Flame retardants are chemical substances added to plastics, textiles, wood, resins, and coatings to make them less flammable or to prevent the spread of flames. They are generally classified into reactive, additive, halogenated, and halogen-free flame retardants. Most are compounds containing nitrogen, phosphorus, antimony, iodine, chlorine, bromine, boron, aluminum, silicon, or platinum. The most commonly used and important are compounds containing phosphorus, bromine, chlorine, iodine, and aluminum. Examples of halogen-free flame retardants include aluminum hypophosphite, diethylaluminum hypophosphite, ammonium polyphosphate, aluminum hydroxide, and red phosphorus. Halogenated flame retardants include decabromodiphenyl ethane, octabromoether, and tetrabromobisphenol A. The mechanism of action of flame retardants can be explained from the following aspects:

1. At combustion temperatures, flame retardants form a non-combustible protective film that covers the material, isolating it from air and thus achieving flame retardancy. On the one hand, some flame retardants decompose at combustion temperatures into a non-volatile, non-oxidizing glassy film that covers the surface of materials, isolating them from air (or oxygen) and reflecting heat away or reducing thermal conductivity, thus achieving a flame-retardant effect. Examples include halogen-free PP flame retardants, halogenated phosphorus, boric acid, and hydrated borates. On the other hand, some flame retardants dehydrate and carbonize the material surface at combustion temperatures, forming a porous, heat-insulating coke layer, thereby preventing heat conduction and achieving a flame-retardant effect. Examples include aluminum hypophosphite, red phosphorus-treated cellulose, and ammonium salt flame retardants.

2. The decomposition products of some flame retardants readily react with active free radicals, reducing the concentration of certain free radicals and disrupting the crucial chain reactions in combustion. For example, the non-flammable gas HX produced by the decomposition of halogenated flame retardants at combustion temperatures can react with the active free radical HO. during combustion, breaking the free radical chain reaction and achieving flame retardancy.

3. Some flame retardants can immediately decompose at moderate temperatures to release non-flammable gases, diluting flammable gases and preventing combustion. For example, organic halogenated flame retardants release the non-flammable gas HX upon heating. Halogenated flame retardants such as bromine and chlorine decompose at high temperatures to generate HBr/HCl, which captures combustion-active free radicals (HO・, H・, O・), terminating the chain reaction; simultaneously, HX is a non-flammable heavy gas that covers and dilutes O₂ on the surface.

4. Some flame retardants decompose violently at high temperatures, absorbing a large amount of heat energy, lowering the ambient temperature, and thus preventing further combustion, such as aluminum hydroxide and magnesium hydroxide.

Flame retardants block the combustion cycle from multiple dimensions through five mechanisms: gas-phase chain scission, condensed-phase char formation, endothermic cooling, dilution and inertness, and synergistic enhancement. Halogen-free (phosphorus, nitrogen, silicon, metal hydroxides) is the mainstream direction, taking into account the needs of flame retardancy, environmental protection, and smoke suppression.