Recent progress in flame retardant strategies for polyamides

Polyamides (PAs) are widely used engineering plastics because their amide-containing backbones and semi-crystalline structures combine mechanical strength, toughness, wear resistance, chemical resistance, and processability. Their flammability, melt dripping, high heat release and toxic-gas generation nevertheless restrict use in fire-safety-critical applications. This review summarizes recent flame-retardant strategies for aliphatic, semi-aromatic and aromatic PAs, with emphasis on aliphatic PA systems that dominate both academic research and industrial applications. Additive approaches are compared with nano-sized synergists, reactive/copolymer FRs and flame-retardant finishing. The mechanisms responsible for gas-phase radical quenching, condensed-phase charring, barrier formation, melt-dripping control and smoke/toxicity regulation are discussed in relation to PA structure, processing stability and end-use requirements. Nano-sized synergists are treated as supporting components rather than primary FRs, because their main role is to reinforce char, improve dispersion and reduce heat/mass transfer at low loadings. For semi-aromatic and aromatic PAs, the review identifies challenges associated with high processing temperatures, compatibility, mechanical retention and cost. Market requirements, including regulatory compliance, comparative tracking index and glow-wire performance, moisture resistance, toxicity, supply-chain stability and circularity, are further summarized. Finally, future directions are proposed toward integrated formulation design that balances fire performance with processability, durability, mechanical/electrical properties, environmental impact and recyclability.