The Fire-retardant Effect of Flame Retardants And The Fire-retardant Mechanisms

The Fire-retardant Effect of Flame Retardants And The Fire-retardant Mechanisms

Flame retardants are a class of additives that can prevent the ignition of plastics or inhibit the spread of flames. Based on their method of use, they can be divided into two categories: additive-type and reactive-type: Additive-type flame retardants are mixed into the plastic during the processing of plastics and are mostly used for thermoplastics.

Reactive-type flame retardants are chemically bonded to the polymer chain as monomers during the polymer synthesis process, and are mostly used for thermosetting plastics. Some reactive-type flame retardants can also be used as additive-type flame retardants.

According to chemical structure, flame retardants can be divided into inorganic and organic classes. Many of these compounds contain halogens and phosphorus, and some contain antimony, boron, aluminum, and other elements.

I. Flame Retardant Effect of Flame Retardants

The function of flame retardants is to prevent or inhibit the rate of physical or chemical changes during the combustion of polymeric materials. Specifically, these effects are manifested in the following ways:

• Heat Absorption Effect

The role of this effect is to make it difficult for the temperature of polymeric materials to rise. For example, borax contains ten molecules of crystalline water, and the release of this crystalline water requires the absorption of 141.8 kJ/mol of heat. Due to this endothermic process, the temperature rise of the material is suppressed, thereby producing a flame retardant effect. The flame retardant effect of hydrated aluminum oxide is also due to the endothermic effect of dehydration upon heating.

Additionally, the melt droplets often produced during the pyrolysis of some thermoplastic polymers can remove reaction heat by leaving the combustion zone, which also provides a certain flame retardant effect.

• Barrier Effect

The role of the barrier effect is to form a stable protective layer at high temperatures, or to decompose and form a foam-like substance that covers the surface of the polymeric material. This layer prevents the heat generated by combustion from penetrating the interior of the material, making it difficult for the combustible gases produced by the thermal decomposition of the polymer to escape. It also acts as a barrier to air, thereby inhibiting the pyrolysis of the material and achieving the effect of flame retardancy. Compounds such as phosphate esters and intumescent fireproof coatings can function according to this mechanism.

• Dilution Effect

Substances of this type can produce a large amount of non-combustible gases when they decompose upon heating. This dilutes the combustible gases produced by the polymeric material and the oxygen in the air to a level below the combustible concentration range, thus preventing the ignition and combustion of the polymeric material.

Gases that can act as diluting agents include CO2, NH3, HCl, and H2O, among others. Phosphoric amines, ammonium chloride, and ammonium carbonate can generate such non-combustible gases when heated.

• Transfer Effect

This effect works by altering the thermal decomposition pattern of polymeric materials, thereby suppressing the generation of flammable gases. For instance, the use of acids or bases to induce dehydration reactions in cellulose, leading to its decomposition into char and water, does not produce flammable gases and thus cannot ignite and burn. Compounds such as ammonium chloride, phosphoric amines, and phosphate esters can decompose to produce substances that catalyze the charring of materials, achieving the goal of flame retardancy.

• Inhibition Effect (Scavenging Free Radicals)

The combustion of polymers is primarily a free radical chain reaction. Some substances can capture active intermediates in the combustion reaction, such as HO•, H•, O•, HOO•, etc., inhibiting the free radical chain reaction and reducing the rate of combustion until the flame is extinguished. Commonly used organic halogen compounds, such as bromine and chlorine, possess this inhibitory effect.

• Enhancement Effect (Synergistic Effect)

The combustion of polymers is primarily a free radical chain reaction. Some substances can capture active intermediates in the combustion reaction, such as HO·, H·, ·O·, HOO·, etc., suppressing the free radical chain reaction and reducing the burning rate until the flame is extinguished. Commonly used organic halogen compounds, including bromine and chlorine, possess this inhibitory effect.

II.Flame Retardant Mechanism

The flame retardancy of materials is often achieved through mechanisms such as gas-phase flame retardation, condensed-phase flame retardation, and heat exchange interruption.

