Development of flame retardant coatings containing hexaphenoxycyclotriphosphazene and expandable graphite for carbon fibre-reinforced polyamide 6 composites

• HPCTP had no effect on T g , T m , or monomer conversion, but increased crystalline fraction and enhanced thermal stability. • HPCTP with EG achieved synergistic effects due to different mechanisms. • HPCTP and EG reduced the pHRR by 68 % and the THR by 29 %. • 0.5 mm PA6/3P%HPCTP/3 %EG coating reduced the composite pHRR by 33 % and THR by 37 %. • FRs formed a stable charred layer on the composites and increased the residual mass. The increasing importance of thermoplastic composites, driven by their enhanced recyclability and production efficiency, has attracted interest in continuous fibre-reinforced thermoplastic. Polyamide 6 (PA6), synthesised via anionic ring-opening polymerisation, is particularly relevant, however, the flammability of PA6 poses significant challenges and a critical concern for their structural application, necessitating effective flame retardancy measures. This study investigates the development of flame retardant coatings for carbon fibre-reinforced PA6 composites, employing hexaphenoxycyclotriphosphazene (HPCTP) and expandable graphite (EG), and evaluates their efficacy in improving the flammability properties of the composites through their combined modes of action. We investigated the impact of FRs on glass transition temperature, crystallinity, thermal stability, monomer conversion and flammability properties. The best-performing formulations (PA6/3P%HPCTP/3%EG and PA6/3P%HPCTP/4%EG) were applied to the surface of carbon fibre-reinforced PA6 composites by in-mould coating. Due to the synergistic effect of HPCTP and EG, the coatings containing 3P% HPCTP and 3 % EG reduced the maximum heat release by 33 % and the total heat release by 37 %.