Brominated Flame Retardants (BFRs)
There are approximately 75 different commercial brominated flame retardants, each with specific properties depending on the nature of the material they are protecting. Some are reacted into the final polymer while others are additives.
The electronics industry accounts for the greatest consumption of BFRs, such as in computers for printed circuit boards and in components such as connectors, plastic cabinets and cables. Other key areas of use are construction products (thermal insulation), carpets, coatings, upholstery and textiles.
Bromine, like chlorine, fluorine and iodine is one of the elements in the chemical group known as halogens. The word halogen is derived from Greek meaning ‘salt-former’; because these elements are commonly found in nature reacted with metals to form salts.
- The effectiveness of brominated flame retardants lies in their ability to release active bromine atoms (called low-energy free radicals) into the gas phase before the material reaches its ignition temperature
- These bromine atoms effectively quench the chemical reactions occurring in the flame, reducing the heat generated and slowing (or even preventing) the burning process; thus preventing the fire cycle being established or sustaining itself.
- Brominated flame retardants dehydrogenate polymers by virtue of abstracting hydrogen atoms needed to produce hydrogen bromide. This process enhances charring of the polymer on expense of volatile combustible products. This contributes to the flame retardancy of the polymer.
Often and when permitted, the addition of metallic compounds such as zinc or antimony oxides will enhance the efficiency of BFRs, by allowing the formation of transition species, so-called metal oxohalides, which allow the deposit of a protective layer of metal oxides. For example:
Antimony trioxide does not have flame retarding properties on its own, but is an effective synergist for bromine and chlorine based (halogenated) flame retardants.
- It acts as a catalyst, facilitating the breakdown of these halogenated flame retardants to active free radicals.
- It also reacts with the halogens to produce volatile antimony halogen compounds, which are themselves directly effective in removing the high energy H∙ and OH∙ radicals that feed the gas phase of the fire, thus strengthening the flame suppressing effect of the flame retardants. When added to PVC, antimony trioxide acts to suppress flames by activating the chlorine present in the plastic itself.
Phosphorous flame retardants
The class of phosphorus-containing flame retardants covers a wide range of inorganic and organic compounds and includes both reactive (chemically bound into the material) and additive (physically integrated into the material) compounds. They have a broad application field and offer very good fire safety performance.
The most important are: phosphate esters, phosphonates, red phosphorous and ammonium polyphosphate.
Phosphorus-containing flame retardants act efficiently in the solid phase of burning materials (Figure a).
- When heated, the phosphorus reacts to give a polymeric form of phosphoric acid. This acid causes the material to char, forming a glassy layer, and so inhibiting the “pyrolysis” process (break down and release of flammable gases), which is necessary to feed flames. By this mode of action the amount of fuel produced is significantly diminished, because char rather than combustible gas is formed.
- The intumescent char plays a specific role in the flame retardant process. It acts as a two-way barrier, both hindering the passage of the combustible gases and molten polymer towards the flame and shielding the polymer from the heat of the flame.
Phosphorous flame retardants are thus able to offer specific performance properties, depending on the required fire performance, processing conditions and mechanical properties of the material.
Certain products contain both phosphorus and chlorine, bromine or nitrogen, thus combining the different flame retarding mechanisms of these elements. They are widely used in standard and engineering plastics, polyurethane foams, thermosets, back coating and textiles.
The mechanisms of action of nitrogen containing flame retardants are not fully understood, but they are believed to act by several mechanisms:
- they are relatively stable compounds at high temperatures, thus physically inhibiting the breakdown of materials to flammable gases, which are needed to feed flames. A mechanism in the gas phase may aid the release of stable nitrogen-containing molecules, which dilute volatile polymer decomposition products.
- in the condensed phase, melamine is transformed into cross-linked molecular structures, which promote char formation.
Three main chemical groups can be distinguished: pure melamine, melamine derivatives (i.e. salts with organic or inorganic acids such as boric acid, cyanuric acid, phosphoric acid or pyro/poly-phosphoric acid) and melamine homologues.
Melamine-based products are the most widely used type of nitrogen flame retardant today, and are used for example in polyurethane foams for furniture, building foams, nylons etc.
Inorganic flame retardants
A wide range of different inorganic compounds are used as flame retardants, or as synergists of flame retardant systems in combination with brominated, phosphorous and/or nitrogen flame retardants.
The inorganic compounds used include metal oxides, hydroxides, borates, stannates (aluminium and magnesium hydroxides, antimony oxides, zinc borate and stannate), inorganic phosphorus compounds (red phosphorus and ammonium polyphosphate) and graphite.
Those used as flame retardants, mainly aluminium and magnesium hydroxides, interfere with the burning process through three main physical processes:
(Figure b below)
- release of inert gases such as water vapour, which dilute the fuel/oxygen mix thus preventing the exothermic radical reaction from taking place in the combustion zone.
- energy absorption through endothermic decomposition (reducing energy available for fire spread) thereby contributing to cooling and retardation of the pyrolysis process.
- production of a non-flammable and resistant layer on the surface of the material, (protective char layer) reducing the release flammable gases by the polymer and the energy transfer to the polymer, which sustains pyrolysis.
Inorganic flame retardants are used to achieve fire safety in plastics, foams, natural and man-made textiles, wood and timber products.
Their mechanisms are, however, of a relatively low efficiency and the products often have to be used in relatively large concentrations, or more usually, in combination with other types of flame retardants.
Specific application forms of these products (for example, within organic coatings) can enable such high concentrations to be added to plastics without modifying their performance properties.
Intumescent coatings are fire protection systems which are used to protect materials such as wood or plastic from fire (prevent burning), but also to protect steel and other materials from the high temperatures of fires (thus preventing or retarding structural damage during fires). The coatings are made of a combination of products, applied to the surface like a paint, which are designed to expand to form an insulating and fire-resistant covering when subject to heat. (Figure c)
The products involved contain a number of essential interdependent components:
- spumific compounds, which (when heated) release large quantities of non-flammable gas (such as nitrogen, ammonia, CO2)
- a binder, which (when heated) melts to give a thick liquid, thus trapping the released gas in bubbles and producing a thick layer of froth
- an acid source and a carbon compound. On heating, the acid source releases phosphoric, boric, or sulphuric acid, which chars the carbon compound (mechanism described under phosphorus flame retardants above) causing the layer of bubbles to harden and producing a fire-resistant barrier. Often the binder can also serve as the carbon compound.
As well as being used to protect flammable materials and structural elements, intumescent systems are now being incorporated into certain plastics, thus providing an inherent fire protection capacity (see phosphorus flame retardants above).
In the event of fire, heat causes the intumescent coating to expand creating an insulating and protective layer. (Figure d)