Gas-phase flame retardation involves the suppression of free radicals that promote the propagation of combustion reactions, thereby exerting a flame retardant function. Condensed-phase flame retardation refers to the delay or prevention of polymer thermal decomposition in the solid phase, which plays a role in flame retardation. Heat exchange interruption flame retardation involves the removal of some of the heat generated by the combustion of polymers, leading to flame retardation.

However, combustion and flame retardation are very complex processes involving many influencing and limiting factors. It is difficult to strictly categorize the flame retardant mechanism of a flame retardant system into just one type. In fact, many flame retardant systems operate simultaneously through several mechanisms.

• Gas-Phase Flame Retardant Mechanism

A gas-phase flame retardant system refers to the flame retardant action that interrupts or slows down the chain combustion reaction in the gas phase. The following types of flame retardation fall under gas-phase flame retardation:

1. When flame retardant materials are heated or burning, they can produce free radical inhibitors, which interrupt the combustion chain reaction.

2. When flame retardant materials are heated or burning, they generate fine particles that promote the combination of free radicals to terminate the chain combustion reaction.

3. When flame retardant materials are heated or burning, they release a large amount of inert gases or high-density vapors. The inert gases can dilute oxygen and gaseous combustibles, and reduce the temperature of the combustible gases, causing the combustion to cease, the high-density vapors cover the combustible material, isolating it from contact with air, thereby suffocating the combustion.

Volatile and low-boiling point phosphorus-containing compounds, such as trialkyl phosphate (R3PO), are classified as gas-phase flame retardants. Mass spectrometry analysis has shown that triphenyl phosphate and triphenyl phosphite decompose into free radical fragments in the flame, which, like halides, capture H· and O· free radicals, thereby suppressing the combustion chain reaction.

During the combustion and pyrolysis of red phosphorus, P· radicals are also formed, which react with oxygen in the polymer to generate a phosphate ester structure.

Additionally, the intumescent flame retardant system may also function in the gas phase. Amine compounds within this system can decompose upon heating to produce NH3, H2O, and NO. The former two gases can dilute the oxygen concentration in the flame zone, while the latter can quench the free radicals necessary for combustion, leading to the termination of the chain reaction.

• Condensed-Phase Flame Retardant Mechanism

This refers to the flame retardant action that slows down or interrupts the thermal decomposition of flame retardant materials in the condensed phase. The following types of flame retardation are all part of condensed-phase flame retardation:

1. Flame retardants slow down or prevent the thermal decomposition in the condensed phase that can produce combustible gases and free radicals.

2. In flame retardant materials, inorganic fillers with a high specific heat capacity absorb and conduct heat, making it difficult for the material to reach its thermal decomposition temperature.

3. Flame retardants decompose endothermically upon heating, which slows down or stops the temperature rise of the flame retardant materials.

4. When flame retardant materials burn, a porous carbon layer is formed on their surface. This layer is difficult to burn, provides insulation, and separates oxygen. It also prevents combustible gases from entering the combustion gas phase, causing the combustion to cease. To maintain continued combustion, a sufficient mixture of oxygen and combustible gases is necessary. If the free radicals generated by thermal cleavage are intercepted and eliminated, the combustion will slow down or be interrupted.

When flame retardant thermoplastics containing organic bromine compounds as flame retardants burn, the following reactions occur:

RH → R· + H· (Initiation of the chain reaction)

HO· + CO = CO2 + H· (Propagation of the chain reaction, a highly exothermic reaction)

H· + O2 = HO· + O· (Branching of the chain reaction)

O· + HBr = HO· + Br· (Transfer of the chain reaction)

HO· + HBr = H2O + Br· (Termination of the chain reaction)

The highly reactive HO·radical plays a crucial role in the combustion process. When HO·is replaced by the less reactive Br·, the radical chain reaction is terminated.

• Interruptive Heat Exchange Flame Retardancy Mechanism

This refers to the process where some of the heat generated by the combustion of flame-retardant materials is removed, preventing the material from maintaining its thermal decomposition temperature. Consequently, the production of combustible gases is not sustained, leading to self-extinguishing of the flame.

