Interface correlation and toughness matching of phosphoric acid functionalized Kevlar fiber and epoxy matrix for filament winding composites

Kevlar fiber was functionalized with phosphoric acid (PA) of different concentrations. The monofilament tensile strength was constant with less than 40 wt% PA functionalization. The amount of surface oxygen functional groups was maximal at 20 wt% PA-functionalized Kevlar fiber, in which the surface roughness of functionalized fiber approached to that of as-received fiber. Using optimally functionalized fiber, various combinations of epoxy and hardener were studied. The composite mechanical properties were far enhanced when 40 wt% DGEBF were added to DGEAC/DDM/DETDA system, which was attributed to interfacial adhesion between functional groups and resin matrix, and toughness matching between fiber and matrix. The interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) were 76 and 79 MPa, respectively, and the fiber strength conversion ratio of Naval Ordnance Laboratory (NOL) Rings reached 95%. Microstructure analysis revealed that better interfacial adhesion resulted in higher improvement of composite mechanical properties.