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Fire retardant treasure book: mechanism of typical flame retardants
- Jan 24, 2019 -

Flame retardants are additives that can prevent plastics from igniting or inhibiting flame propagation. According to its usage, it can be divided into two types: additive type and reactive type. According to chemical structure, flame retardants can be divided into inorganic and organic. The flame retardant mechanism of various flame retardants is necessary and necessary for today's flame retardant engineers. Today, we will summarize and introduce the flame retardant mechanism of several typical flame retardants.

Flame Retardant Mechanism of Halogen Flame Retardants

Halogen flame retardants include bromine and chlorine flame retardants. Halogen flame retardant is one of the most productive organic flame retardants in the world. Most of halogen flame retardants are bromine flame retardants. The brominated flame retardants produced in industry can be divided into three types: additive type, reactive type and polymer type, and there are many varieties. There are more than 20 kinds of additive brominated flame retardants, more than 10 kinds of macromolecule brominated flame retardants and more than 20 kinds of reactive brominated flame retardants on the domestic and foreign markets. Additive flame retardants are mainly decabromodiphenyl ether (DBDPO), tetrabromobisphenol A, bis (2,3-dialkylpropyl) ether (TBAB), octabromodiphenyl ether (OBDPO), reactive flame retardants are mainly tetrabromobisphenol A (TBBPA), 2,4,6-tribromophenol, etc. Polymer flame retardants are mainly brominated polystyrene, brominated epoxy, tetrabromobisphenol A carbonate oligomers, etc.

Brominated flame retardants are popular because of their high flame retardant efficiency and moderate price. Because of the lower bond energy of C-Br bond, the decomposition temperature of most brominated flame retardants ranges from 200 to 300 C, which is also the decomposition temperature range of common polymers. So when the polymer is decomposed, the bromine flame retardant also begins to decompose, and can capture the free radicals when the polymer is decomposed, thus delaying or inhibiting the natural burning chain reaction. At the same time, the released HBr is a kind of non-flammable gas, which can cover the surface of the material and play the role of blocking and diluting oxygen concentration. This kind of flame retardant is used with antimony series (antimony trioxide or antimony pentoxide) without exception, and the flame retardant effect is obviously improved by synergistic effect.

Halogen flame retardants mainly play a flame retardant role in the gas phase. Because the hydrogen halide gas produced by the decomposition of halides is non-flammable gas and has dilution effect. It has a large proportion, forming a layer of gas film, covering the solid surface of polymer materials, which can isolate air and heat, and play a covering effect. More importantly, hydrogen halide can inhibit the chain reaction of combustion of polymer materials and scavenge free radicals.

Bromine-containing flame retardants added in polymer materials decompose into free radicals Br. They react with polymer materials to form hydrogen bromide. Hydrogen bromide reacts with highly active OH radicals. On the one hand, it regenerates Br, on the other hand, reduces the concentration of OH radicals, inhibits the chain reaction of combustion, and slows down the combustion rate until extinction.

However, when a fire occurs, due to the decomposition and combustion of these materials, a large number of soot and poisonous corrosive gases are produced, resulting in "secondary disasters", and the combustion products (halides) have a long atmospheric life. Once entering the atmosphere, it is difficult to remove them, which seriously pollutes the atmospheric environment and destroys the ozone layer. In addition, the combustion and pyrolysis products of flame retardant polybrominated diphenyl ethers contain toxic polybrominated dibenzodioxane (PBDD) and polybrominated dibenzofuran (PBDF). In September 1994, the United States Environmental Protection Agency (EPA) assessed that these substances were toxic to humans and animals.

Flame Retardant Mechanism of Phosphorus and Phosphorus Compounds

Phosphorus and phosphorus compounds have been used as flame retardants for a long time. The flame retardant mechanism of phosphorus compounds in different reaction zones can be divided into flame retardant mechanism in condensed phase and flame retardant mechanism in steam phase. Organophosphorus flame retardants play a flame retardant role in condensed phase.

The flame retardant mechanism is as follows: during combustion, phosphorus compounds decompose into non-flammable liquid membranes of phosphoric acid, whose boiling point can reach 300 C. At the same time, phosphoric acid is further dehydrated to produce metaphosphoric acid, and metaphosphoric acid is further polymerized to produce polymetaphosphate. In this process, not only the covering layer formed by phosphoric acid plays a covering effect, but also the polymetaphosphate formed by phosphoric acid is a strong acid and a strong dehydrating agent, which makes the polymer dehydrate and carbonize, changes the mode of the combustion process of the polymer and forms carbon film on its surface to insulate the air, thus playing a stronger flame retardant effect.