For example, when flame-retardant materials are subjected to intense heat or combustion, they can melt. The molten material tends to drip, thereby carrying away most of the heat and reducing the amount of heat fed back to the main body. This slows down the combustion process, and eventually, the burning may cease.

Therefore, materials that are prone to melting typically have lower combustibility. However, the dripping hot droplets can ignite other substances, increasing the risk of fire.

III. Mechanisms of Several Typical Flame Retardants

• Halogen Flame Retardants

Halogen flame retardants include brominated and chlorinated flame retardants. They are among the most widely produced organic flame retardants in the world.

Most halogen flame retardants are brominated flame retardants. Industrially produced brominated flame retardants can be divided into three major categories: additive, reactive, and polymer types, with a wide variety of products.

There are more than 20 types of additive brominated flame retardants, over 10 types of polymer brominated flame retardants, and more than 20 types of reactive brominated flame retardants available in the domestic and international markets.

The main additive flame retardants include decabromodiphenyl ether (DBDPO), tetrabromobisphenol A, bis(2,3-dibromopropyl) ether (TBAB), and octabromodiphenyl ether (OBDPO), reactive flame retardants mainly include tetrabromobisphenol A (TBBPA) and 2,4,6-tribromophenol, polymer flame retardants mainly include brominated polystyrene, brominated epoxy, and tetrabromobisphenol A carbonate oligomers.

The popularity of brominated flame retardants is primarily due to their high flame retardancy efficiency and moderate pricing.

Due to the low bond energy of the C-Br bond, most brominated flame retardants decompose at temperatures between 200°C and 300°C, which coincides with the decomposition temperature range of commonly used polymers. Therefore, when polymers decompose, brominated flame retardants also begin to decompose. They can capture the radicals produced during the decomposition of polymeric materials, thereby delaying or inhibiting the combustion chain reaction. At the same time, the HBr released is a non-flammable gas that can cover the surface of the material, acting as a barrier and diluting the concentration of oxygen.

These flame retardants are invariably used in combination with antimony compounds (antimony trioxide or antimony pentoxide), and the synergistic effect significantly enhances the flame retardancy. Halogen flame retardants primarily exert their flame retardant action in the gas phase. The hydrogen halide gas produced by the decomposition of halogenated compounds is a non-flammable gas with a diluting effect. Its higher density forms a gas film that covers the surface of the solid phase of polymeric materials, isolating air and heat, thus providing a shielding effect.

More importantly, hydrogen halide can inhibit the chain reaction of polymeric material combustion, acting to eliminate free radicals. Taking brominated compounds as an example, the mechanism for inhibiting free radical chain reactions is as follows:

Brominated flame retardant→ Br·

Br·+RH→R·+HBr

HO·+HBr=H2O +Br·

When brominated flame retardants are added to polymeric materials and exposed to fire or heat, they undergo decomposition reactions, generating bromine radicals (Br·). These radicals react with the polymeric materials to produce hydrogen bromide (HBr). HBr then reacts with highly reactive hydroxyl radicals (OH·), which not only regenerates Br but also reduces the concentration of OH· radicals. This process inhibits the combustion chain reaction, slows down the burning rate, and eventually extinguishes the fire.

However, during fires, the decomposition and combustion of these materials produce large amounts of smoke and toxic corrosive gases, leading to "secondary disasters." The combustion products, which are halogenated compounds, have a long atmospheric lifetime and are difficult to remove once released into the atmosphere. This severely pollutes the atmospheric environment and depletes the ozone layer. Additionally, the combustion and pyrolysis products of polymeric materials containing brominated flame retardants, such as polybrominated diphenyl ethers, contain toxic substances like polybrominated dibenzo-p-dioxins (PBDD) and polybrominated dibenzofurans (PBDF). In September 1994, the U.S. Environmental Protection Agency evaluated these substances and confirmed their toxicity to humans and animals.

