The main types and functions of common flame retardants

Flame retardants are functional additives that endow flammable polymers with flame retardancy. They are mainly designed for the flame retardancy of polymer materials. Gas asphyxiation, etc. Most flame retardants achieve the purpose of flame retardancy through the joint action of several mechanisms.

endothermic effect

The heat released by any combustion in a short period of time is limited. If a part of the heat released by the fire source can be absorbed in a short period of time, the temperature of the flame will decrease, radiate to the combustion surface and act on the gasified gas. The heat of flammable molecules cracked into free radicals will be reduced, and the combustion reaction will be inhibited to a certain extent.
Under high temperature conditions, the flame retardant has a strong endothermic reaction, absorbs part of the heat released by combustion, reduces the surface temperature of combustibles, 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 increase the heat capacity of the polymer so that it absorbs more heat before reaching the 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.

Coverage

After adding flame retardants to combustible materials, the flame retardants can form a glassy or stable foam covering layer at high temperature, which can isolate oxygen and have the functions of heat insulation, oxygen isolation, and prevention of combustible gas from escaping, so as to achieve flame retardancy Purpose. For example, organophosphorus flame retardants can produce cross-linked solid substances or carbonized layers with a more stable structure when heated. On the one hand, the formation of the carbonized layer can prevent further pyrolysis of the polymer, and on the other hand, it can prevent the internal thermal decomposition products from entering the gas phase to participate in the combustion process.

chain reaction of inhibition

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 it terminates. Such as halogen-containing flame retardant, its evaporation temperature is the same or close to the decomposition temperature of the polymer. When the polymer is decomposed by heat, 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 chain reaction of combustion.

Asphyxiation of non-combustible gases

When the flame retardant is heated, it decomposes into non-combustible gas, and dilutes the concentration of combustible gas decomposed from combustibles to below the lower limit of combustion. 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 retardancy.
The vast majority of polymers are composed of hydrocarbons and other elements, which are highly flammable. During the combustion process, it is a complex chain reaction process of free radicals, which will release a large amount of heat energy, causing direct damage and rapidly increasing the intensity of the fire.

With the continuous progress of society and the continuous development of science, the application range of various polymer materials in various fields has continued to expand, but the danger and hazard of fire have increased significantly.
The flame retardant treatment of polymer materials is one of the important measures to reduce fires. How to improve the flame retardancy of polymer materials has become one of the technical problems that scientists around the world need to solve urgently. major bottleneck.
The development of new polymer flame retardant additives to improve flame retardancy has become an urgent task in the development of polymer materials.

Inorganic flame retardant

The flame retardant effect of inorganic flame retardants is mainly to use the heat storage and thermal conductivity properties of large specific volume fillers, so that the material is not easy to reach the decomposition temperature, or the flame retardant is decomposed by heat to absorb heat, thereby alleviating or terminating the heating process of the main material. Its flame retardant mechanism is to release crystal water when heated, evaporate, decompose and release water vapor.
This reaction process needs to absorb a large amount of combustion heat energy, thereby greatly reducing the surface temperature of the material, so that the probability of thermal decomposition and combustion of the polymer material is greatly reduced.
Halogen flame retardant

Halogenated flame retardants are currently one of the most produced organic flame retardants in the world, and the most widely used halogenated flame retardants are bromine-containing and chlorine-containing flame retardants.
Most of the halogenated flame retardants are organic and have good compatibility with the main polymer material. As a flame retardant additive, the halogenated flame retardant will not have an essential impact on the physical and chemical properties of the polymer material itself. In addition, the halogenated flame retardant The flame retardant can be added in a small amount, but it can achieve an extremely excellent flame retardant effect.
Bromine-containing halogenated flame retardants include aliphatic, alicyclic, and aromatic bromine-containing compounds. Common ones include decabromodiphenyl ether, decabromodiphenylethane, and tetrabromobisphenol A. Chlorine-containing flame retardants The main agent is chlorinated paraffin.
The flame retardant mechanism of bromine and chlorine is similar: at high temperature, the carbon-halogen bonds in halogen flame retardants can be broken, releasing halogen free radicals and effectively capturing free active free radicals produced by thermal degradation of polymer materials, which can Effectively reduce the concentration of free radicals, thereby alleviating or terminating the free radical chain reaction of combustion.