The flame retardant effect of phosphorus flame retardant is mainly embodied in the decomposition stage of polymer in the early stage of fire, because it can promote the dehydration and carbonization of polymer, thereby reducing the amount of flammable gases produced by thermal decomposition of polymer, and the carbon film generated can also isolate the outside air and heat. In general, phosphorus flame retardants have the best effect on oxygen-containing polymers, which are mainly used in hydroxyl-containing cellulose, polyurethane, polyester and other polymers. For hydrocarbon polymers without oxygen, the effect of phosphorus flame retardants is relatively small.

Phosphorus-containing flame retardants are also free radical trapping agents. It was found by mass spectrometry that PO formed in any phosphorus-containing compounds during polymer combustion. It can bind with hydrogen atoms in the flame region and inhibit the flame. In addition, the moisture produced by phosphorus flame retardants in the flame retardant process can reduce the temperature of the condensed phase on the one hand, and dilute the concentration of combustibles in the gas phase on the other hand, so as to play a better flame retardant role.

Flame Retardant Mechanism of Inorganic Flame Retardants

Inorganic flame retardants include aluminium hydroxide, magnesium hydroxide, expanded graphite, borate, aluminium oxalate and zinc sulfide. Aluminum hydroxide and magnesium hydroxide are the main kinds of inorganic flame retardants. They have the characteristics of non-toxicity and low smoke. Because thermal decomposition absorbs a large amount of heat in the combustion zone, the temperature in the combustion zone decreases to below the critical temperature of combustion and the combustion self-extinguishes. Most of the metal oxides produced after thermal decomposition have high melting point, good thermal stability, and cover the solid surface of combustion to block heat conduction and radiation, thus playing a flame retardant role. At the same time, decomposition produces a large amount of water vapor, which can dilute flammable gases and also play a flame retardant role.

Alumina hydrate has the advantages of good thermal stability, can be converted into AlO (OH) by heating at 300 C for 2 hours. It will not produce harmful gases after contacting with flame, and can neutralize the acid gases released during pyrolysis of polymers. It has the advantages of less smoke and low price. Therefore, it has become an important kind of inorganic flame retardants. When alumina hydrate is heated, it releases chemically bound water, absorbs combustion heat and reduces combustion temperature. Two crystalline water acts as the main flame retardant. In addition, the dehydration product is activated alumina, which can promote the carbonization of some polymers during combustion, so it has the flame retardant effect of condensed phase. From this mechanism, it can be seen that using alumina hydrate as flame retardant, the amount of addition should be larger.

Magnesium element flame retardant is mainly magnesium hydroxide, which is a kind of flame retardant being developed at home and abroad in recent years. It begins to undergo endothermic decomposition reaction at about 340 to produce magnesium oxide. The weight loss reaches the maximum at 423 and the decomposition reaction ends at 490 C. Magnesium hydroxide has better thermal stability and smoke suppression ability than alumina hydrate. However, due to its large surface polarity and poor compatibility with organic matter, it needs surface treatment to be an effective flame retardant. In addition, its thermal decomposition temperature is on the high side, which is suitable for flame retardant polymers with higher decomposition temperature, such as thermosetting materials.

At high temperature, the embedded layer in expansible graphite is easy to decompose when heated, and the gas generated rapidly enlarges the spacing of graphite layers to tens to hundreds of times of the original. When expansible graphite is mixed with polymer, tough carbon layer can be formed on the surface of polymer under the action of flame, thus playing the role of flame retardant.

Borate flame retardants include borax, boric acid and zinc borate. At present, zinc borate is mainly used. Zinc borate begins to release crystalline water at 300 C. Under the action of halogen compounds, boron halide and zinc halide are formed to inhibit and capture free hydroxyl groups and prevent chain reaction of combustion. At the same time, a solid phase covering layer is formed to isolate the surrounding oxygen, prevent the flame from continuing to burn and suppress smoke. Zinc borate can be used alone or in combination with other flame retardants. At present, the main products are fine zinc borate, heat-resistant zinc borate, anhydrous zinc borate and high-water zinc borate.