• The Flame Retardancy Mechanism of Phosphorus and Phosphorus Compounds

Phosphorus and its compounds have been used as flame retardants for a long time, and their flame retardancy mechanisms have been studied extensively. Initially, it was observed that materials treated with phosphorus-containing flame retardants produce a significant amount of char during combustion, reducing the production of combustible volatile substances and significantly lowering the thermal weight loss during burning. However, the smoke density during combustion of the flame-retardant materials increases compared to untreated materials.

Based on these observations, several flame retardancy mechanisms have been proposed. The flame retardancy effects of phosphorus compounds can be divided into mechanisms in the condensed phase and the vapor phase. Organic phosphorus flame retardants primarily exert their effects in the condensed phase, and their mechanism is as follows:

During combustion, phosphorus compounds decompose to form a non-flammable liquid film of phosphoric acid, which has a boiling point of up to 300°C. Phosphoric acid then further dehydrates to form pyrophosphoric acid, which polymerizes to produce polyphosphoric acid.

In this process, not only does the covering layer formed by phosphoric acid provide a shielding effect, but the polyphosphoric acid, being a strong acid and a powerful dehydrating agent, causes the polymer to dehydrate and char. This changes the mode of the polymer's combustion process and forms a carbon layer on its surface to isolate air, thereby exerting a stronger flame retardant effect.

The flame retardancy of phosphorus flame retardants is mainly manifested in the initial stage of polymer decomposition during a fire. They promote the dehydration and charring of polymers, reducing the amount of combustible gases produced by thermal decomposition and forming a carbon layer that isolates external air and heat.

Typically, phosphorus flame retardants are most effective for oxygen-containing polymers and are primarily used in polymers containing hydroxyl groups, such as cellulose, polyurethane, and polyester. For hydrocarbon polymers that do not contain oxygen, the effectiveness of phosphorus flame retardants is relatively low.

Phosphorus flame retardants also act as radical scavengers. Mass spectrometry has revealed that any phosphorus-containing compound forms PO· during polymer combustion. It can combine with hydrogen atoms in the flame region, playing a role in suppressing the flame.

Additionally, the water produced during the flame retardancy process of phosphorus flame retardants can, on one hand, lower the temperature of the condensed phase and, on the other hand, dilute the concentration of combustible substances in the vapor phase, thereby enhancing the flame retardant effect.

• The Flame Retardancy Mechanism of Inorganic Flame Retardants

Inorganic flame retardants include those based on aluminum hydroxide, magnesium hydroxide, expandable graphite, borate salts, aluminum oxalate, and zinc sulfide.

Aluminum hydroxide and magnesium hydroxide are the main types of inorganic flame retardants, characterized by their non-toxicity and low smoke emission. They absorb a large amount of heat from the combustion zone upon thermal decomposition, lowering the temperature below the critical combustion temperature, leading to self-extinguishing. The metal oxides generated after decomposition typically have high melting points and good thermal stability, forming a barrier on the surface of the combustion solid phase to block heat conduction and radiation, thereby exerting a flame-retardant effect. Additionally, the decomposition produces a large amount of water vapor, which dilutes combustible gases and also contributes to flame retardancy.

Hydrated aluminum oxide has good thermal stability and can be transformed into AlO(OH) when heated at 300°C for 2 hours. It does not produce harmful gases upon contact with flames and can neutralize acidic gases released during the pyrolysis of polymers. It has the advantages of low smoke emission and low cost, making it an important variety among inorganic flame retardants. When heated, hydrated aluminum oxide releases chemically bound water, absorbing combustion heat and lowering the combustion temperature. The two crystalline waters mainly play a role in flame retardancy, and the dehydrated product is active aluminum oxide, which can promote the formation of a dense char layer in some polymers during combustion, thus providing a condensed phase flame retardant effect. From this mechanism, it is known that a larger amount of hydrated aluminum oxide should be used as a flame retardant.

