How+PBDE's+and+other+Fire-Retardants+Work


 * 2. Sources of PBDEs and How They Work**


 * 2.1 Mechanisms of Flame Retardants**

Although seemingly a simple concept, flame retardation is a complex process in which a wide variety of factors must be taken into account before applying a would-be retardant. The complexities of the types of reactions discussed in this section stem from the raw nature of a combustion reaction which uses oxygen to fuel a mass oxidation of every compound within its midst at high temperatures, releasing all types of radical species and chemical by-products into the air. So when designing a flame retardant, not only does the process of flame retardation need to be considered, but also do all of the combusted species, as well as the properties of the latent compounds themselves (toxicity, ignition temperature, thermal degradation temperature). Previously used flame retardants have operated using one of a few key mechanisms. The first being //cooling// or endothermic degradation which employs the use of a compound which when provided with sufficient activation energy will undergo a large endothermic reaction, ultimately resulting in the intake of energy (thermal) decreasing the temperature of the surrounding area. The second method is by //formation of a protective layer// in which when combusted, a polymer layer typically carbonaceous forms and blocks oxygen from reaching the protected material thus eliminating combustion. Another method of retardation is by //dilution// in which gases or liquids released from the flame retardant dilutes the fuel for combustion, in a similar fashion to a fire extinguisher. Finally there exists the method of //gas phase radical quenching// in which radicals produced (typically from halogenated arenes) quench the reactive oxidative radicals produced throughout the process of combustion. (Horrocks et al, 2001; Ampacet).


 * 2.2 Mechanisms of PBDEs**

PBDEs react using the gas phase radical quenching mechanism briefly discussed in section 2.1. To completely understand the action of these compounds, the chemistry of the flame itself must first be understood. As mentioned in section 2.1, fire is an uncontrolled oxidation of any hydrocarbon which acts as a fuel. This uncontrolled oxidation yields and propagates largely through radical species (scheme 2.1).




 * Scheme 1:** Overall reaction scheme depicting the “radical train” produced through the combustion of a hydrocarbon. [1] Is the initiation step followed by [2] which is the branching step and followed finally by the highly exothermic (heat yielding) propagation step, [3] (Horrocks et al, 2001). This unveils the mechanism with which PBDEs perform their flame attenuating reactions (Scheme 2). In order for the retardant to effectively reduce the fire’s intensity there must be a radical species produced which can intercept the hydroxyl radical (in reaction [3] shown in Scheme 1) to interrupt the most exothermic portion of the “radical train” formed through the combustion of hydrocarbons. This is where the aromatic halides in Poly chlorinated and poly brominated phenyls come into play. Considering a source of radical species is needed, halogenated aryl compounds present themselves as a wonderful candidate. Aliphatic compounds are far too stable with C---Br bond dissociation energy of 288KJ/mol (UWaterloo, n.d.); however the aromatic C---Br bond dissociation energy is greatly decreased at 88kJ/mol (Tang et al, 2003). This is due to the inherent stability of the 6-membered aromatic system which can stabilize the radical resulting from dissociation much more adequately (Scheme 2).


 * Scheme 2:** Reaction mechanism for the attenuation of radical propagation through the proliferation of bromine radicals from the dissociation of PDBEs. Reaction [1] displays the dissociation to generate the radical bromine and initiate the flame retardant’s action. Reaction [2] shows the halide’s reaction with a hydrocarbon to form the active hydrogen bromide which is believed to be the major compound in the disruption of the radical changes. Reactions [3] and [4] display the subsequent reactions to form hydrogen gas and water. (Horrocks et al, 2001; Ampacet)

Although these PBDEs possess a suitable mechanism for flame retardation many properties which make them an efficient retardant also are the properties which make them toxic to us. For example the large phenyl rings gives the compounds adequate lipophilicity to penetrate the plasma membrane, thus promoting bioaccumulation (Boer et al, 2005).

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