Exploitable results
The experimental results and the modelling performed in this study show that the efficiency of fluorocarbons (FC), hydrofluorocarbons (HFC) or hydrocarbon fluorocarbons (HCFC) in fire extinguishing is a function of the number of fluorine atoms present in the molecule. That is to say that, for the same molecular weight, the efficiency of FCs, HFCs or HCFCs is the same. Taking into account environmental considerations, the potential substitutes will then have to be chosen from amongst HFCs. An optimum has to be found between a maximum number of fluorine atoms in the molecule and a maximum volatility of the compound. Up to now, the best candidates seem to be C(3)F(7)H (HFC 227ea) and C(2)F(5)H (HFC 125).
This study has also shown that, for the same efficiency in fire extinguishing, it will be necessary to use quite larger amounts of HFCs compared to those of brominated compounds. The fact that important amounts of fluorinated compounds are required in fire extinction raises the problem of the formation of potential toxins (HF, CF(2)O, etc) during the combustion of these compounds. This study has clearly shown that the effect of brominated compounds is due to a chemical cycle where the free radicals which propagate the combustion are destroyed (by combination reactions). But it is not yet fully understood if the inhibiting effect of fluorinated compounds is largely due to a physical action, that is to say to a heat removal in the flame due to their important heat capacity, or if the chemical influence, induced by radical scavengers which stop the branching reaction propagations, is the dominant factor.
Indeed, even if this study has permitted substantial progress in the knowledge of the reactivity of the key CF(2) radicals, there are still too many inaccuracies, concerning the reaction channels potentially identified as influential, for the modelling results to be accurate and meaningful enough to reach a conclusion about the inhibiting action of the additives. Therefore further investigations are needed.
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