The main variety of magnesium-based flame retardants is magnesium hydroxide, which has been developed in recent years both domestically and internationally. It begins to undergo an endothermic decomposition reaction at around 340°C, reaching maximum weight loss at 423°C, and the decomposition reaction terminates at 490°C. Calorimetric studies have shown that the reaction absorbs a large amount of heat energy (44.8 kJ/mol), and the water generated also absorbs a significant amount of heat, lowering the temperature to achieve flame retardancy. Magnesium hydroxide has better thermal stability and smoke suppression capabilities than hydrated aluminum oxide. However, due to its high surface polarity and poor compatibility with organic materials, it needs to be surface-treated to be an effective flame retardant. Additionally, its thermal decomposition temperature is relatively high, making it suitable for flame retardancy in thermosetting materials and other polymers with higher decomposition temperatures.

At high temperatures, the intercalated layers in expandable graphite are easily decomposed, and the gases produced rapidly expand the layer spacing to dozens or even hundreds of times its original size. When expandable graphite is mixed with polymers, a tough carbon layer is formed on the surface of the polymer under the action of flames, thereby exerting a flame-retardant effect.

Borate flame retardants include borax, boric acid, and zinc borate. Currently, zinc borate is the most widely used.

Zinc borate begins to release crystalline water at 300°C. In the presence of halogen compounds, it forms halogenated boron and zinc compounds, which inhibit and capture free hydroxyl radicals, preventing the combustion chain reaction. It also forms a solid-phase covering layer that isolates surrounding oxygen, preventing the flame from continuing to burn and providing a smoke suppression effect.

Zinc borate can be used alone or in combination with other flame retardants. The main products currently available include fine-particle zinc borate, heat-resistant zinc borate, anhydrous zinc borate, and high-water-content zinc borate.

Aluminum oxalate is a crystalline substance derived from aluminum hydroxide with low alkaline content. When polymers containing aluminum oxalate burn, they release H2O, CO, and CO2 without generating corrosive gases. Aluminum oxalate also reduces smoke density and smoke generation rate. Due to its low alkaline content, it does not affect the electrical performance of materials when used for flame retardancy in wire and cable coatings.

Five types of flame retardants based on zinc sulfide have been developed, four of which are used for rigid PVC and one for flexible PVC, polyolefins, and nylon. These flame retardants can improve the material's resistance to aging and have good compatibility with glass fibers, enhancing the thermal stability of polyolefins.

• The Synergistic Flame Retardancy Mechanism of Mixed Flame Retardants

When halogen-containing flame retardants are used in combination with phosphorus-containing flame retardants, a significant synergistic effect is observed. For the halogen-phosphorus synergistic effect, it is proposed that the combination of halogens and phosphorus can promote each other's decomposition and form halogen-phosphorus compounds and their transformation products such as PBr3, PBr·, and POBr3, which have stronger flame retardancy than when used alone.

Research using methods such as pyrolysis gas chromatography, differential thermal analysis, differential scanning calorimetry, oxygen index measurement, and flame retardant temperature programming observation has shown that when halogens and phosphorus are used together, the decomposition temperature of the flame retardants is slightly lower than when used alone, and the decomposition is very intense. The smoke cloud formed by the chlorophosphorus compounds and their hydrolysis products in the combustion zone can linger in the combustion zone for a longer time, forming a powerful gas-phase isolation layer.

The interaction mechanism between phosphorus and nitrogen is not fully understood. It is generally believed that nitrogen compounds (such as urea, cyanamide, guanidine, dicyandiamide, and hydroxymethyl melamine) can promote the phosphorylation reaction of phosphoric acid with cellulose. The formed phosphoric amine is more likely to react with cellulose to form an ester, and this ester has better thermal stability than phosphoric ester. The phosphorus-nitrogen flame retardant system can promote the decomposition of sugars at lower temperatures to form char and water, and increase the production of char residues, thereby improving the flame retardancy. Phosphorus and nitrogen compounds form an expanding char layer at high temperatures, which acts as an insulating and oxygen-blocking protective layer. Nitrogen compounds act as foaming agents and char enhancers. Basic element analysis shows that the residues contain nitrogen, phosphorus, and oxygen, which form thermally stable amorphous substances at flame temperatures, like glass, serving as an insulating protective layer for cellulose.

