mechanical properties are achieved for critical engineering applications. In this work, five types of self-emulsifying sizing agents, featuring organic polymer–inorganic nano zinc oxide (ZnO) core-shell structures with varying crosslinked polymer densities in the core, were synthesized using self-emulsifying technology through a one-pot, in situ synthetic process. This study revealed that these sizing agents exhibited a uniform particle size distribution within the range of 100–200 nm, along with excellent storage stability and thermal stability up to 300 °C. The optimized sizing agent significantly enhanced the surface properties of CFs, achieving a surface roughness of 6.04 nm and a surface energy of 74.81 mJ/m2. Moreover, the interlaminar shear strength (ILSS) and flexural strength of CF/epoxy resin (EP) composites modified with the synthesized sizing agent increased by 86% and 86.43%, respectively, compared to unoptimized composites. These improvements in mechanical properties are attributed to enhanced stress transfer at the CF/EP interface, facilitated by the interlocking mechanism of the nano ZnO particle shell and the superior anti-pressure resistance provided by the crosslinked organic polymer core.