Most products are made of composite plastics, but some products require flame retardancy. What are the principles of flame retardant plastics?
1) Endothermic action
The heat released by any combustion in a short time is limited. If a part of the heat released by the fire source can be absorbed in a short time, the flame temperature will be reduced, and the heat radiated to the combustion surface and acting on the cracking of gasified combustible molecules into free radicals will be reduced, and the combustion reaction will be restrained to a certain extent.
At high temperature, the flame retardant has a strong endothermic reaction, absorbing part of the heat released from combustion, reducing the surface temperature of combustible materials, effectively inhibiting the formation 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 can absorb more heat before reaching the thermal decomposition temperature, so as to improve its flame retardant performance. This kind of flame retardant can give full play to its heat absorption characteristics when combined with steam, and improve its flame retardant ability.
2) Covering effect
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, organic phosphorus flame retardants can produce more stable cross-linked solid substances or carbonized layers when heated.
On the one hand, the formation of carbonization layer can prevent the further pyrolysis of polymer, on the other hand, it can prevent the internal thermal decomposition products from entering the gas phase to participate in the combustion process.
3) Inhibitory chain reaction
According to the chain reaction theory of combustion, free radicals are needed to maintain combustion. The flame retardant can act on the gas phase combustion zone to capture the free radicals in the combustion reaction, so as to prevent the spread of flame, reduce the flame density in the combustion zone, and finally reduce the combustion reaction speed until the end.
For example, the evaporation temperature of halogen-containing flame retardants is the same or similar to that of polymer decomposition temperature. When the polymer is heated to decompose, the flame retardant also volatilizes. At this time, the halogen-containing flame retardant and thermal decomposition products are in the gas-phase combustion zone at the same time, and the halogen can capture the free radicals in the combustion reaction, thus preventing the spread of the flame, reducing the flame density in the combustion zone, and finally reducing the combustion reaction speed until the end.
4) Asphyxiation of non combustible gas
When the flame retardant is heated, the non combustible gas will be decomposed, and the concentration of the combustible gas decomposed from the combustible material will be diluted to below the lower combustion limit. At the same time, it can also dilute the oxygen concentration in the combustion zone, prevent the combustion from going on, and achieve the flame retardant effect.