Antimony trioxide cannot be used alone as a flame retardant (except for halogen-containing polymers), but when used in combination with halogen flame retardants, it has a significant synergistic enhancement effect. This is because, in the presence of halogens, antimony trioxide generates halogenated antimony compounds such as SbCl3 and SbBr3 during combustion. These compounds have a high relative density and cover the polymer surface, providing a shielding effect, and also capture free radicals in the gas phase. For example, when antimony trioxide is used with chlorinated flame retardants, the chlorinated compounds decompose to release hydrogen chloride, which reacts with antimony trioxide to form antimony trichloride and chlorooxide antimony. The chlorooxide antimony further decomposes upon heating to continue forming antimony trichloride.

Hydrated zinc borate has a good synergistic effect when used in combination with halogen flame retardants. Under combustion conditions, they and their pyrolysis products interact with each other, allowing almost all flame retardant elements to exert their flame retardant effects. Hydrated zinc borate reacts with halogen flame retardants to form dihalogenated zinc and trihalogenated boron, which can capture HO· and H· in the gas phase and form a glassy isolation layer in the solid phase, providing thermal insulation and oxygen isolation. The generated water dilutes the oxygen in the combustion zone and carries away the reaction heat, thus exerting a significant flame retardant effect.

• The Flame Retardancy Mechanism of Intumescent Systems

Intumescent flame retardants are primarily composed of three parts: a charring agent (carbon source), a charring catalyst (acid source), and a blowing agent (gas source).

The charring agent serves as the carbon source for the formation of an expanded, porous carbon layer. It is typically a substance rich in carbon with multiple functional groups (such as —OH), and pentaerythritol (PER) and its diols and triols are commonly used as charring agents.

The charring catalyst is generally a compound that can release inorganic acid under heating conditions. The inorganic acid should have a high boiling point and not be too strong in oxidizing power. Ammonium polyphosphate (APP) is a commonly used charring catalyst.

The blowing agent is a compound that releases inert gases upon heating, typically amine and amide compounds, such as urea, melamine, dicyandiamide, and their derivatives.

The selection criteria for each component are as follows:

Acid source: To be practical, the acid source must be able to dehydrate polyols containing carbon. We do not want the dehydration reaction to occur before a fire, so commonly used acid sources are salts or esters. The release of acid from the acid source must occur at a lower temperature, especially below the decomposition temperature of the polyol. If the organic part helps in charring, using organic phosphorus compounds is more effective.

Carbon source: The effectiveness of the carbon source is related to its carbon content and the number of active hydroxyl groups. The carbon source should react with the catalyst at a lower temperature before its own or the matrix decomposition.

Gas source: The foaming agent must decompose at an appropriate temperature and release a large amount of gas. Foaming should occur after melting and before solidification. The appropriate temperature depends on the system. For specific intumescent flame retardant polymer systems, sometimes all three components do not need to be present simultaneously, sometimes the polymer itself can act as one of the elements. Using the above criteria can predict the effectiveness of most systems.

When intumescent flame retardants are heated, the charring agent dehydrates to form carbon under the action of the charring catalyst. The carbonized material forms a porous, closed-structure carbon layer due to the gas released from the decomposition of the blowing agent. Once formed, it is non-combustible and can weaken the heat conduction between the polymer and the heat source, as well as prevent gas diffusion. Once combustion does not receive enough fuel and oxygen, the burning polymer will self-extinguish.

The formation of this carbon layer involves the following steps:

At lower temperatures, the acid source releases inorganic acid that can esterify polyols and act as a dehydrating agent.

At a temperature slightly higher than the acid release, esterification occurs, and the amine in the system can act as a catalyst for esterification.

The system melts before or during esterification.

The water vapor produced by the reaction and the non-combustible gases generated by the gas source cause the molten system to expand and foam.

As the reaction approaches completion, the system gels and solidifies, finally forming a porous foam carbon layer.

