What better way to kick off the week than with a discussion of chemistry! And in keeping with the fire theme from last week I though it would be good to talk about the chemistry behind fire, and try to answer the question, what is fire?. The question has been asked throughout history. Aristotle considered fire one of the major elements of the universe, along with water, earth and air. Alchemists and early chemists, namely Johann Joachim Becher, believed fire was caused by the liberation of a substance called phlogiston. The phlogiston theory posited that all flammable materials contain phlogiston, a massless, odorless, colorless, tasteless substance that is liberated upon burning. The idea of phlogiston was eluded to in the Star Trek The Next Generation episode, Thine Own Self, where Data crash lands on a planet and loses his memory and is forced relearn everything from this pre-industrial society, which has an Aristotelian view of the universe. Data shoots down that theory quite nicely. Just as Data was able to poke holes in that theory, so too was Antoine-Laurent Lavoisier able to poke holes in the phlogiston theory. In 1777, Lavoisier demonstrated that burning is a process that involves the combination of a substance with an element in the air, which he named oxygen (previously described as “dephlogisticated air” by Joseph Priestly). Lavoisier explained combustion not as the removal of phlogiston, but rather as the addition of oxygen, a process called oxidation. When oxidation reactions occur at high temperatures, and in the presence of fuel, fire is produced. Thus, fire is the visible, tangible side effect of matter changing form as part of a chemical reaction that releases heat and light.
In the combustion process the following steps happen; First an energy source (heat, incandescent material or a small flame) acts as the initial ignition source. The energy is transmitted by the ignition source to the material (wood, polymer etc.), where pyrolysis takes place. Pyrolysis is a process that degrades the long-chain molecules in the material into smaller hydrocarbon molecules, which in turn release into the gas phase. In the condensed phase, the result is an inert carbonised material called char. It is in the gas phase where the combustible gases released from the pyrolysis reaction combine with oxygen, producing an exothermic chemical reaction (flame), which involves high-energy free radicals (H• and OH•). Incomplete combustion products are emitted as smoke, and the energy emitted during the exothermic reactions is transmitted back onto the material and reinforces pyrolysis. As you can tell from its cyclical nature, that left unchecked this chemical reaction has potential for great destruction. And that is where flame retardants come into play!
The term “flame retardant” describes a function and not a chemical class. Flame retardants act in products, such as plastics, textiles and foams, to make them less likely to ignite, and if they are ignited, to burn much less efficiently. There are more than 175 different types of flame retardants currently in commerce, and they are commonly divided into four major groups; inorganic, organophosphorus, nitrogen-containing and halogenated flame retardants. Halogenated flame retardants (HFRs) have historically been favored because of their accessibility, low cost and efficacy. The performance of halogens as flame retardants is rated I > Br > Cl > F. Bromine and chlorine compounds are the only halogen compounds having commercial significance as flame-retardant chemicals. Fluorine compounds are generally ineffective because the C-F bond is too strong. Iodine compounds, although quite effective, are too unstable to be useful as the iodine is liberated by even a negligible energy supply. The brominated flame retardants (BFRs) are much more numerous than the chlorinated types because of their higher efficacy due to the weaker bonding to carbon as compared to chlorine. BFRs are either reacted with or added to a base polymer. Reactive flame retardants become a part of the polymer either by becoming a part of the backbone or by grafting onto the backbone. The strength of this bond limits the potential for outgassing and maintains fire safety properties over the lifetime of the product. Conversely, with additive incorporation the flame retardant does not form a chemical bond with the material and as such may be more prone to enter the environment and decrease flame retardant efficacy over the lifetime of the product. Regardless of the method of incorporation the general mechanism for BFRs is the same. BFRs chemically interact with the gas phase during combustion by emitting low-energy halogen radicals, Br•. Bromine is effective because Br• is liberated over a narrow temperature range so that it is available at a high concentration in the flame zone. These low energy radicals will interfere with the radical chain mechanism by substituting for high-energy free radicals (H• and OH•) in the gas phase, quenching the exothermic reaction which leads to flame formation. The absence or reduction of energy release retards the burning process, thus slowing the spread, or preventing the establishment and sustainment, of the fire cycle. BFRs, do not stop a fire from starting but they do slow down its spread, which I will talk about tomorrow.