What are the flame retardant mechanisms of flame retardants?

The mechanism of action of flame retardants is relatively complicated and is not yet fully understood. It is generally believed that halogen compounds undergo a decomposition reaction when exposed to fire and heat, and the decomposed halogen ions react with polymer compounds to produce hydrogen halide. The latter reacts with a large number of active hydroxyl radicals (HO·) that proliferate during the combustion of polymer compounds, reducing its concentration and slowing the burning speed until the flame is extinguished. Among the halogens, bromine has greater flame retardancy than chlorine. The role of phosphorus-containing flame retardants is that they form metaphosphoric acid when they burn, and metaphosphoric acid polymerizes into a very stable multimeric state, which becomes a protective layer of plastics and isolates oxygen.
Flame retardants exert their flame retardant effects through several mechanisms, such as endothermic effect, covering effect, inhibition of chain reaction, and suffocation of non-combustible gases. Most flame retardants achieve the purpose of flame retardancy through the joint action of several mechanisms.

1. Heat absorption
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 lowered, radiating to the combustion surface and acting on the vaporized The heat of pyrolysis of combustible molecules into free radicals will decrease, and the combustion reaction will be suppressed to a certain extent. Under high temperature conditions, the flame retardant undergoes a strong endothermic reaction, absorbs part of the heat released by combustion, reduces the surface temperature of combustibles, effectively inhibits the generation of combustible 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 type of flame retardant gives full play to its large heat absorption characteristics when combined with water vapor, and improves its own flame retardant ability.
2. Covering
After adding the flame retardant to the combustible material, the flame retardant can form a glassy or stable foam covering layer at high temperature, which can insulate oxygen, have the function of heat insulation, oxygen insulation, and prevent combustible gas from escaping, so as to achieve flame retardancy purpose. For example, organic phosphorus flame retardants can produce cross-linked solid substances or carbonized layers with a more stable structure when heated. The formation of the carbonized layer can prevent the polymer from further pyrolysis, and on the other hand, it can prevent the thermal decomposition products inside it from entering the gas phase to participate in the combustion process.
3. Inhibit chain reaction
According to the combustion chain reaction theory, free radicals are needed to maintain combustion. Flame retardants can act on the gas phase combustion zone to capture free radicals in the combustion reaction, thereby preventing the spread of flames, reducing the flame density in the combustion zone, and ultimately reducing the combustion reaction speed until it stops. For example, halogen-containing flame retardants have the same or similar evaporation temperature as the decomposition temperature of the polymer. When the polymer is decomposed by heat, the flame retardant will also volatilize at the same time. At this time, the halogen-containing flame retardant and the thermal decomposition product are in the gas phase combustion zone at the same time, and the halogen can capture the free radicals in the combustion reaction and interfere with the combustion chain reaction.
4. Non-combustible gas asphyxiating effect
Flame retardants decompose incombustible gas when heated, and dilute the concentration of combustible gas from combustibles to below the lower combustion limit. At the same time, it also dilutes the oxygen concentration in the combustion zone, prevents the combustion from continuing, and achieves a flame retardant effect.