Brominated Fire Retardants

Highly brominated organic compounds are common commercial fire retardants. Large amounts of these fire retardants are used worldwide, and because of their persistence, they now are accumulating in the environment and have even been detected in the Arctic, to which they presumably migrated by the LRTAP mechanism discussed above. Based upon animal studies, they may have potential for liver toxicity, thyroid hormone-level disruption, and reproduction and development effects.

Many brominated organic compounds function as fire retardants because, when heated to 200-300°C—the approximate temperature range in which many polymers begin to decompose—they release free bromine atoms, which react with the free radicals of combustion and thereby quench any fire. For example, when a molecule of such a compound has absorbed sufficient energy from the fire and released Br atoms from one or more of its C—Br bonds, the atom may react with one of the free H atoms associated with the free-radical mechanism of combustion:

The hydrogen bromide, HBr, molecule so formed may subsequently react with a free hydroxyl radical, which otherwise would also have continued to propagate the combustion:

The net reaction of these two steps is the formation of a water molecule by the reaction of H and OH, thereby reducing the concentration of highly reactive free radicals.

In this way, energy is withdrawn from the propagation mechanism of the combustion process, and the fire is quenched. The decomposition temperature of brominated fire retardants lies just below those of the polymers that they protect.

Iodine organic compounds would be even more effective in withdrawing energy, but since the C—I bond is so weak, they decompose at too low a temperature. Fluorinated compounds are generally unsuitable as fire retardants because most C—F bonds are so strong that free fluorine atoms would not be released and because, once formed, HF molecules are so stable that they would not participate in further reaction.

PBDEs: A New Persistent Pollutant

Brominated diphenyl ethers, especially at levels of 5-30%, are incorporated into polyurethane foam, textiles, ready-made plastic products, and certain electronic equipment to prevent them from ever catching on fire. They are found in common domestic products such as carpet padding, mattresses, curtains, and upholstered couches and chairs.

From a conceptual viewpoint, the molecular structure of diphenyl ether is analogous to biphenyl, except that an ether oxygen atom joins the two benzene rings. Bromine atoms can occupy any of the 10 other positions on the rings, analogous to the chlorine atoms in PCBs, again giving 209 possible congeners. Molecules with fewer than four bromines are generally not present in commercial mixtures.

diphenyl ether

Br a PBDE

diphenyl ether

Br a PBDE

As a class, these compounds are known as polybrominated diphenyl ethers, PBDEs,

PBDE molecules are of particular environmental concern because some migration of them has occurred from their commercial products into the environment, where they now are widely distributed. Like PCBs, they are persistent and lipophilic, they bioaccumulate, and some of them are toxic. PBDEs have been detected in U.S. sewage sludge (much of it destined to be spread on agricultural land), in some fish caught in the wild, and even in sperm whales, which normally feed only in deep ocean waters. Also like PCBs, the commercial products are mixtures of congeners, not pure compounds, although the number of congeners present in each product is relatively small. Unlike most PCBs, PBDEs are solids under ambient conditions rather than liquids.

The acute toxicity of PBDEs decreases as the number of bromines per molecule increases. Consequently, the least toxic PBDE is the fully brominated congener decabromodiphenvl ether. It is the almost exclusive ingredient (> 97%) in Dec a, the commercial mixture that is the predominant PBDE product on the market and that is used as the flame retardant in plastic components in computers and TV housings. Some scientists suspect that the PBDEs in Deca, while not themselves highly toxic, may degrade in the environment by loss of some bromine, thereby increasing the toxicity of the mixture, since PBDEs having intermediate bromine content are more toxic than is decabromodiphenyl ether. Indeed, there is growing evidence that Deca can undergo debromination by photochemical decomposition in sunlight; by reduction with elemental iron; and by metabolic processes in fish, such as carp and rainbow trout, and in rats. It is also debrominated anaerobically in sewage sludge if certain other chemicals are also present.

PROBLEM 12-6

Deduce the structures of (a) the three unique PBDEs formed by loss of one bromine atom, and (b) the twelve PBDEs formed by loss of two bromine atoms, from a molecule of decabromodiphenyl ether. Assume that the two rings cannot rotate relative to each other around the intermediate oxygen atom.

