Reaction Mechanism of 5 Commonly Used Flame Retardants

Reaction Mechanism of 5 Commonly Used Flame Retardants

Flame retardants are functional additives that confer flame retardancy to flammable polymers. They are mainly designed for the flame retardancy of polymer materials and play their flame retardant roles through a number of mechanisms, such as heat absorption, coverage, inhibition of chain reaction, and asphyxiation of incombustible gas. Most of the flame retardant is through a number of mechanisms work together to achieve the purpose of flame retardant.

The role of flame retardant

1. Heat absorption

Any combustion in a relatively short period of time the heat released is limited, if the fire source can be absorbed in a relatively short period of time part of the heat released, then the flame temperature will be reduced, radiation to the combustion surface and the role of combustible molecules that have been vaporized to crack into free radicals will reduce the amount of heat, combustion reaction will be inhibited to a certain extent.

Under high temperature conditions, the flame retardant undergoes a strong heat-absorbing reaction, absorbs part of the heat released by combustion, reduces the temperature of the surface of the combustible material, effectively inhibits the generation 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, so that it absorbs more heat before reaching the thermal decomposition temperature. This kind of flame retardant gives full play to its property of absorbing large amount of heat when combining with water vapor, so as to improve its own flame retardant ability.

2. Covering effect

After adding flame retardants into combustible materials, the flame retardants can form a glassy or stable foam cover layer at high temperature to isolate the oxygen, which has the function of heat insulation, oxygen insulation and preventing the combustible gases from escaping to the outside, so as to achieve the purpose of flame retardancy. Such as organophosphorus flame retardants can produce a more stable structure when heated cross-linked solid material or carbonized layer. The formation of carbonized layer on the one hand can prevent further pyrolysis of polymers, on the other hand, can prevent its internal thermal decomposition of products into the gas phase to participate in the combustion process.

3. Inhibition of chain reaction

According to the chain reaction theory of combustion, free radicals are required to maintain combustion. Flame retardants can act in the gas-phase combustion zone to capture the free radicals in the combustion reaction, thus preventing the propagation of the flame, so that the flame density in the combustion zone decreases, and ultimately, the combustion reaction rate decreases until termination. Such as halogen-containing flame retardants, its evaporation temperature and polymer decomposition temperature is the same or similar, when the polymer thermal decomposition, the flame retardant is also volatilized at the same time. At this time, halogen-containing flame retardants and thermal decomposition products at the same time in the gas-phase combustion zone, halogen will be able to capture the free radicals in the combustion reaction, interfering with the chain reaction of combustion.

4. Asphyxiating effect of non-flammable gas

When the flame retardant is heated, it decomposes the non-flammable gas, which will dilute the concentration of combustible gases from the decomposition of combustible materials 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 combustion from continuing and achieving the effect of flame retardant.

The vast majority of polymers are composed of elements such as carbon and hydrogen, which are highly flammable, and in the combustion process is a complex chain reaction of free radicals, which will release a large amount of heat energy, causing direct damage, but also rapidly increase the fire.

5 common flame retardants flame retardant mechanism

1. Inorganic flame retardant

The flame retardant effect of inorganic flame retardant is mainly to use the heat storage and thermal conductivity of large specific volume filler, so that the material is not easy to reach the decomposition temperature, or through the thermal decomposition of the flame retardant heat absorption, so as to alleviate or terminate the warming process of the main material. The flame retardant mechanism is to release crystalline water when heated, evaporate, decompose and release water vapor. This reaction process needs to absorb a large amount of combustion heat energy, thus significantly downgrading the surface temperature of the material, so that the probability of thermal decomposition and combustion of polymer materials is greatly reduced.

