Preparation of self‐emulsifying waterborne polyamide‐imide sizing for carbon fiber and improvement on mechanical performances of carbon fiber reinforced thermoplastic polymer

Abstract Recently, Carbon fiber reinforced thermoplastic polymer (CFRTP) has been widely used in automotive, aviation, and aerospace. However, the traditional epoxy Carbon fiber (CF) sizing for thermosetting composites was unsuitable for high‐performance CFRTP due to the high‐molding temperatures and incompatibility between them. Herein, a novel semi‐aromatic polyamide‐imide (PAI) backbone structure with multi‐ethoxy pendant groups was designed and synthesized via click‐reaction and solvent‐free polymerization. Waterborne PAI sizing was prepared via self‐emulsifying without surfactants. CF/PEEK (polyether‐ether‐ketone) and CF/PEI (polyether‐imide) composites were manufactured. Results showed that novel PAI had excellent thermal stability and solubility, PAI sizing was stable after 6‐month‐storage. Mechanical performances and hydrothermal aging resistance of composites were improved. Compared with commercial‐sized CF (eCF)/PEI, the flexural strength of sCF/PEI increased by 35.2% to 753.47 MPa, and retention rates of them with aging also increased. Noted, mechanisms of interface enhancement were revealed by quantum chemical calculations, ππ stacking and hydrogen bonding between PAI and PEI, PEEK was found.