In many occasions, materials are required to have flame retardancy. The most commonly used occasions are electronic appliances. Automobile industry also has flame retardant requirements, but they are generally low. Flame retardant can be achieved by adding flame retardants. Most plastics are flammable. With wide application of plastics in construction, furniture, transportation, aviation, aerospace, electrical appliances, etc., improving flame retardancy of plastics has become a very urgent topic.

 

Flame retardants, also known as flame retardants, fire retardants or fire retardants, are functional additives that impart flame retardancy to flammable polymers; most of them are compounds of VA (phosphorus), VIIA (bromine, chlorine) and IIIA (antimony, aluminum) elements in periodic table.
Molybdenum compounds, tin compounds, and iron compounds with smoke-suppressing effects also belong to category of flame retardants. They are mainly used for plastics with flame retardant requirements to delay or prevent burning of plastics, especially polymer plastics, to increase ignition time, self-extinguishing, and difficult to ignite.

Heat released by any combustion in a relatively short period of time is limited. If a part of heat released by fire source can be absorbed in a relatively short period of time, flame temperature will be lowered, radiating to combustion surface and acting on vaporized gas. Heat of pyrolysis of combustible molecules into free radicals will be reduced, and combustion reaction will be inhibited to a certain extent.
Under high temperature conditions, flame retardant undergoes a strong endothermic reaction, absorbing part of heat released by combustion, reducing surface temperature of combustibles, effectively inhibiting generation of combustible gases, and preventing spread of combustion. Flame retardant mechanism of Al(OH)3 flame retardant is to increase heat capacity of polymer so that it can absorb more heat before reaching thermal decomposition temperature, thereby improving its flame retardant performance. This kind of flame retardant gives full play to its characteristic of absorbing a large amount of heat when combined with water vapor, and improves its own flame retardant ability.

After adding a flame retardant to combustible materials, flame retardant can form a glass-like or stable foam covering layer at high temperatures to isolate oxygen, have function of heat insulation, oxygen insulation, prevent combustible gas from escaping outward, so as to achieve purpose of flame retardancy.
For example, when organic phosphorus-inhibiting flame retardant is heated, it can produce a cross-linked solid substance or a carbonized layer with a more stable structure.
Formation of carbonized layer can prevent polymer from further pyrolysis, and on the other hand, it can prevent thermal decomposition products inside it from entering gas phase to participate in combustion process.

According to combustion chain reaction theory, free radicals are needed to maintain combustion. Flame retardant can act on gas phase combustion zone to capture free radicals in combustion reaction, thereby preventing spread of flames, reducing flame density in combustion zone, and ultimately reducing combustion reaction speed until it stops.
For example, a halogen-containing flame retardant, its evaporation temperature is same or similar to decomposition temperature of polymer. When polymer is decomposed by heat, flame retardant will also volatilize at the same time. At this time, halogen-containing flame retardant and thermal decomposition products are in gas phase combustion zone at the same time, halogen can capture free radicals in combustion reaction, thereby preventing spread of flame, reducing flame density in combustion zone, and finally reducing combustion reaction speed until it stops. .

Flame retardants decompose incombustible gas when heated, and dilute concentration of combustible gas from combustibles to below lower limit of combustion. At the same time, it also dilutes oxygen concentration in combustion zone, prevents combustion from continuing, and achieves effect of flame retardancy.