Synergistic flame retardant coatings for carbon fibre-reinforced polyamide 6 composites based on expandable graphite, red phosphorus, and magnesium oxide

Continuous fibre-reinforced polyamide 6 (PA6) composites are predominantly prepared by anionic ring-opening polymerisation of low viscosity ε-caprolactam monomer. PA6 melts easily when exposed to flames, which can lead to rapid fire spread due to dripping. For this reason, the use of flame retardants is necessary. We investigated the effects of magnesium oxide (MgO), red phosphorus (RP), and expandable graphite (EG) on the glass transition temperature, crystalline fraction, and thermal stability of PA6. After investigating the flammability properties, we selected the best compositions (MgO and RP combined with EG) and applied them as coatings on carbon fibre-reinforced polyamide 6 composites. A synergistic effect was achieved when RP or MgO were used together with expandable graphite. The 0.5 mm thick coating containing 5% RP and 5% EG reduced the peak heat release rate (pHRR) of the composite by 21% and the total heat release (THR) by 28%, while 5% MgO and 5% EGES100 reduced the pHRR by 27% and the THR by 37%.