Fire protection knowledge: the flame retardancy mechanism of several typical flame retardants

Halogen - based flame retardants include bromine and chlorine - based flame retardants. Halogen flame retardant is one of the largest organic flame retardants in the world at present. Most of the halogen flame retardants are brominated flame retardants. The bromine flame retardants produced in industrial production can be divided into three kinds, adding type, reactive type and polymer type, and there are many varieties. There are more than 20 kinds of brominated flame retardants, more than 10 kinds of brominated flame retardants, more than 20 kinds of reactive brominated flame retardants at home and abroad. Flame retardants are the main type of decabromodiphenyl oxide (DBDPO). Four tetrabromobisphenol A bis (2,3 one or two alkyl propyl ether) (TBAB), eight bromo two phenyl ether (OBDPO); there are four main reactive flame retardant tetrabromobisphenol A (TBBPA), 2,4,6- type flame retardant polymer tribromophenol; mainly bromide polystyrene, brominated epoxy, four bromo bisphenol A carbonate oligomers. The main reason for the popularity of bromine - based flame retardants is that the flame retardant efficiency is high and the price is moderate. Because the bond energy of C-Br bond is low, the decomposition temperature of most brominated flame retardants is at 200 c -300 C, which is exactly the decomposition temperature range of commonly used polymers. So in polymer decomposition, brominated flame retardants also began to decompose, and decomposition of polymer materials when capturing free radicals, thereby delaying or inhibiting the firing chain reaction, while the release of the HBr itself is a kind of difficult burning gas, can cover the surface of the material, to isolate the oxygen concentration and dilution effect. This kind of flame retardant has no exception to the antimony system (three oxidation two antimony or five oxidation two antimony), and the flame retardant effect is greatly improved by the synergistic effect.

Halogen - based flame retardants mainly play a role in flame retardancy in gas phase. The hydrogen halide gas, produced by halide decomposition, is a nonflammable gas and has a dilution effect. It has a large proportion, forming a layer of gas film covering the solid surface of polymer material, which can cut off air and heat, and play a covering effect. More importantly, halogenated hydrogen can inhibit the chain reaction of the combustion of polymer materials and remove the free radicals. Taking the bromide as an example, the mechanism of the inhibition of free radical chain reaction is as follows:

Brominated flame retardants, Br. Br. +RH to R. +HBr

Brominated flame retardant added HO +HBr=H2O+Br polymer materials, fire heat decomposition reaction, generate Br radical, it and polymer reaction of hydrogen bromide, OH free radical reaction and the activity of hydrogen bromide is very strong, on the one hand makes the regeneration of Br, concentration of OH. Free radical chain reaction reduced, the combustion is suppressed, the combustion speed slows down, until extinguished.

But when a fire occurs, due to the decomposition and combustion produces a large amount of smoke and toxic corrosive gases caused by "two disaster" of these materials, and the combustion products (halide) have very long atmospheric lifetimes, once into the atmosphere is very difficult to remove, seriously pollutes the atmospheric environment, the destruction of the ozone layer. In addition, the combustion and pyrolysis products of polybrominated two phenyl ether flame retardant polymer materials contain toxic polybrominated two benzo two oxane (PBDD) and polybrominated two benzofuran (PBDF). In September 1994, the United States Environmental Protection Agency (EPA) assessed that these substances are toxic to humans and animals.

Flame retardancy mechanism of phosphorus and phosphorus compounds

Phosphorus and phosphorus compounds have long been used as flame retardant, flame retardant mechanism of its research was initially found earlier, combustion of phosphorus containing flame retardant materials will generate a lot of coke, and reduces the production of combustible volatile substances, burning flame retardant materials heat loss is greatly reduced, but the smoke density of flame retardant materials when burning than non flame retardant increased. According to the above facts, some mechanism of flame retardancy is put forward. The flame retardancy of phosphorus compounds in different reaction zones can be divided into flame retardant mechanism in condensed phase and flame retardation mechanism in steam phase. Organic phosphorus flame retardants play a role of flame retardancy in condensed phase, and their flame retardance mechanism is as follows:

During combustion, the phosphorus compounds are decomposed to produce a non flammable liquid membrane of phosphoric acid, with a boiling point of up to 300 degrees C.