2. Halogenated flame retardants

Halogenated flame retardants is currently the world's largest production of organic flame retardants, the application of more halogenated flame retardants are bromine-containing and chlorine-containing flame retardants. Most of the halogenated flame retardants are organic, and the main polymer material has good compatibility, as a flame retardant additive halogenated flame retardants will not have an intrinsic effect on the physicochemical properties of the polymer material itself, in addition, halogenated flame retardants can be satisfied with the amount of their additions is very small, but it can be achieved with excellent flame retardant effect. Halogenated flame retardants containing bromine include aliphatic, alicyclic, aromatic and other brominated compounds, such as decabromodiphenyl ether, decabromodiphenylethane and tetrabromobisphenol A. Chlorinated flame retardants are mainly chlorinated paraffins. The flame retardant mechanism of bromine and chlorine is similar: at high temperature, the carbon-halogen bond in the halogen flame retardant can be broken, releasing halogen radicals and effectively capturing the free active radicals generated by the degradation of polymer materials due to heat, which can effectively reduce the concentration of free radicals, thus alleviating or terminating the free radical chain reaction of combustion. In addition, the hydrogen halide released from the decomposition of halogenated flame retardants has the property of not being easy to burn, effectively blocking oxygen and inhibiting the combustion reaction at the same time. However, once added halogenated flame retardant polymer material combustion, will produce a large number of hydrogen halide gas, this kind of gas is toxic and corrosive, but also very easy to adsorb the moisture in the air to form a strong corrosive hydrohalic acid, accompanied by a large number of smoke, these smoke, toxic gases and corrosive gases will be hazardous to human health, but also to the extinguishing of fires, escape and recovery work has brought great obstacles.

3. Treated Al(OH)3 Flame Retardant

Aluminum hydroxide, also known as aluminum oxide trihydrate (ATH), with the molecular formula of Al(OH)3, is one of the earliest inorganic flame retardants, which can have synergistic effects with a variety of substances and is non-toxic and non-corrosive. At present, the use of aluminum hydroxide flame retardant accounts for more than 80% of the total amount of inorganic flame retardants, and is widely used in a variety of polymer plastic products. The addition of aluminum hydroxide to polymer materials reduces the concentration of flammable polymers. When the polymer material is heated (around 250℃), aluminum hydroxide undergoes dehydration reaction and absorbs a large amount of heat energy, effectively inhibiting the polymer material from heating up. At the same time, the water vapor generated by decomposition can dilute the flammable gas and oxygen concentration generated by combustion, which inhibits the continuous spread of combustion. At the same time the decomposition of another metal oxide produced by the aluminum oxide (Al2O3) due to the high catalytic activity, can catalyze the polymer thermal crosslinking reaction, thus forming a layer of dense carbonized film on the surface of the polymer, which can effectively slow down the combustion of the heat transfer, thus playing a role in flame retardant. Aluminum oxide can also adsorb particles and inhibit soot. In general, the higher the added aluminum hydroxide content, the better the flame retardant effect, but too much filling will significantly reduce the strength of the polymer material and other properties. Aluminum hydroxide also has another shortcoming, that is, the decomposition temperature is low, between 245 ~ 320 ℃ can occur dehydration reaction, so the addition of flame retardant aluminum hydroxide also limits the processing temperature of polymer materials.

4. Phosphorus flame retardant

According to the nature and composition of phosphorus flame retardants can be divided into inorganic phosphorus flame retardants and organic phosphorus flame retardants. Inorganic phosphorus flame retardants include red phosphorus, ammonium phosphate and ammonium polyphosphate, etc. Organic phosphorus flame retardants include phosphate esters, phosphite and so on. Phosphorus flame retardants are also a kind of highly efficient, stable and widely used flame retardants, and their flame retardant mechanism is mainly to form isolation film to achieve flame retardant effect.

As a typical inorganic phosphorus flame retardant, red phosphorus has a unique and complex reaction process. When red phosphorus is heated, a series of thermal decomposition reactions will occur first. In the lower temperature stage, red phosphorus begins to transform slowly, it will gradually decompose to generate phosphoric acid (H₃PO₄), the process of chemical bond breaking and reorganization releases a certain amount of heat, but at the same time, the generated phosphoric acid on the surface of the material has a key role.

Phosphoric acid is further dehydrated under the influence of high temperature and converted into a series of condensed phosphate products such as pyrophosphoric acid (H₄P₂O₇) and metaphosphoric acid (HPO₃). These condensed phosphoric acids are highly hygroscopic, and they rapidly form a glassy liquid film on the surface of the combustibles that is rich in phosphorus and oxygen, the so-called barrier film. The existence of this layer of isolation film is significant, on the one hand, it can isolate the oxygen, so that the combustion area of the oxygen supply is cut off or greatly reduced, because oxygen is essential to maintain the combustion of the elements, the lack of sufficient oxygen supply, the combustion reaction will be difficult to continue to carry out, the fire will also be suppressed.

On the other hand, this film also prevents the diffusion of flammable volatiles into the flame area. In the process of combustion, the combustible volatile components generated by the decomposition of the material can continue to enter the flame area, as for the flame to add a steady stream of “fuel”, while the glassy liquid membrane is like a barrier, effectively blocking these volatile components in the membrane, preventing them from further to support the combustion reaction.

