Covalent modification of Aramid fibers' surface via direct fluorination to enhance composite interfacial properties

• Aromatic CF bonds can be generated in the direct fluorination of Aramid fiber.
• The substitution reaction of aromatic CF bond is utilized in the modification of Aramid fiber.
• The traditional effective coupling agents of glass fiber are introduced onto Aramid fiber in covalent bond.
• The interfacial enhancement can be achieved and the corresponding mechanism is discussed.

Covalently introducing amine and epoxy groups on the surface of inert fiber will make a great contribution to the interfacial properties of fiber/epoxy composite. In this study, direct fluorination was utilized to introduce aromatic CF bonds on the surface of Aramid fiber. The F atom was proved to be linked with phenyl based on the analysis of fluorinated model compounds of the fiber. Those CF bonds were used as the active sites for further functionalization of SiOH by the nucleophilic substitution of these active CF bonds with 3-aminopropyltriethoxysilane (APTES). On the basis of the reactive SiOH groups, these traditional coupling agents, APTES and 3-glycidoxypropylthrimethoxysilane (GPTES) were respectively grafted on the fiber surface and thus successfully realized the introduction of amine and epoxy groups. Compared to that of untreated fiber, the interfacial shear strength of the fiber with amine/epoxy modified had a dramatical increase of 46.7% and 40.0%, respectively. It is considered that the formation of chemical bond between epoxy resin and amine/epoxy group on fiber surface plays a determining role in improving the interfacial adhesion.

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