The combustion of polymer is a very intense and complex thermal oxidation reaction, which is characterized by emitting thick smoke or burning flame. The general process of combustion is that under the constant heating of external heat sources, the polymer first undergoes a free radical chain degradation reaction with the oxygen in the air to produce volatile combustibles, which will ignite and burn when reaching a certain concentration and temperature. Part of the heat released from combustion is supplied to the polymer being degraded, further intensifying its degradation and generating more combustible gases, The flame will spread rapidly in a short time and cause a big fire.

Flame retardants are additives that can prevent the ignition of plastics or inhibit the spread of flame. It can be divided into additive type and reactive type according to its use method. According to the chemical structure, flame retardants can be divided into inorganic and organic. The flame retardancy mechanism of various flame retardants can be said to be necessary and necessary for today's flame retardant engineers. Today, we will summarize and introduce the flame retardancy mechanism of several common typical flame retardants.

Flame Retardant Mechanism of Several Typical Flame Retardants

1. Flame retardant mechanism of silicone flame retardant
The research on 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 polycarbonate and polymethyl siloxane blends. Although the research and development of organosilicon flame retardants lags 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, molding processability and environmental friendliness. Silicone flame retardants include silicone oil, silicone resin, polysiloxane with functional groups, polycarbonate siloxane copolymer acrylate siloxane composites and silicone gel. For polymer materials with silicone compounds as flame retardants, silicone flame retardants will most likely migrate to the surface of the materials, forming polymer gradient materials with silicone enriched layers on the surface.

Once burned, it will generate an inorganic insulation insulation protective layer with a Si-O bond and a Si-C bond unique to silicone, which not only prevents the escape of decomposition products generated by combustion, but also inhibits the thermal decomposition of polymer materials, achieving the goal of high flame retardancy, low smoke generation and low harmfulness. At present, the silicone flame retardants developed and applied include "D.C.RM" series flame retardants developed and commercialized by DowCorning Company of the United States, "XC-99-B6645" silicone flame retardants jointly developed by NEC of Japan and GE Toshiba Organosilicon Company, and SFR104 silicone resin developed by GE of the United States.

2. Flame Retardant Mechanism of Halogen Flame Retardants
Halogen flame retardants include bromine flame retardants and chlorine flame retardants. Halogen flame retardant is one of the largest organic flame retardants in the world. Most halogenated flame retardants are brominated flame retardants. Brominated flame retardants produced in industry can be divided into additive, reactive and polymer types, and there are many varieties. There are more than 20 kinds of additive bromine flame retardants, more than 10 kinds of polymer bromine flame retardants and more than 20 kinds of reactive bromine flame retardants on the domestic and international markets. The additive flame retardants mainly include decabromodiphenyl ether (DBDPO), tetrabromobisphenol A, bis (2,3-dimethylpropyl) ether (TBAB), octabromodiphenyl ether (OBDPO), etc; Reactive flame retardants mainly include tetrabromobisphenol A (TBBPA), 2,4,6-tribromophenol, etc; Polymeric flame retardants mainly include brominated polystyrene, brominated epoxy, tetrabromobisphenol A carbonate oligomer, etc.

3. Flame retardant mechanism of phosphorus and phosphorus compounds
Phosphorus and phosphorus compounds have been used as flame retardants for a long time, and their flame retardancy mechanisms have also been studied earlier. The flame retardancy of phosphorus compounds in different reaction zones can be divided into the flame retardancy mechanisms in the condensed phase and the flame retardancy mechanisms in the vapor phase. Organic phosphorus flame retardants play a flame retardancy role in the condensed phase. The flame retardant mechanism is as follows:

During combustion, phosphorus compounds decompose to form a noncombustible liquid membrane of phosphoric acid, and its boiling point can reach 300 ℃. At the same time, phosphoric acid is further dehydrated to generate metaphosphoric acid, and metaphosphoric acid is further polymerized to generate polymetaphosphoric acid. In this process, not only the covering layer generated by phosphoric acid plays a covering effect, but also the poly (metaphosphoric acid) generated is a strong acid and a strong dehydrating agent, which makes the polymer dehydrate and carbonize, changing the mode of polymer combustion process and forming a carbon film on its surface to isolate air, so as to play a stronger flame retardant effect.

4. Flame Retardant Mechanism of Inorganic Flame Retardant
Inorganic flame retardants include aluminum hydroxide, magnesium hydroxide, expanded graphite, borate, aluminum oxalate and zinc sulfide based flame retardants. Aluminum hydroxide and magnesium hydroxide are the main varieties of inorganic flame retardants, which are non-toxic and low smoke. Because the thermal decomposition absorbs a large amount of heat in the combustion zone, the temperature of the combustion zone is reduced to below the critical combustion temperature, and the combustion self extinguishes. Most of the metal oxides generated after decomposition have high melting point, good thermal stability, and cover the surface of the combustion solid phase to block heat conduction and radiation, thus playing a role in flame retardance. At the same time, the decomposition generates a large amount of water vapor, which can dilute the combustible gas and also play a role in flame retardancy.

5. Synergistic Flame Retardant Mechanism of Flame Retardants
Halogen containing flame retardants and phosphorus containing flame retardants can produce significant synergistic effect when used together. For the synergistic effect of halogen phosphorus flame retardancy, it is proposed that the combined use of halogen phosphorus can promote the decomposition of each other and form halogen phosphorus compounds and their conversions PBr3, PBr ·, POBr3, etc., which have stronger flame retardancy than the single use. The study on the synergistic effect of halogen phosphorus by pyrolysis gas chromatography, differential thermal analysis, differential scanning calorimetry, oxygen index measurement and flame retardant temperature programmed observation shows that the decomposition temperature of flame retardant when used in combination with halogen phosphorus is slightly lower than that when used alone, and the decomposition is not always violent. The smoke cloud formed by chlorine phosphorus compounds and their hydrolysates in the combustion zone can stay in the combustion zone for a long time, A strong gas phase isolation layer is formed.