Aluminum oxalate is a crystalline product derived from aluminium hydroxide with low alkali content. When the polymer containing aluminium oxalate burns, it emits hydrogen peroxide, carbon dioxide and carbon dioxide without producing corrosive gases. Aluminum oxalate can also reduce smoke density and smoke speed. Because the alkali content of aluminium oxalate is low, the electrical properties of the material are not affected when the flame retardant cladding material for wires and cables is used.

Zinc sulfide based flame retardants can be used in PVC, polyolefin diameter and nylon. This kind of flame retardant can improve the anti-aging performance of the material, and has good compatibility with glass fiber and improve the thermal stability of polyolefin.

Flame Retardant Mechanism of Ammonium Salt

Ammonium salts have poor thermal stability and release ammonia when heated. Ammonia is a non-flammable gas, which dilutes oxygen in the air; H2SO4 formed acts as a catalyst for dehydration and carbonization. The latter is generally considered to be the main role. In addition, experiments show that NH3 reacts in fire as follows:

NH 3+O 2_N 2+H 2

It is also accompanied by deep oxidation products such as N2O4, from which we can see that NH3 not only has physical flame retardant effect, but also has chemical flame retardant effect.

Flame Retardant Mechanism of Silicone Flame Retardant

The study of silicone compounds as flame retardants began in the early 1980s. In 1981, Kamber et al. published a research report on the improvement of flame retardancy of blends of polycarbonate and polymethylsiloxane. Although the research and development of organosilicon flame retardants lag behind halogen and phosphorus flame retardants, organosilicon flame retardants, as a new type of halogen-free flame retardants, are unique for their excellent flame retardancy, moulding processability and environmental friendliness. Silicone flame retardants include silicone oil, silicone resin, functional polysiloxane, polycarbonate siloxane copolymer acrylate siloxane composite material, and silicone gel. Most of the silicone flame retardants will migrate to the surface of the material to form a polymer gradient material with a silicone enrichment layer.

Once burned, silicone-specific inorganic insulating protective layer containing Si-O bond and Si-C bond will be formed, which not only prevents the decomposition products from escaping from combustion, but also inhibits the thermal decomposition of polymer materials, and achieves the goal of high flame retardancy, low smoke emission and low harmfulness. At present, the silicone flame retardants developed and applied include the "D.C.RM" series flame retardants developed and commercialized by Dow Corning Company of the United States, the "XC-99-B6645" silicone flame retardant developed jointly by NEC and GE Toshiba Silicone Company of Japan, and SFR104 silicone resin developed by GE Company of the United States.

Flame Retardant Mechanism of Nanocomposite Flame Retardant Materials

Nanocomposites are proposed separately. Although they are all composite flame retardants, their principles are somewhat different. Nanocomposites refer to the dispersion of one or more components in the matrix of another component in nano-size or molecular level. Experiments show that the properties of various nanocomposites are better than those of corresponding macro or micro-composites because of the existence of nano-materials in ultra-fine size, and the thermal stability and flame retardancy of the materials will also be greatly improved.

Some flaky inorganic materials can be fragmented into nano-sized structural micro-regions under the physical and chemical action. The spacing between flakes is usually a few nanometers from zero. They can not only make some polymer intercalation into nano-sized sandwich space, forming "intercalated nanocomposites", but also make inorganic sandwiches propped up by polymers to form monoliths with large aspect ratio. The inorganic substances are evenly dispersed in the matrix of the polymer to form "layered nanocomposites". Inorganic/polymer nanocomposites are prepared by using the properties of porous or layered inorganic compounds. During thermal decomposition and combustion, carbon and inorganic salt multilayer structures may be formed, which can insulate heat and prevent flammable gases from escaping, thus making the polymers flame retardant.

Flame Retardant Mechanism of Intumescent Flame Retardant System

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

The acid source is usually inorganic acid or the compound that produces inorganic acid when heated to 100-250 C, such as phosphoric acid, sulfuric acid, boric acid, various ammonium phosphate, phosphate ester and borate.

Carbon source (carbon forming agent) is the basis for forming the carbonized layer of the foam, generally carbon rich polyhydroxyl compounds, such as starch, pentaerythritol and its two polymers, trimers and organic resins containing light radicals.

Gas source (foaming source) is mainly amine or amide compounds, such as melamine, dicyandiamide, polyphosphate amine, etc.

