Hydrogen halide, combustion

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. 

The addition of chlorinated waxes will trap free radicals formed during the basic combustion process, by combination with a hydrogen halide, usually HC1. 

The Dohrmann DX 20B system is based on combustion of the sample to produce the hydrogen halide, which is then swept into a microcoulometric cell and estimated. It is applicable at total halide concentrations up to lOOOpl-10 with a precision of 2% at the lOpg L-1 level. The detection limit is about 0.5pg L-1. Analysis can be performed in 5 min. A sample boat is available for carrying out analysis of solid samples. The instrument has been applied to waste waters, soils and sediments. 

Mitsubishi also supply a microprocessor-controlled automatic total halogen analyser (model TOX-IO) (Fig. 1.8 (b)) which is very similar in operating principles to the Dohrmann instruments discussed above, i.e. combustion at 800-900°C followed by coulometric estimation of hydrogen halide produced. 

Bromine-containing compounds on a weights basis are at least twice as effective as chlorine-containing ones. Because of the smaller quantities of bromine compounds needed, their used hardly influences the mechanical properties of the base resins and reduces markedly the hydrogen halide content of the combustion gases. Aliphatic, cycloaliphtic, as well as aromatic and aromatic-aliphatic bromine compounds are used as flame retardants. 

The hydrogen halides neutralize combustion-fostering radicals H and HO, producing halide radicals that continue their combustion-interfering activities, following the same cycle. 

Halogens appear in the combustion gases as hydrogen halides (mainly HCl from the most common halogen chlorine), while sulphur produces sulpW dioxide (SO2). 

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