Flame Retardant Definition & Classification

Flame Retardant Definition

Combustion could occur when a substance is heated to its flammable temperature in the presence of oxygen. Flame retardants are generally designed to provide a particular level of resistance to ignition or flame spread. This is achieved by suppressing oxygen availability, building up of char at the surface, inhibiting the combustion reaction within the flame area, or by other mechanisms .

Flame Retardant Classification

Flame retardant can be classified by different methods：

By whether it contains halogens;

Flame retardant can be classified into halogen free (non–halogened) flame retardant and halogen flame retardant. Halogen flame retardant contains chlorine and brominated flame retardant. Halogen-free FRs contains Nitrogen FRs, Phosphorus FR, Antimony trioxide, silicon, etc.

Halogen Flame Retardant

Halogen flame retardant works in a gas phrase and reacts with the radicals formed during the combustion. Chlorine and bromine are the only halogens used as FRs in synthetic plastic formulations since fluorine and iodine are unsuitable for this application. Brominated flame retardants (BFRs) are by far the most widely used. Halogen FRs as they are more effective cost less and have wider application. And could use together with other flame retardants to achieve a synergistic effect. However, BFRs have been targeted as an environmental hazard by persistence and bio-accumulation, when heated they can also form toxic by-products includes COand smoke.

By Organic and Inorganic Types;

Flame retardant can also be classified into organic and inorganic flame retardant which mechanism of action is quite different.

Inorganic Flame Retardants

Inorganic flame retardants include aluminum trioxide Al(OH) 3, magnesium hydroxide Mg(OH) 2, Antimony trioxide(sb2o3), ammonium polyphosphate, and red phosphorus, boric acid. This group represents about 50% by volume of the global flame retardant production. The remaining categories are organic FRs.

Inorganic FRs do not evaporate by effect of the combustion heat but decompose in non-flammable gases, mostly water by endothermic reactions. Currently, aluminum hydroxide is the most commonly employed IFR, due to low cost and good compatibility with most plastic materials, especially PVC and PE. Magnesium hydroxide (Mg2(OH)4) can be employed in engineering polymers, such as PPs and polyamides (PA), which are usually processed at higher temperatures.

The endothermic decomposition of aluminum hydroxide primarily leads to the cooling of polymer and the formation of a protective layer of aluminum oxide. Moreover, the formation of water vapour decreases the oxygen concentration near the surface, hindering the combustion reaction. Simple and high volume fire retardant additives are hydroxides of aluminum and magnesium. Upon decomposition, they remove heat and release water vapor due to their endothermic nature. However, large amounts of these additives are needed to achieve the desired effect. Major disadvantages of inorganic FRs are their relatively low decomposition temperature and requirement of a large fraction of material to give sufficient flame retardant performance in most polymers.

By chemical category:

By chemical category, FRs can be classified into categories are as the table below:

• Halogenated additives in which brominated FRs are most widely used, halogenatedFRs work in the gas phase above the burning polymer surface by chemically interrupting the flame propagation mechanism.

• Intumescent additives include phosphorus FRs that react with the polymer substrate to produce a char layer which forms an effective barrier between heat source and oxygen and the fuel derived from pyrolysis of the polymer. The efficiency with which a polymer can be flame retarded is related to its inherent tendency to char.

• Spumific additives include inorganic FRs which decompose at the combustion temperature of the polymer to produce inert gases, such as CO2 andH2O, which dilute the combustion gases and hinder burning.

By either reactive or additive.

Reactive flame retardants which used mainly in thermosets(epoxy resin, UP, polyurethane) are chemically bound to the polymer; Additive FRs which used primarily in thermoplastics(ABS, PC/ABS, HIPS, EPS, PP, PE, PA, PC, PBT) are physically mixed with the resin during or after polymerisation.

Trend of Flame Retardant

Flame retardants can reduce processability and interfere with other additives. The ideal FR additive is colorless, easily blended, compatible with the substrate (no blooming and plate-out), has no deleterious effects on (mechanical)properties of the substrate, allows all finished article colours, remains effective throughout the service life of the products, is thermally and light-stable, resistant to aging and hydrolysis, odorless, does not cause corrosion, is highly effective in small quantities, generates a very low amount of smoke, has minimal toxicological effects, is economic and recyclable. This goal is still to be reached. Nearly all the present-day commercial FRs and smoke suppressants can be divided into several chemical groups as the table below.