The Chemistry of Flame Retardants: Part Two-The Environmental Impact of Brominated Flame Retardants

Brominated flame retardants show some of the complexities of the problems faced with many of the materials that we use in life. Obviously, the effects of fires are terrible. There is severe, acute danger to the ease of ignition and flammability associated with the many materials that our daily life is so dependent on. The incorporation of flame retardants immediately reduces this problem. But the use of flame retardants isn't without its own pitfalls. The second most common flame retardant in commercial use is polybrominated diphenylethers (PBDE). These compounds can have up to ten bromine atoms attached to them, and their use is dependent on the number of bromines that are attached to the diphenylether. In chemistry, a difference of one atom can make massive changes to its chemical behaviour. For example, cyanide is one carbon atom triple bonded to one nitrogen atom, and is extremely poisonous. But the atomospere is 75% N2, which is one nitrogen atom triple bonded to one nitrogen atom, and is completely innocuous. These two compounds differ only be one atom. The three most common PBDEs are deca, penta, and octa (10, 5, and 8 bromine atoms). One of the problems with PBDEs is that they are not chemically bonded to any of the materials that they are incorporated into, they are simply physically mixed in. This means that they can be leached from the material and into the environment. 

To examine the problem of their presence in the environment, we need to look at what characteristics makes the chemicals good flame retardants. The chemicals need to be stable and they need to last long. If they weren't, the chemicals wouldn't stay around long enough in the materials that they are incorporated into and eventually those materials would become easily flammable again. So PBDEs are very stable and will last a long time without degrading. This means that if they leach into the environment they will not break down, but persist for years. The other downside of their ability to leach out of materials is a decrease in flame retardance over time. 

PBDE have been detected in arctic life. This suggests that they can be transported through the environment a long way from where they were initially released into it. This is termed "long-range transport". There is also evidence of "bioaccumulation". The chemical is taken up by organisms low on the food chain, and those organisms are in turn take up by organisms higher up on the food chain. The result is that what was a small amount of chemical in an organism low on the food chain becomes a much larger amount of chemical in organisms higher up on the food chain. This process happens when a particular chemical is not broken down in the digestive system of organisms, but rather stored, in fats usually. Mercury is an example of another chemical that is know to bioaccumulate. Beyond that, more labile PBDEs, like deca-PBDE, will break down into its more persistent cousins, penta- and tetra (four bromines) -PBDE. PBDEs have also been shown to degrade overtime, using heat and light, to toxic chemicals: polybrominated dibenzodoxins and polybrominated dibenzofurans. So even though they are not acutely toxic, PBDEs may prove to have chronic effects.

The evidence of environmental impact of PBDEs have prompted legislation against them. In Canada there is legislation against PBDE under subsection 93(1) of the Canadian Environmental Protection Act, 1999. In the United States, there is no federal legislation, but many states have bans against PBDEs. The European Union has banned the use of PBDEs.

What I find interesting about the case of brominated flame retardants is that it highlights many of the complexities associated with the problems with the chemicals in our life. These chemicals are not good for the environment, and we shouldn't use them; however, the results of not using flame retardants are equally damaging. Solutions are being researched to find effective flame retardants that are not environmentally damaging. It is important to understand that these materials weren't designed to be damaging or done by "evil scientists in labs who don't care about the environment". They were designed to solve a problem. That problem was the flammability of materials. Unfortunately, they also created a problem. Every action will have a reaction. 

References:

See part one for the references.

The Chemistry of Flame Retardants: Part One-What is a Flame Retardant?

Here in Northern Alberta, Canada we have had a disaster unprecedented in our province: wild fires. We have been hit with numerous fires that have impacted over 10 000 people in the province. Most notably are the residents of the town of Slave Lake, Alberta. On May 15, 2011 the fires actually entered the town, destroying one third of the town. After being forced to flee their homes, many residents returned to find that they had lost their homes and businesses to the fire. This tragedy caught the eyes of the world and even prompted a stop by the Duke and Duchess of Cambridge during their Canadian visit to meet with those affected by this disaster. So I dedicate this post to those affected by the Northern Alberta fires. Anyone wishing to support the many victims of this disaster can do so by making donations to the Canadian Red Cross. Information on the relief and recovery effort of this disaster is also available on the Canadian Red Cross website: www.redcross.ca

I am part of a team of disaster management volunteers and was deployed on May 15th to assist the victims of the Northern Alberta fires. As I drove north, on my way to High Prairie, Alberta, the landscape became an eerie red colour. The road, the treeline, the areas recovering from fires ten years past, all were covered in a red film. That red film was flame retardant and that image was the inspiration for today's blog entry: chemistry of flame retardants.  

