Two-stage interface enhancement of aramid fiber composites: Establishment of hierarchical interphase with waterborne polyurethane sizing and oxazolidone-containing epoxy matrix

Hydroxy-terminated waterborne polyurethane (WPU) and oxazolidone-containing epoxy (OXEP) matrix were designed to realize two-stage interface enhancement of WPU sized aramid fiber/oxazolidone-containing epoxy (WAF/OXEP) composite, and the interfacial properties and interphase reinforcing mechanism were investigated. Compared with commercial aramid fiber (CAF), the surface roughness and energy of WAF were increased, and the fracture toughness and elongation at break of OXEP matrix were improved relative to those of general epoxy (GEP) matrix. In contrast to CAF/GEP and CAF/OXEP composites, the flexible interphase with high bonding capability from WPU sizing was constructed in WAF/GEP composite, which released interphase stress through deformation and improved interfacial adhesion. The hierarchical interphase encompassing inner modulus intermediate layer and outer flexible layer was established in WAF/OXEP composite, which could prevent crack propagation onto fiber surface and induce crack deflection through the enhancement of modulus and deformability of hierarchical interphase.