Fabrication of flame-retardant PA6/three-dimensional braided glass fiber composites via in situ polymerization

Reaction injection molding method is employed to improve flame retardancy and mechanical properties of three-dimensional braided glass fiber (3D-braided-GF) reinforced in situ polymerized polyamide 6 (PA6) composites. The kernel lies in adding flame retardants, hexaphenoxycyclotriphosphazene (HPCTP), to the reactive monomers to allow it to be in situ filled into composites during preparing PA6 through anionic polymerization, which enables flame retardants to achieve better dispersion. In addition, extremely low-viscosity reactive monomers are easier to immerse in 3D-braided-GF, ameliorating interfacial adhesion between GF and PA6. The preliminary research indicates that samples with 10 wt.% HPCTP concentration can achieve the best mechanical properties and a satisfied flame-retardant grade (UL 94 HB). However, with the incorporation of 15 wt.% HPCTP into samples, the optimum flame-retardant grade (UL 94 V2) and the reduced mechanical performance are obtained. Furthermore, a dual flame retardancy mechanism in both gaseous and condensed phases of samples is further elucidated. This study can provide a reference for scenarios with high requirements for the flame-retardant and mechanical properties, such as high-speed rail and aircraft.