Based on the above discussion, it may seem that any compound containing these functional groups can foam, just to different extents, but this is incorrect. To foam, each step of the reaction must occur almost simultaneously but must proceed in a strict order.

Intumescent flame retardants may also have a gas-phase flame retardant effect, as the phosphorus-nitrogen-carbon system may produce NO and NH3 upon heating, which can also combine with free radicals to terminate the combustion chain reaction.

The main components of the intumescent flame retardant system can be divided into three parts: acid source, carbon source, and gas source:

The acid source is generally inorganic acid or a compound that generates inorganic acid when heated to 100-250°C, such as phosphoric acid, sulfuric acid, boric acid, various ammonium phosphate salts, phosphate esters, and borate salts, etc.

The carbon source (charring agent) is the basis for forming a foamed carbonized layer and is generally a carbon-rich polyhydroxy compound, such as starch, pentaerythritol, and its dimers and trimers, as well as organic resins containing hydroxyl groups, etc.

The gas source (blowing source) is mostly amine or amide compounds, such as melamine, dicyandiamide, and ammonium polyphosphate, etc.

The structure of the intumescent carbon layer is complex and influenced by many factors. The chemical structure and physical properties of the polymer matrix, the composition of the intumescent flame retardant, the conditions during combustion and pyrolysis (such as temperature and oxygen content), and the reaction rate of crosslinking, among many other factors, can all affect the structure of the intumescent carbon layer.

The thermal protective effect of the intumescent carbon layer depends not only on the yield of coke, the height of the carbon layer, the structure of the carbon layer, and the thermal stability of the protective carbon layer, but also on the chemical structure of the carbon layer, especially the appearance of cyclic structures increases thermal stability, in addition to the strength of chemical bonds and the number of crosslinking bonds.

The flame retardancy mechanism of the gas source intumescent flame retardant system is generally considered to be a condensed phase flame retardancy. First, ammonium polyphosphate decomposes upon heating to generate phosphoric acid and pyrophosphoric acid with strong dehydrating effects, which cause pentaerythritol to esterify and then dehydrate and char. The water vapor formed by the reaction and the ammonia gas released by the decomposition of melamine cause the carbon layer to expand, eventually forming a porous carbon layer, thereby isolating air and heat conduction, protecting the polymer matrix, and achieving the flame retardant purpose.

Intumescent flame retardants added to polymer materials must possess the following properties:

Good thermal stability, able to withstand temperatures above 200°C during the polymer processing,since thermal degradation releases a large amount of volatile substances and forms residues, this process should not adversely affect the foaming process, these flame retardants are uniformly distributed in the polymer and can form an expanded carbon layer that completely covers the surface of the material during combustion, the flame retardant must have good compatibility with the polymer to be flame retarded, should not have adverse effects with the polymer and additives, and should not excessively deteriorate the physical and mechanical properties of the material.

The advantage of intumescent flame retardants over general flame retardants is that they are halogen-free and antimony oxide-free: low smoke, less toxic, and non-corrosive gases, the carbon layer generated by the intumescent flame retardant can adsorb the molten burning polymer, preventing its dripping and spreading the fire.

• The Flame Retardancy Mechanism of Ammonium Salts

Ammonium salts have poor thermal stability and release ammonia gas when heated. For example, the decomposition process of ammonium sulfate ((NH4)2SO4) is as follows:

(NH4)2SO4→NH4HSO4

NH4HSO4→H2SO4+NH3↑

The released ammonia gas is a non-combustible gas that dilutes the oxygen in the air. The formed H2SO4 acts as a dehydration and charring catalyst. It is generally believed that the latter effect is the main one.

Additional experiments have shown that NH3 also undergoes the following reaction in a fire:

NH3 +O2→N2+H2O

This reaction is accompanied by the formation of deep oxidation products such as N2O4. From this, it can be seen that NH3 not only has a physical flame retardant effect but also a chemical flame retardant effect.

