The mechanism of action of flame retardants is relatively complex and not yet fully understood. It is generally believed that the halogen compound undergoes a decomposition reaction when exposed to fire and heat, and the decomposed halogen ion reacts with the polymer compound to generate hydrogen halide. The latter reacts with a large number of active hydroxyl radicals (HO ·) that multiply during the combustion of the polymer compound, reducing its concentration and slowing the combustion rate until the flame is extinguished. Among halogens, bromine has a greater flame retardancy than chlorine. The role of phosphorus-containing flame retardants is that they form metaphosphoric acid when they burn. Metaphosphoric acid polymerizes into a very stable polymer state, which becomes a protective layer for plastics and isolates oxygen.

Flame retardants exert their flame retardant effects through several mechanisms, such as endothermic effect, covering effect, chain reaction inhibition, and asphyxiating effect of non-combustible gases. Most flame retardants achieve flame retardant purposes through a number of mechanisms.

1. Endothermic effect

The amount of heat emitted by any combustion in a short period of time is limited. If a part of the heat emitted by the ignition source can be absorbed in a short period of time, the flame temperature will decrease, radiate to the combustion surface and act on the gasified gas. The heat of flammable molecules to be broken down into free radicals will be reduced, and the combustion reaction will be suppressed to a certain extent. Under high temperature conditions, the flame retardant undergoes a strong endothermic reaction, absorbing part of the heat released by combustion, reducing the temperature of the surface of the combustible material, effectively suppressing the generation of combustible gases, and preventing 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 absorbs more heat before reaching the thermal decomposition temperature, thereby improving its flame retardancy. This type of flame retardant takes full advantage of its large amount of heat absorption when combined with water vapor to improve its own flame retardant ability.

2. Coverage

After the flame retardant is added to the combustible material, the flame retardant can form a glassy or stable foam cover at high temperature to isolate oxygen, and has the functions of heat insulation, oxygen barrier, and prevention of flammable gas from escaping to achieve flame retardancy purpose. For example, organophosphorus flame retardants can produce cross-linked solid substances or carbonized layers with more stable structures when heated. The formation of the carbonized layer can prevent the polymer from further pyrolysis on the one hand, and prevent the internal thermal decomposition products from entering the gas phase to participate in the combustion process.

3.Inhibition of chain reaction

According to the theory of combustion chain reaction, free radicals are required to maintain combustion. The flame retardant can act on the gas phase combustion zone to capture the free radicals in the combustion reaction, thereby preventing the flame from propagating, reducing the flame density in the combustion zone, and ultimately reducing the combustion reaction rate until termination. For example, the halogenated flame retardant has the same or similar evaporation temperature as the polymer decomposition temperature. When the polymer is thermally decomposed, the flame retardant is also volatilized 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 progress of the combustion chain reaction.

4. Asphyxiating effect of non-combustible gas

The flame retardant decomposes non-combustible gas when heated, and dilutes the concentration of combustible gas decomposed by the combustible material to below the lower combustion limit. At the same time, it also has a dilution effect on the oxygen concentration in the combustion zone, preventing the combustion from continuing and achieving a flame retardant effect.