The structure of char formation in expansive system is complex, and there are many influencing factors. The chemical structure and physical properties of polymer, the composition of intumescent flame retardant, the conditions of combustion and pyrolysis (such as temperature and oxygen content), the reaction rate of cross-linking, and so on, all have effects on the structure of intumescent carbon. The thermal protection effect of expanded carbon layer depends not only on coke yield, carbon layer height, carbon layer structure and thermal stability of the protective carbon layer, but also on the chemical structure of the carbon layer, especially the appearance of ring structure, which increases thermal stability. In addition, the strength of chemical bonds and the number of cross-linking bonds are also determined.

The flame retardant mechanism of gas-source intumescent flame retardant system is generally considered as condensed phase flame retardant. Firstly, polyphosphamide is decomposed by heat to form phosphoric acid and pyrophosphate with strong dehydration effect, which can esterify pentaerythritol, then dehydrate and carbonize. The water vapor formed by reaction and ammonia decomposed by melamine expand the carbon layer, and finally form a multi-porous carbon layer, which can insulate air and heat conduction, and conserve the carbon layer. Protect polymer body to achieve flame retardant purpose.

When intumescent flame retardants are added to polymer materials, they must have the following properties: good thermal stability and can withstand the high temperature of over 200 C in the process of polymer processing; because thermal degradation releases a large number of volatile substances and forms residues, the process should not have adverse effects on the expansion and foaming process; such flame retardants are uniformly distributed in the polymer and can be burned when the material is burned. A layer of expanded carbon covering the surface of the material is formed. The flame retardant must have good compatibility with the flame retardant polymer, can not have adverse effects with the polymer and additives, and can not deteriorate the physical and mechanical properties of the material too much.

The advantages of intumescent flame retardants over general flame retardants lie in halogen-free and antimony oxide-free: low smoke, less toxicity and no corrosive gas; the carbon layer formed by intumescent flame retardants can absorb the melting ignited polymer and prevent its dropping and spreading fire.

Synergistic flame retardant mechanism of flame retardants

The synergistic effect of halogen-containing flame retardant and phosphorus-containing flame retardant can be remarkable. For the synergistic effect of halogen-phosphorus flame retardant, it was proposed that the combination of halogen and phosphorus could promote each other's decomposition and form halogen-phosphorus compounds and their convertants PBr3, PBr * and POBr3 with stronger flame retardant effect than that of single use. The results show that the decomposition temperature of halogen-phosphorus flame retardant is slightly lower than that of halogen-phosphorus flame retardant used alone, and the decomposition is very severe. The flue gas cloud formed by chlorophosphorus compounds and their hydrolysates in the combustion zone can stay in the combustion zone for a long time and form a strong gas phase isolation layer.

The mechanism of phosphorus-nitrogen interaction is not perfect. It is generally believed that nitrides (such as urine, cyanamide, guanidine, dicyandiamide, hydroxymethyl melamine, etc.) can promote phosphorylation of phosphoric acid with cellulose. The formed phosphate amine is easier to esterify cellulose, and the thermal stability of this ester is better than that of phosphate ester. Phosphorus-nitrogen flame retardant system can promote the decomposition of carbohydrates to form coke and water at lower temperature, and increase the production of coke residues, thereby improving the flame retardant effect. Phosphides and nitrides form expansive coke layers at high temperatures, which act as thermal insulation and oxygen barrier protective layers. Nitrogen-containing compounds act as foaming agents and coke enhancers. The basic element analysis shows that the residues contain three elements, nitrogen, phosphorus and oxygen, which form thermostable amorphous substances at flame temperature, just like vitreous body, as an adiabatic protective layer of cellulose.

Antimony trioxide can not be used as flame retardant alone (except halogenated polymers), but it has a great synergistic effect with halogenated flame retardants. This is because SbCl3, SbBr3 and other antimony halides produced by combustion of antimony trioxide in the presence of halides have a high relative density, covering effect on the surface of polymers, and also capture free radicals in the gas phase. For example, when antimony trioxide is used with chlorine flame retardants, hydrogen chloride is decomposed due to the heating of chloride. Hydrogen chloride and antimony trioxide react to form antimony trichloride and antimony oxychloride. Antimony oxychloride is decomposed into antimony trichloride by heating.

Zinc borate hydrate has good synergistic effect with halogen flame retardant. Under combustion conditions, almost all flame retardant elements can play a flame retardant role through interaction between them and their pyrolysis products. Zinc borate hydrate reacts with halogenated flame retardants to form zinc dihalide and boron trihalide. They can capture HO and H in gas phase, form glass-like isolation layer in solid phase, insulate and isolate oxygen. The water generated dilutes oxygen in combustion zone and takes away reaction heat, so they can play a greater role in flame retardancy.


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