Moreover, red phosphorus produces some phosphorus-containing reactive intermediates with free radical trapping ability, like PO・, HPO・, and other free radicals during the decomposition process. In the chain reaction of combustion, free radicals play a key role in transmitting and maintaining the combustion reaction, and these phosphorus-containing reactive intermediates can quickly react with the highly reactive free radicals (e.g., hydroxyl radical, H, OH, etc.) generated during the combustion process and convert them into relatively stable compounds, thus interrupting the chain reaction of combustion and fundamentally hindering the continuous development of combustion.

In addition, the decomposition products such as phosphoric acid formed by red phosphorus in the combustion area can also catalyze the carbonization reaction on the surface of the material, leading to the rapid formation of a relatively dense carbon layer on the surface of the material. This charcoal layer is also an excellent heat-insulating layer, which can block the further transfer of heat to the inside of the material, slowing down the speed of the internal material to reach the ignition point, and also enhances the physical barrier effect of the whole material system, and further improves the flame retardant effect together with the isolation film, which makes the red phosphorus show excellent flame retardant properties, and play an important role in many fields requiring fire protection and retardant protection.

5.Silicon flame retardant

Silicone flame retardants include inorganic silicon and silicone, of which inorganic silicon mainly includes silica, silica gel, silicate and talc, etc., which are commonly used as fillers; silicone flame retardant is a new type of halogen-free flame retardant, but also a charcoal smoke suppressant, mainly refers to silicone resins, polysiloxanes (silicone oil, silicone resins, silicone rubber and a variety of silicone copolymers, etc.), polysilanes, etc., which are the most rapidly developing. The most rapid development is polysiloxane. The flame retardant mechanism is mainly reflected in the condensed phase flame retardant mechanism, that is, through the generation of cracked carbon layer and improve the antioxidant properties of the carbon layer to achieve its flame retardant effect. After the addition of organosilicon flame retardants to polymer materials, most of the organosilicon flame retardants will migrate to the surface of the material, and react at high temperature to form a carbon-containing silicate layer on the surface of the polymer, which has the effect of slowing down or preventing the escape of flammable gases and the generation of free radicals. At the same time, the flame retardant will also promote the carbonization of the polymer, thereby reducing the degradation rate of the polymer, so that it is not easy to thermal decomposition at high temperatures. On the other hand, silicone-based flame retardants will also undergo thermal decomposition when subjected to heat, and this process requires the absorption of a large amount of heat, which can achieve the effect of slowing down or discontinuing the warming of flame retardant materials.

After discussing the reaction mechanism of the five commonly used flame retardants, we can pay special attention to the red phosphorus flame retardant products promoted by YINSU Flame Retardant Company, which specializes in providing highly efficient and environmentally friendly flame retardant solutions, and the main red phosphorus products promoted by YINSU Flame Retardant Company include:

Coated red phosphorus flame retardant masterbatch FRP-950 series: This is a kind of microcapsule coated red phosphorus as the main material of the flame retardant, provided in the form of dust-free granules, which improves the safety of storage, transportation and use.FRP-950 series flame retardant has the characteristics of low additive amount, good anti-separation performance, better mechanical properties, etc., and it is suitable for a variety of engineering plastics system, such as PA6/PA66, PE, PBT, PPO/HIPS alloys, etc. FRP-8050: This is a product developed by YINSU Flame Retardant Company for the flame retardant needs of low smoke and halogen free wires and cables. It has very small particle size and the additive amount is between 10-12%, which can provide excellent flame retardant effect. Red Phosphorus Flame Retardant FRP-750 Series: This is a highly concentrated flame retardant masterbatch containing stabilized red phosphorus and new material compatibilizer carrier, which is supplied in the form of granules, customer service has eliminated the hazardous nature of red phosphorus powder, and made the transportation and use safer and more reliable.FRP-750A series products have excellent environmental protection flame retardancy and heat resistance, and the ignition point reaches more than 300℃. A series of red phosphorus flame retardant products such as red phosphorus paste PG-50.

YINSU Flame Retardant's products meet the market demand for halogen-free flame retardants with their environmentally friendly halogen-free, low-smoke, and low-toxicity properties, while providing excellent flame retardancy and physical and mechanical properties, making them ideal for use in a wide range of resins, plastics, rubbers, coatings, and other products. Through these products, YINSU Flame Retardant demonstrates its expertise and innovation in the field of flame retardants.