Phase-differentiated interfacial engineering enabled by a cationic dual-component waterborne sizing for CF/PA6 composites

Conventional single-component sizing agents for carbon fiber–reinforced thermoplastic composites typically form interfacial layers governed by a single interaction mechanism, making it difficult to simultaneously achieve interfacial spreading and structural stability during processing. To overcome this limitation, a cationic dual-component waterborne sizing strategy based on main-chain hydrophilic structural design is proposed. A flexible waterborne polyurethane (WPU) and a rigid naphthalene-containing polyamide (PNA) are co-deposited on carbon fibers to construct a phase-differentiated interfacial layer in CF/PA6 composites. The identical charge characteristics enable stable aqueous coexistence and uniform deposition. The treatment increases fiber surface polarity and improves wettability toward PA6 melt. Simulations show that WPU, with higher fractional free volume, promotes interfacial spreading, whereas PNA, with a rigid backbone and lower free volume, maintains structural integrity at elevated temperature. Their synergy induces a phase-differentiated architecture and phase-structure-driven mechanical interlocking interface. As a result, interlaminar shear strength and flexural strength reach 73.6 MPa and 1054 MPa, respectively, with failure mode shifting from interfacial debonding to interfacial/matrix cooperative failure. This dual-component, phase-structure-regulated strategy provides an effective route to surpass the intrinsic limits of single-component sizing systems.