The mechanism of action of flame retardants is more complex and not very clear. It is generally believed that a halogen compound undergoes a decomposition reaction when exposed to fire, and the decomposed halogen ion reacts with a polymer compound to generate a hydrogen halide. Flame retardants exert their flame retardant effects through several mechanisms, such as endothermic effect, covering effect, inhibition of chain reaction, and asphyxiation of non-combustible gases. Most flame retardants achieve the purpose of flame retardancy through the joint action of several mechanisms.
    
1. Endothermic effect
The heat released by any combustion in a relatively short period of time is limited. If a part of the heat released by the fire source can be absorbed in a relatively short period of time, the flame temperature will be reduced, and the radiation will be on the combustion surface and act on the combustible molecules that have been vaporized. The heat of cracking into free radicals will be reduced, and the combustion reaction will be suppressed to a certain extent.
Under high temperature conditions, the flame retardant has a strong endothermic reaction, absorbs part of the heat released by combustion, reduces the temperature of the combustible surface, effectively inhibits the generation of flammable gases, and prevents the spread of combustion. The flame retardant mechanism of Al(OH)3 flame retardant is to increase the heat capacity of the polymer, so that it can absorb more heat before reaching the thermal decomposition temperature, thereby improving its flame retardant performance. This kind of flame retardant gives full play to its characteristics of a large number of heat absorption when combined with water vapor, and improves its own flame retardant ability.
2. The role of coverage
After the flame retardant is added to the combustible material, the flame retardant can form a glassy or stable foam coating at high temperature, isolate oxygen, have heat insulation, oxygen barrier, and prevent the escape of combustible gas, so as to achieve the purpose of flame retardant. For example, when the organic phosphorus flame retardant is heated, it can produce a cross-linked solid substance or a carbonized layer with a more stable structure. On the one hand, the formation of the carbonized layer can prevent the further pyrolysis of the polymer, on the other hand, it can prevent the internal thermal decomposition organisms from entering the gas phase to participate in the combustion process.