The mechanism of action of flame retardants is complex and not well understood. It is generally believed that halogen compounds undergo decomposition reaction when heated by fire, and the decomposed halogen ions react with polymer compounds to produce hydrogen halide. The latter is associated with a large number of active hydroxyl free radicals (HO·) proliferated during the combustion of polymer compounds reaction, causing it to decrease its concentration and slow down the rate of combustion until the flame is extinguished. Among halogens, bromine has a greater flame retardant effect than chlorine. The function of phosphorus-containing flame retardants is that when they burn, they form metaphosphoric acid, which polymerizes into a very stable polymeric state, which becomes a protective layer for plastics and isolates oxygen.
Flame retardants exert their flame retardant effects through several mechanisms, such as endothermy, covering, inhibition of chain reaction, and asphyxiation of non-combustible gases. Most flame retardants are flame retardant through the joint action of several mechanisms.
1. Endothermy
If a part of the heat emitted by the ignition source can be absorbed in a short time, the flame temperature will be reduced, the heat radiating to the combustion surface and acting on the pyrolysis of combustible molecules that have been vaporized into free radicals will be reduced, and the combustion reaction will be inhibited to a certain extent. Under high temperature conditions, the flame retardant undergoes a strong endothermic reaction, which absorbs part of the heat released by combustion, reduces the temperature of the surface of the combustible, 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 improve the flame retardant performance by increasing the heat capacity of the polymer so that it can absorb more heat before reaching the thermal decomposition temperature. This type of flame retardant gives full play to its characteristics of absorbing a large amount of heat when combined with water vapor, and improves its own flame retardant ability.
2. Coverage
After adding flame retardant to the combustible material, the flame retardant can form a glass-like or stable foam covering layer at high temperature, isolate oxygen, and have the functions of heat insulation, oxygen insulation, and preventing combustible gas from escaping outward, so as to achieve the purpose of flame retardant. For example, organophosphorus flame retardants can produce cross-linked solid substances or carbonized layers with more stable structures when heated. On the one hand, the formation of the carbonization 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 chain reaction theory of combustion, what is needed to sustain combustion are free radicals. Flame retardants can act on the gas-phase combustion zone to capture free radicals in the combustion reaction, thereby preventing the spread of the flame, reducing the flame density in the combustion zone, and finally reducing the combustion reaction speed until termination. For example, halogen-containing flame retardants, its evaporation temperature is the same or similar to the polymer decomposition temperature, and when the polymer is decomposed by heat, the flame retardant also volatilizes 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 chain reaction of combustion.
4. Non-flammable gas suffocation
When the flame retardant is heated, it decomposes non-combustible gas, and dilutes the concentration of combustible gas decomposed by combustible materials below the lower combustion limit. At the same time, it also has the effect of diluting the oxygen concentration in the combustion zone, preventing the continuation of combustion and achieving the effect of flame retardant.
