Design of Water‐Based Sizing Agent Based on Polyamide Containing Hydrophilic and Hydrophobic Segments and Its Effect on Interfacial Properties of CF / PA612 Composites

ABSTRACT Carbon fiber (CF) reinforced thermoplastic composites are limited in applications like the automotive industry due to the weak interfacial strength between the fiber and matrix, which adversely affects overall composite performance. In this work, a novel series of water‐based polyamide (WPA) sizing agents were designed and synthesized from polyetheramine D230, 5‐sulfoisophthalic acid sodium salt, 1,6‐hexamethylenediamine, and dodecanedioic acid to improve the interfacial properties of CF fabric reinforced polyamide 612 (CF/PA612) composites. Their chemical structures were characterized by FT‐IR and ¹ H NMR spectra. Furthermore, their thermal stability, hydrophilicity, and dispersion stability were systematically characterized via TGA, contact angle measurements, and dynamic light scattering, respectively. After sizing treatment with the WPA sizing agent, the surface‐modified CFs exhibited an increase in surface roughness from 175.4 to 61.4 nm and in dynamic contact angle from 85.56° to 55.25°. Furthermore, for the WPA‐sized CF‐reinforced PA612 thermoplastic composites, the flexural strength and interlaminar shear strength (ILSS) increased from 505 MPa and 43.9 MPa to 676 MPa and 55.8 MPa, respectively, representing increases of 27.11% and 33.68%. This water‐based sizing agent design and CF surface modification strategy provide new insights for enhancing the interfacial properties of CF reinforced polyamide‐type thermoplastic composites.