Flame retardancy can be achieved by reducing the generation of flammable gases, converting reactive free radicals that cause combustion into inactive free radicals to make them non-flammable, isolating flammable materials from oxygen, and lowering the temperature of the combustion zone. The flame retardant principle of flame-retardant nylon includes endothermic cooling, a covering and isolation mechanism, termination of free radical reactions, and dilution of flammable gases. Typically, flame retardants are added and blended with polyamide during spinning to achieve a limiting oxygen index greater than 27%, thus exhibiting flame retardancy. Nylon has a decomposition temperature of 310–380℃, an auto-ignition temperature of 424℃, and an ignition temperature of 530℃. Because the amide bonds in nylon macromolecules are relatively reactive, they easily react chemically with some flame retardants at the melting temperature (215–220℃). Some additives produce hydrogen halides, which can promote the degradation of polyamide. Therefore, the selection of flame retardants is crucial. Commonly used flame retardants include magnesium hydroxide, antimony trioxide, chlorinated polyethylene, and synergists such as oxides of Ca, Mo, and Zn, ammonium polyphosphate, and melamine, among other nitrogen-containing flame retardants. Multi-component flame retardants containing halogens, antimony, and boron can also be used to improve flame retardancy. Brominated flame retardants (such as decabromodiphenyl ether) often work synergistically with antimony compounds, retardant through endothermic and gas dilution mechanisms, but their use is limited due to environmental controversies (potential for dioxin formation). Halogen-free flame retardants mainly include red phosphorus (low cost, high efficiency, but with safety hazards and color restrictions), melamine salts (such as MCA and MPP, low toxicity, high efficiency, enabling PA to achieve UL94 V-0 rating), and phosphonates.
The amount of flame retardant added varies depending on the type of PA, generally ranging from 5% to 15%. In recent years, new types of flame retardants have emerged, such as ionic liquid flame retardants for PA6/PA66, and the development of exudation-resistant flame retardant solutions for reinforced PA6/PA66. Flame retardants are often compounded (e.g., halogen-phosphorus, phosphorus-nitrogen synergy) to improve efficacy and reduce costs. Copolymer flame retardant modification achieves molecular-level dispersion through chemical bonding, exhibiting good flame retardant stability and representing an important research direction. Under environmental trends, halogen-free flame retardants (such as phosphorus-based, nitrogen-based, silicon-based, and nano-flame retardants) have become mainstream. Flame-retardant nylon is widely used in carpets, interior decoration materials, military uniforms, tents, etc.
