The mechanism of flame retardant is complex and unclear. It is generally believed that halogen compounds decompose when heated under fire, and halogen ions decomposed react with polymer compounds to produce hydrogen halide. The latter reacts with the active hydroxyl radicals (HO.) which proliferate in the combustion process of macromolecule compounds, reducing their concentration and burning speed until the flame extinguishes. In halogens, bromine is more flame retardant than chlorine. The function of phosphorus-containing flame retardants is that they form metaphosphoric acid when burning, and metaphosphoric acid polymerizes into a very stable polymer, which becomes the protective layer of plastics and isolates oxygen. [1]

Flame retardants play their flame retardant role through several mechanisms, such as endothermic effect, covering effect, chain reaction inhibition, asphyxiation of non-combustible gases and so on. Most flame retardants achieve their flame retardant purposes through the interaction of several mechanisms.

1. Endothermic action

The heat released by any combustion in a shorter period of time is limited. If a part of the heat released by the fire source can be absorbed in a shorter period of time, the flame temperature will be lowered. The heat radiated to the combustion surface and acting on cracking the gasified combustible molecules into free radicals will be reduced, and the combustion reaction will be obtained. A certain degree of inhibition. 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 formation of combustible gases and prevents the spread of combustion. The flame retardant mechanism of Al(OH)3 flame retardant is to improve the flame retardant performance by increasing the heat capacity of the polymer to absorb more heat before reaching the thermal decomposition temperature. This kind of flame retardant can give full play to its heat absorption characteristics when combined with water vapor, and improve its own flame retardant ability.

2. Coverage

After adding flame retardants in combustible materials, flame retardant can form glass or stable foam covering at high temperature, and isolate oxygen, which has the function of heat insulation, oxygen isolation and preventing flammable gas from escaping outward, so as to achieve the purpose of flame retardancy. For example, when organophosphorus flame retardants are heated, they can produce more stable cross-linked solid substances or carbide layers. The formation of carbide layer can prevent the further pyrolysis of polymer on the one hand, and prevent the pyrolysis products from entering the gas phase to participate in the combustion process on the other hand.

3. Inhibition chain reaction.

According to the chain reaction theory of combustion, free radicals are needed to maintain combustion. Flame retardants can act on the gas phase combustion zone to capture free radicals in combustion reaction, thus preventing flame propagation, reducing the flame density in the combustion zone, and ultimately reducing the combustion reaction rate until termination. For example, the evaporation temperature and decomposition temperature of halogen-containing flame retardants are the same or similar. When the polymer is decomposed by heat, the flame retardants also volatilize at the same time. When halogen-containing flame retardants and thermal decomposition products are in the gas phase combustion zone, halogen can capture the free radicals in the combustion reaction and interfere with the chain reaction of combustion.

4. Asphyxiation of non-combustible gases

When the flame retardant is heated, it decomposes the non-combustible gas and dilutes the concentration of the combustible gas decomposed from the combustible matter to below the lower limit of combustion. At the same time, it can dilute the oxygen concentration in the combustion zone, prevent the combustion from proceeding and achieve flame retardant effect.