Aqueous sizing agent prepared via ethyl acetate-induced polyamide acid conformation twist for CFRTPs enhancement

Sizing agents are essential for carbon fiber (CF), serving to protect the fiber, improve processability, and enhance the performance of carbon fiber reinforced thermoplastics composites (CFRTPs). However, sizing agents capable of withstanding the processing temperatures of CFRTPs are typically require the assistance of highly toxic additives for dissolution. To address this issue, a novel strategy is proposed that enhances the water solubility of the sizing agent by modulating the polymer conformation using a small amount of environmentally friendly compounds. The resulting sizing agent exhibited excellent dispersibility and long-term stability, with a shelf life exceeding 12 months and a particle size around 20 nm. Theoretical simulations revealed the potential interaction mechanism between ethyl acetate (EA) molecules and sulfonated polyamide acid (SA-PAA) chain segments and further elucidated the solvation behavior of the polymers in binary solvent systems. Thermal imidization transformed SA-PAA into sulfonated polyimide (SA-PI) on CF surface, endowing it with high thermal resistance and excellent compatibility. As a result, carbon fiber reinforced polyetheretherketone (CF/PEEK) exhibited outstanding mechanical properties, including an interlaminar shear strength (ILSS) of 87.7 MPa, an impact strength of 42.7 MPa, and a flexural strength of 973.7 MPa, representing increases of 40.5 %, 35.5 %, and 33.6 %, respectively. A similar enhancement was observed when reinforcing polyetherimide. This design provides a novel direction and theoretical foundation for the application of thermoplastic sizing agent.