• The Flame Retardancy Mechanism of Nanocomposite Flame Retardant Materials

Nanocomposite materials, while falling under the category of composite flame retardants, have distinct mechanisms. These materials involve dispersing one or more components at the nanoscale or molecular level within another component's matrix. Research in this area has a history of only a few decades.

Experiments have shown that due to the ultra-fine dimensions of nanomaterials, the performance of various types of nanocomposite materials is significantly improved compared to their macroscopic or micrometer-level counterparts. This improvement includes enhanced thermal stability and flame retardancy. Certain lamellar inorganic materials can fracture into nanoscale structural microzones under physical and chemical actions. The interlayer spacing of these materials is typically a few to several nanometers. They not only allow certain polymers to intercalate into the nanoscale interlayer spaces, forming "intercalated nanocomposites," but also cause the inorganic layers to be expanded by the polymer, forming "exfoliated nanocomposites" with a high aspect ratio, which are uniformly dispersed in the polymer matrix.

By utilizing the characteristics of porous or layered inorganic compounds, inorganic/polymer nanocomposites can be prepared. During thermal decomposition and combustion, these materials may form multi-layer structures of carbon and inorganic salts, which act as thermal barriers and prevent the escape of combustible gases, thereby achieving flame retardancy. Additionally, inorganic/polymer nanocomposites also possess properties such as corrosion resistance, leak prevention, and wear resistance. Significant progress has been made in the research of nanocomposites such as nylon/Clay Nanocomposite, PS/Clay Nanocomposite, PET/Clay Nanocomposite, PBT/Clay Nanocomposite, and PP/Clay Nanocomposite.

• Organic Silicon Flame Retardants

The study of using silicone compounds as flame retardants began in the early 1980s. In 1981, Kamber et al. published a research report showing that blending polycarbonate with polydimethylsiloxane could improve flame retardancy.

Although the development of organic silicon flame retardants lags behind that of halogen and phosphorus flame retardants, organic silicon flame retardants, as a new type of halogen-free flame retardant, are unique due to their excellent flame retardancy, processability, and environmental friendliness.

Organic silicon flame retardants include silicone oils, silicone resins, polyorganosiloxanes with functional groups, polycarbonate-siloxane copolymers, acrylate-siloxane composite materials, and silicone gels. When used as flame retardants in polymers, organic silicon flame retardants tend to migrate to the surface of the material, forming a gradient polymer material with a silicone-rich surface layer.

During combustion, a unique inorganic insulating and heat-resistant protective layer containing Si-O and Si-C bonds is formed. This layer not only prevents the escape of combustible decomposition products but also inhibits the thermal decomposition of the polymer, achieving high flame retardancy, low smoke emission, and low toxicity.

After understanding the flame retardant effects and mechanisms of several typical flame retardants, it is especially worth mentioning the flame retardants provided by YINSU Flame Retardant Company, such as red phosphorus flame retardant series, antimony composite T series, and antimony trioxide replacement T-30. These products are halogen-free, environmentally friendly and highly effective.These innovative R&D results of YINSU Flame Retardant Company meet the diversified needs of the market, provide a variety of options to meet the different needs of customers, and contribute to the development of halogen-free and environmentally friendly flame retardants.

Conclusion

YINSU Flame Retardant Company has successfully developed a series of highly efficient flame retardant products based on its profound R&D strength in the field of flame retardants. These flame retardants fully integrate the various flame retardant mechanisms mentioned above, such as red phosphorus flame retardant for condensed vapour phase flame retardancy, and are able to provide targeted solutions to the problems encountered by different materials in the combustion process. For example, red phosphorus flame retardants can effectively inhibit the combustion reaction of materials and reduce the amount of smoke and toxic gases by capturing free radicals, generating a stable cover layer and absorbing heat to decompose.

YINSU Flame Retardant Company's flame retardant products not only have excellent flame retardant properties, but also take into account the physical properties and processing properties of the materials, providing customers with solutions to reduce costs and increase efficiency while realizing efficient flame retardancy, which are widely used in a variety of industries such as wires and cables, electronics and electrical appliances, construction materials, etc., and help customers to stand out in the market competition.