The term ‘flame retardant’ describes a function rather than a chemical class. Overall, more than 200 different types of flame retardants exist, which producers have classified according to their major constituent elements. The different elements govern their chemical reaction with fire and so determine their suitability in different applications. The most common elements used are: bromine, phosphorous, nitrogen, and chlorine. Minerals, such as certain phosphates, are also used extensively.
Bromine, like chlorine, fluorine and iodine, is one of the elements in the chemical group known as halogens. The word halogen derives from Greek, meaning ‘salt-former’, because these elements are commonly found in nature, in the form of natural salts. For example, sodium chloride or table salt, is the most common halogen salt.
A common use of bromine is in making flame retardants. Due to its unique chemical interaction with the combustion process bromine is an extremely efficient element, meaning that a relatively small amount is needed to achieve the needed fire resistance. Brominated flame retardants are used to protect a wide variety of applications.
Most people are aware of chlorine use in cleaning products, or as a disinfectant in swimming pools. However, chlorine also has properties that make it valuable in the fight against fire.
Chlorine is found primarily as a component of the salt that is deposited in the earth, or dissolved in the oceans – about 1.9% of the mass of seawater is chlorine. Even higher concentrations of chloride are found naturally in underground brine deposits and other specific locations.
Chlorinated paraffins and chlorinated phosphates are used to prevent a whole range of materials from catching fire, including leather, paints and coatings, rubbers, textiles, foam fillings for furniture and other materials. It is also due to its chlorine content that polyvinylchloride (PVC) has some intrinsic fire-resistant properties.
Inorganics and minerals
A wide range of inorganic and mineral compounds are used as flame retardants or as elements of flame retardant systems in combination with bromine, phosphorus or nitrogen. The inorganic compounds include those based on nitrogen (melamine compounds), graphite (as used in pencils), silica (as in glass and sand) and inorganic phosphates (ammonium phosphate and polyphosphate). Mineral compounds include certain phosphates, metal oxides, hydroxides, and other metal products (aluminium, zinc, magnesium, molybdenum, boron, antimony).
Some inorganic and mineral compounds can be used as part of a flame retardant system, in combination with other elements, to achieve fire safety in plastics, foams, textiles, wood and timber products.
The large majority of the world’s phosphorus is found in mines. Phosphorus is used to produce liquid and solid organic or inorganic flame retardants which are extensively used in polyurethane foams to make fire resistant furniture, mattresses and thermal insulation materials.
Phosphorous is also commonly used in intumescent or fire resistant coatings, as well as in flexible PVC, which is employed as insulation for electric cables. It is also applied in electronics and in high temperature plastics used for manufacturing switches, connectors and in certain less flammable plastics used for casings.
Nitrogen is the largest single constituent of the Earth’s atmosphere and is present in all living organisms. Nitrogen compounds comprise a relatively small group of flame retardants. Today their main applications are in nylons, in polyolefins (a type of hard plastic), in polyurethane (synthetic) foams, in intumescent coatings (fire resistant paints), textiles and wallpapers.