In addition, the hydrogen halide released by the decomposition of the halogen flame retardant has the property of non-combustibility, effectively blocks oxygen, and inhibits the progress of the combustion reaction.
However, once the polymer material added with halogen flame retardant is burned, a large amount of hydrogen halide gas will be produced, which is toxic and corrosive, and it is also very easy to absorb moisture in the air to form a highly corrosive Hydrohalic acid, accompanied by a large amount of smoke, these smoke, toxic and corrosive gases will endanger human health, but also bring great obstacles to fire fighting, escape and recovery work.
Treated AI(OH)3 Flame Retardant

Aluminum hydroxide is also called alumina trihydrate (ATH), and its molecular formula is Al(OH)3. It is one of the earliest inorganic flame retardants. It can produce synergistic effects with various substances, and it is non-toxic and non-corrosive sex.
At present, the use of aluminum hydroxide flame retardants accounts for more than 80% of the total inorganic flame retardants, and is widely used in various polymer plastic products. After adding aluminum hydroxide to the polymer material, the concentration of flammable polymer can be reduced.
When the polymer material is heated (around 250°C), aluminum hydroxide undergoes a dehydration reaction and absorbs a large amount of heat energy, effectively inhibiting the temperature rise of the polymer material. At the same time, the water vapor generated by the decomposition can dilute the flammable gas and oxygen concentration generated by the combustion, and inhibit the continuous spread of the combustion.
At the same time, another metal oxide, aluminum oxide (Al2O3), which is decomposed at the same time, can catalyze the thermal crosslinking reaction of the polymer due to its high catalytic activity, thereby forming a dense carbonized film on the surface of the polymer. It can effectively slow down the heat transfer during combustion, thus playing a flame retardant role.
Alumina can also adsorb particles and play a role in suppressing smoke. In general, the higher the content of aluminum hydroxide, the better the flame retardant effect, but too much filling will greatly reduce the strength and other properties of the polymer material.
Aluminum hydroxide also has another disadvantage, that is, the decomposition temperature is low, and dehydration reaction can occur between 245 ° C and 320 ° C, so the addition of flame retardant strong alumina also limits the processing temperature of polymer materials

Phosphorus Flame Retardant

According to the nature and composition of phosphorus-based flame retardants, they can be divided into inorganic phosphorus-based flame retardants and organic phosphorus-based flame retardants.
Among them, inorganic phosphorus flame retardants include red phosphorus, ammonium phosphate and ammonium polyphosphate, etc., and organic phosphorus flame retardants include phosphoric acid esters, phosphite esters, etc. Phosphorus-based flame retardants are also a type of highly efficient, stable, and widely used flame retardants. The flame retardant mechanism is mainly to form an isolation film to achieve the flame retardant effect.
There are two different methods for the formation of the isolation membrane:
(1) Flame retardant effect on oxygen-containing polymers: The thermal degradation products of flame retardants are used to promote rapid dehydration and carbonization of the polymer surface, thereby forming a carbonized layer. Since elemental carbon does not perform evaporative combustion and decomposition combustion that generate flames, it has a flame-retardant effect.
The chemical reaction that takes place internally is the thermal decomposition of phosphorus-containing compounds and the end product is polymetaphosphoric acid, which is a strong dehydrating agent.
(2) Phosphorus-based flame retardants decompose into non-volatile glass-like substances at the combustion temperature, which can be wrapped on the surface of the polymer, and this dense protective layer acts as an isolation layer.
The stage where organophosphorus flame retardants play a role is mainly in the initial stage of fire in the decomposition stage of polymer materials.

It can promote the dehydration and carbonization of polymer materials, so that the polymer materials cannot produce flammable gases, and because the non-volatile phosphorus compound acts as a coagulant, the carbonized materials form a protective carbon film to isolate the outside air and heat.

Silicon flame retardant

Silicon-based flame retardants include inorganic silicon and organic silicon, of which inorganic silicon mainly includes silicon dioxide, silica gel, silicate and talcum powder, etc., such flame retardants are often used as fillers; organic silicon flame retardant is a new type of Halogen-free flame retardant is also a charcoal-forming smoke suppressant, mainly referring to silicone Resin, polysiloxane (silicone oil, silicone resin, silicone rubber and various siloxane copolymers, etc.), polysilane, etc., among which the development The most rapid are polysiloxanes.
The flame retardant mechanism is mainly reflected in the condensed phase flame retardant mechanism, that is, the flame retardant effect is realized by generating a cracked carbon layer and improving the oxidation resistance of the carbon layer.
After the silicone flame retardant is added to the polymer material, most of the silicone flame retardant will migrate to the surface of the material, react at high temperature, and form a carbon-containing silicate layer on the surface of the polymer, which has the function of delaying or preventing the escape of flammable gases. out and the generation of free radicals.
At the same time, the flame retardant will also promote the carbonization of polymers, thereby reducing the degradation rate of polymers and making them less prone to thermal decomposition at high temperatures.
On the other hand, silicon-based flame retardants will also undergo thermal decomposition reactions when heated. This process needs to absorb a large amount of heat, which can achieve the effect of slowing down or stopping the temperature rise of flame-retardant materials.