The product called Penta is a mixture mainly of PBDEs having four or five bromine atoms. It was used as a fire retardant in polyurethane foams, such as those used in furniture upholstery and padding in vehicles. The Penta mixture constitutes up to 30% by mass of some polyurethane foams, products that easily deteriorate by outdoor weathering and break into small, easily transportable fragments that can eventually find their way into natural waters. From this source, PBDE molecules can enter the aquatic food chain. Indeed, it is tetra- and pentabromodiphenyl ethers that are most widely distributed in the environment and that are the most bioaccumula-tive and the most toxic. The commercial PBDE product called Octa, which consists mainly of congeners with six or seven bromine atoms, is used in thermoplastics,

PBDEs with a large amount of bromine (more than six Br atoms per molecule) probably do not bioaccumulate because they are not readily incorporated by organisms; instead they bind to particles and accumulate in sediments. However, congeners with four to six bromines are taken up by organisms and have the potential to biomagnify in the food chain. They may in the future represent a danger to human health due to our exposure to them in food, especially fish.

Although some human exposure to PBDEs occurs through the food we eat, inadvertent digestion of household dust appears to be an even more important source, especially in North America. The difference in exposure to this source may account for the higher blood levels of PBDEs found in Americans and Canadians compared to Europeans.'One recent small-scale study found a definitive link between PBDE levels in human breast milk and their levels in the dust of the women's homes.

The concentrations of PBDEs in human blood, milk, and tissues had risen exponentially for three decades, until at least the early 2000s, with a doubl itig time of about five years. Though few data are available, the concentration of PBDEs in human breast milk is known to have risen sharply in the 1990s in both U.S. and Canadian women—by more than a factor of 10 from 1982 to 2002 for the latter—and is approaching that of PCBs, though the levels in milk samples from European women are much lower. The main human health concern is that PBDEs having relatively few bromine atoms may affect hormone and liver systems and interfere with neurodevelopment.

PROBLEM 12-7

The concentration of PBDEs in herring gull eggs from the Great Lakes was about 1100 ppb in 1990 and about 7000 ppm in 2000. What is the doubling time for the PBDEs in this source? If past trends continue, what will be the concentration in 2010? [Hint: For exponential growth Aekt, the doubling time is equal to 0.69/k,]

Because of environmental concerns, the European Union banned the Penta and Octa products in 2006. The sole North American manufacturer of Penta and Octa voluntarily ceased production of these products at the end of 2004- However, the Deca product is still available and widely used in Europe and North America, though there is much controversy within the European Union concerning whether or not it should be banned. The argument against banning brominated fire retardants is that they are vital in reducing losses of human lives and of property in fires.

Other Brominated Fire Retardants

Two non-PBDE brominated organic compounds are also widely used as fire retardants. Indeed, the retardant of largest usage volume of all is TBBPA, tetrabromobis/jhenol-A, a compound composed of molecules in which all four carbon atoms that stand ortho to the two hydroxyl groups of bisphenol-A (see Figure 12-2) have been brominated:

Commonly, TBBPA is incorporated chemically into the structure of polymers by covalent coupling at the two hydroxyl groups; retardants that are cova-lently bonded to polymers are called reactive ones. Printed circuit boards are a major reactive use for TBBPA. When incorporated into materials in this manner, retardants are much less likely to leach or volatilize into the environment compared to those such as PBDEs, which are only physically dissolved in materials and are called additive substances. In some products, however, TBBPA is used as an additive rather than a reactive retardant. The compound itself is not very toxic (its LD50 is several grams per kilogram), and although it has been found in some biota, it decomposes in air, water, and sediment in weeks or a few months.

The other important brominated fire retardant is the half-brominated cyclic hydrocarbon hexabromocyclododeeane, HBCD, which is used primarily as an additive in polystyrene foams used in building materials and upholstery, though it is also finding new applications in replacing PBDEs. It is not used as a reactive retardant, since it contains no reactive groups that can bond to the polymer chain. Like TBBPA, it is of low acute toxicity. However, it is now a ubiquitous contaminant in the environment, and it undergoes biomagnification in top predators such as birds of prey and marine mammals. To date, it has been detected only in much lower levels in humans. Its environmental levels in Europe are higher than those in North America, owing to its greater use there.

TBBPA

TBBPA

PROBLEM 12-8

Draw the structure of HBCD, given that its name is 1,2,5,6,9,10-hexabromo-cyclododecane. Then, knowing that Br- molecules will add across C—C bonds, deduce the structure of the cyclic triene that could be brominated to produce HBCD. [Hint: Dodeca means "twelve."]

Polybrominated biphenyls, PBBs, were also used as fire retardants but are now banned in some countries, including the United States. A 1973 industrial accident in Michigan resulted in the widespread contamination of the food supply there with PBBs.

Continue reading here: Perfiuorinated Sulfonates

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