Fire Retardancy in Polymers

Plastics are made either from petroleum oil or from natural products. As such, when heated, they may bum. An important objective in modern polymer science relates to methods of reducing flammability and rate of fire spread. 

Combustion in polymeric materials involves two steps First, through a series of mostly thermal decomposition free radical chemical reactions (109), the solid polymer is reduced to monomer, dimer, trimer, and other components. These small molecules vaporize. Heat energy is absorbed in these steps from the surroundings. 

in the gaseous state, these small molecules react with oxygen, an exothermic step, producing carbon dioxide, water vapor, and other small molecules depending on the composition of the original polymer. Under fire conditions, part of the heat evolved is returned to the condensed phase polymer to continue the degrading process. 

One of the major objectives in fire retardancy emphasizes a reduction in the peak heat release rate. This, in turn, reduces the fire propagation rate. 

The most important halogen used is bromine. Some chlorine compounds are also used. The bromine and chlorine compounds used decompose in the temperature range of 150 to 350°C, well above ordinary temperatures, but below 500°C, the lowest decomposition temperature of the carbon-hydrogen bonds. Iodine compounds are not sufficiently stable to be generally useful, and fluorine compounds are too stable. 

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The use of fire retardants in polymers has become more complicated with the realisation that more deaths are probably caused by smoke and toxic combustion products than by fire itself. The suppression of a fire by the use of fire retardants may well result in smouldering and the production of smoke, rather than complete combustion with little smoke evolution. Furthermore, whilst complete combustion of organic materials leads to the formation of simple molecules such as CO2, H2O, N2, SO2 and hydrogen halides, incomplete combustion leads to the production of more complex and noxious materials as well as the simple structured but highly poisonous hydrogen cyanide and carbon monoxide. 

S.K. Brauman and A.S. Brolly, "Sb203 - Halogen Fire Retardance in Polymers I. General Mode of Action,"... 

Chamberlain, "Sb203-Halogen Fire Retardance in Polymers IV. Combustion Performance, "Journal of Fire Retardant Chemistry, 2, 225 (1976). 

Contributory effects, which combine to determine the overall mechanism of fire retardancy in polymers are discussed in the following text. 

Hornsby, P.R. and Watson, C.L., Mechanistic aspects of smoke suppression and fire retardancy in polymers containing magnesium hydroxide filler, Plast. Rubber Process. Appl., 11,45-51, 1989. 

Fire retardancy in polymers can be achieved by one of three ways ... 

Firstly there are forms of polymers, such as polytetrafluoroethylene, which are intrinsically fire retardant. The second types are rendered fire retardant by the inclusion of a suitable additive in the formulation. These include additives based on antimony, bromine, nitrogen, phosphorus and silicon. An essential requirement for fire retardant polymers used in enclosed spaces is that they do not release any toxic products upon combustion. Ffowever, antimony containing additives are going out of favour due to the release of toxic antimony volatiles upon combustion. The properties and mechanisms by which these polymers operate are discussed in Chapters 1 and 6. The third group of polymers consist of intumescent materials and these are being increasingly used as a means of imparting fire retardancy in polymers and this is discussed in Chapter 7. 

M. Lewin in Fire Retardancy in Polymers the use of Intumescence, Eds., M. LeBras, G. Camino, S. Bourbigot and R. Delobel, Special Publication No.224, Royal Society of Chemistry, Cambridge, UK, 1998, p.2. 

Intrinsically fillers can be divided into two types, reactive and inert. Reactive fillers will react with their environment. A good example of this is gibbsite (aluminium hydroxide), which will react with both acidic and basic substances. Aluminium hydroxide also loses its water of crystallisation at around 200 °C and this enables it to provide fire retardancy in polymer formulations. The silicate minerals (kaolin, mica, talc, quartz, etc.), are, in classical chemical terms, virtually inert, only being attacked by very strong acids and alkalis. The carbonate minerals and the hydroxide minerals are very reactive to acids. 

Boron compounds such as borax and boric acid are well known fire retardants for cellulosic products [1]. However, the use of boron compounds such as zinc borate, ammonium pentaborate, boric oxide, and other metallo-borates in the plastics industry has become prominent only since the late 1970s. This entry will review the manufacturing, chemical and physical properties, end-use applications, as well as modes of action of major boron compounds as fire retardants in polymers. The subject is also mentioned in the section entitled Flame retardants inorganic oxide and hydroxide systems. ... 

Nelson, G.L. The changing nature of fire retardancy in polymers, in A.F. Grand and C.A. Wilkie, Eds., Fire Retardancy of Polymeric Materials. Marcel Dekker, New York, 2000, pp. 1-26. 

Ecological considerations and investigations of fire hazards such as CO and smoke production target the inert filler characteristics of nanocomposites. The rather physical mechanisms proposed for nanocomposites are advantageous for such considerations. Nanocomposites appear to be a promising eco-friendly approach to fire retardancy in polymers. 

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