Flame retardants are actually the second most common additive to the polymers that make up the various materials on which our modern western culture has become so reliant: these include polystyrenes, polyesters, epoxy resins, polyethylenes, polyurethanes. Take a look around your home: chances are you are currently using a computer, the circuitry, the wiring, and the casing is comprised of these polymers. Many textiles: curtains, upholstery, and clothing are also comprised of these polymers. Anything in your household that is "plastic" is made of these polymers. Why might adding a flame retardant be so important? These polymers are made of hydrocarbons-they come from petroleum, just like the fuel used in combustion engines. Anything made of a hydrocarbon (a chemical that is rich with hydrogen-carbon bonds) burns really well.  Flame retardants are, therefore, used to prevent or minimise the risk of fire.  The use of flame retardants have been documented as early as 450 BC, when Egyptians used alum (potassium aluminum sulfate hydrate) to reduce the flammability of wood. In the 17th century, Parisian theatre curtains were made "incombustible" by soaking them in a mixture of clay and gypsum. In 1735, the first patent was taken out on fire retardants. 

There are four classes of flame retardants: inorganic, halogenated organic, organophophorous, and nitrogen-based. I don't actually know what flame retardant was sprayed  on the land during the Northern Alberta fires. (A quick Google search suggests that it could be some mixture of ammonia based compounds-take that at face value since there is no verification on its make up and therefore I cannot comment on any environmental impact.) The flame retardants that I will write about today are halogenated organic, specifically brominated flame retardants. In chemistry, the term "organic" refers to chemicals that are rich in the element carbon. Halogens are in the 17th column (the second last) of the periodic table. The elements are, in descending order, fluorine, chlorine, bromine, iodine, and astatine. A halogenated organic compound is one where halogens are bonded to carbons.

How might flame retardants prevent combustion? Combustion is an oxidative (this is why the presence of oxygen in fires is so critical) gas phase reaction. The process of combustion can be broken down into four steps: preheating, volatilisation/decomposition (the reaction takes place in the gas phase-volatilisation is this phase change), combustion, and propagation. A flame retardant will target any one of these steps to prevent combustion. Halogenated flame retardants target the propagation step. In the propagation step, many free radicals are produced, which are what continues the chemical reactions in the burning process. A free radical is a chemical that has an unpaired electron-which makes them really reactive. I stated earlier that halogens were in the second last column of the periodic table; the last column is the noble gas column. (Helium, neon, argon, krypton, xenon, radon.) These gases are considered to be "inert". What separates the halogens from noble gases is one electron. If a halogen can get one electron then they can be as inert (and therefore happy) as a noble gas. If radicals lose one electron, then they have all their electrons paired and are super happy and unreactive too. 

Halogenated flame retardants are added to a material. If this material is lit on fire then the carbon-halogen bond will break, releasing the halogen into the gas phase-which is where the combustion is taking place. The halogen will find and capture radicals (the term used to describe this process is "scavenge") and will prevent the fire from continuing. Certain halogens are better at scavenging radicals than others. Iodine is actually the most efficient halogen at radical capture, fluorine is least efficient. But this is not the only property considered. The strength of the carbon-halogen bond is also important to consider. The halogen-carbon bond has to be strong enough not to degrade before the combustion temperature is reached, otherwise the halogen will be lost and therefore unable to capture any radicals. But if the bond is too strong then the halogen will never be released into the gas phase, and also will be unable to capture any radicals. It turns out that the carbon-fluorine bond is too strong and degrades at too high of a temperature. Fluorine is not a good flame retardant choice-it has poor efficiency, and too strong of a bond. Iodine is also a poor choice as a flame retardant. Though it has good radical capture ability, the carbon-iodine bond is too weak, and therefore degrades at too low of a temperature. This leaves chlorine, and bromine. While both are used in flame retardants, bromine is the halogen of choice because of its increased radical capture efficiency over chlorine, and the carbon-bromine bonds break at a lower, more ideal temperature for use as a flame retardant. 

The carbon part of the flame retardant is not part of the "flame retardant ability". It is present to hold the bromine (or chlorine) in a manner that doesn't interfere with the polymer function, but will allow it to be released upon decomposition in the flame. Some of the flame retardants are actually chemically bonded into the polymer, these are things like brominated styrenes, or tetrabromobisphenol-A. Other brominated flame retardants are simply mixed into the polymer and are not chemically bonded to the polymer, these are things like polybrominated diphenylethers, or hexabromocyclodocanes.  

Coming next: The Chemistry of Flame Retardants: Part Two-Environmental Impact of Brominated Flame Retardants.

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