An overview of commercially used brominated flame retardants, their applications, their use patterns in different countries/regions and possible modes of release
Introduction
Advances in polymer science over the past 50 years has led to the introduction of a large number of polymers with different properties and applications. As a result, we are surrounded by a wide variety of polymers in clothing and furniture, to electronics, vehicles and computers. In fact modern cars contain in excess of 100 kg of various polymers. Most of these polymers are petroleum-based and hence are flammable. In order to meet fire safety regulations, flame retardants are applied to combustible materials such as plastics, wood, paper, and textiles. Flame retardants (FR) are materials added or applied to a material to increase the fire resistance of that product (EHC-192, 1997). A list of annual production figures of major polymers along with the brominated flame retardants (BFRs) commonly used to increase their fire resistance is given in Table 1 (Arias, 2001).
With the increasing usage of polymeric materials in construction, electronic and computer equipment, global market demand for the use BFRs continues to grow substantially; for example the global market demand for BFRs in 1990 was 145 000 tonnes (Pettigrew, 1994), grew to over 310 000 tonnes in 2000 (BSEF, 2000), which represents a growth of over 100% over the past decade.
The idea of flame retardant materials dates back to about 450 BC, when the Egyptians used alum to reduce the flammability of wood. The Romans (about 200 BC) used a mixture of alum and vinegar to reduce the combustibility of wood (Hindersinn, 1990). Today, there are more than 175 chemicals classified as flame retardants. The four major groups are inorganic, halogenated organic, organophosphorus and nitrogen-based flame retardants which account for 50%, 25%, 20% and >5% of the annual production, respectively (EHC-192, 1997).
To understand the modes of action of flame retardants, it is essential to become familiar with the combustion process. Combustion is a gas phase reaction involving a fuel source and oxygen. As illustrated in Fig. 1, the four steps involved in the combustion process are preheating, volatilization/decomposition, combustion and propagation (Troitzch, 1990).
Depending on the mode of action, flame retardants can act at any of the four steps involved in the combustion process, and prevent their occurrence. For example, at temperatures above 200 °C the dehydration of aluminum hydroxide takes place, which is an endothermic reaction resulting in diluting and lowering the temperature of the flame. Another effective method is to capture free radicals (highly oxidizing agents) that are produced during the combustion process; which are essential elements for the flame to propagate. Halogens are very effective in capturing free radicals, hence removing the capability of the flame to propagate. All four halogens are effective in eliminating free radicals, and the trapping efficiency increases with the size of the halogen (i.e., I>Br>Cl>F). Therefore, all organohalogen compounds could be a good form of storage and delivery of halogens to be used as flame retardants. However, not all of the halogens are suitable for use in flame retardants. Fluorinated compounds are very stable and decompose at much higher temperatures than most organic matter burns, delivering their halogens too late to be effective as a FR. On the other hand, iodinated compounds are not stable and decompose at slightly elevated temperatures. Consequently, only organochlorine and organobromine compounds are used as flame retardants. With higher trapping efficiency and lower decomposing temperature, organobromine compounds have become more popular as a flame retardant than their organochlorine counterparts. Since bromine is the major component of a BFR, there is no particular restriction on the structure of the backbone. The main criteria for the usage of a compound as flame retardant are stability during the lifetime of the product and compatibility with the polymer. As a result, there are more than 75 different aliphatic, aromatic and cyclo-aliphatic compounds used as brominated flame retardants. Since bromine is the principle ingredient for BFRs, it is necessary to review the production and applications of bromine as an industrial chemical.
Section snippets
Bromine production and applications
Bromine is a member of group VII elements (halogens) and was discovered in 1826 by Antoine Balard. Bromine is a dense, mobile, dark red liquid at room temperature. Like other members of group VII, bromine is a reactive element, consequently, it is mostly found in the form of inorganic salts of the alkalis and alkaline earth metals mainly in seawater, saline lakes, and earth crust. Therefore, bromine is extracted from brines around the world. The production of bromine begins with the oxidation
Brominated flame retardants
BFRs are divided into three subgroups depending on the mode of incorporation of these compounds into the polymers: brominated monomers, reactive and additive. A brominated monomer such as brominated styrene or brominated butadiene is used in the production of brominated polymers, which are then blended with nonhalogenated polymers or introduced into the feed mixture prior to polymerization, resulting in a polymer containing both brominated and non-brominated monomers. Reactive flame retardants,
Polybrominated biphenyls
Polybrominated biphenyls (PBBs) were introduced as flame retardants in the early 1970s. The commercial production of PBBs in the form of Firemaster® in the United States continued until 1976 and approximately 6071 tonnes of PBBs were produced during those 6 years. In 1973, Firemaster BP-6® and FF-1® were unintentionally mixed into cattle feed at a production site and distributed in rural Michigan. The widespread contamination of Michigan farm products that resulted from this accident led to the
Tetrabromobisphenol A
Tetrabromobisphenol A (TBBPA) [79-94-7] is a reactive flame retardant with a global consumption of 210 000 tonnes, which makes TBBPA the highest volume BFR on the market. TBPPA is produced via bromination of bisphenol A in an organic solvent.
Approximately 90% of TBBPA is used as a reactive intermediate in the production of epoxy and polycarbonate resins. The main application of epoxy resins is in the manufacturing of printed circuit boards that contain approximately 20% bromine. The remaining
Polybrominated diphenyl ethers
Polybrominated diphenyl ethers (PBDEs) are additive flame retardants and the next highest production group of BFRs currently in use. PBDEs are produced by bromination of diphenyl ether in the presence of a Friedel–Craft catalyst (i.e., AlCl3) in a solvent such as dibromomethane. Diphenyl ether molecules contain 10 hydrogen atoms, any of which can be exchanged with bromine, resulting in 209 possible congeners. The structure of PBDEs is similar to that of PCBs, hence the nomenclature proposed by
Hexabromocyclododecane
Hexabromocyclododecane (HBCDD) [25637-99-4] is a white crystalline powder, with 74.7% bromine. HBCDD is a cyclic compound produced from bromination of cyclododecatriene (a butadiene trimer), which results in the formation of three isomers (α, β and γ), with the γ isomer being the predominant product. The structures of the three isomers of HBCDD are presented in Fig. 4 (Chemical Abstracts, 1996). HBCDDs are susceptible to thermal degradation (Barontini et al., 2001). Furthermore, these congeners
Additional information
This manuscript has provided an overview of the use and application of BFRs. Additional information on BFRs is available in a number of reports prepared by the World Health Organization, a general overview is presented in Environmental Health Criteria 192 (EHC-192, 1997), and on specific BFRs such as TBBPA and derivatives, PBBs and PBDEs are presented in Environmental Health Criteria 172 (EHC-172, 1995), 152 (EHC-152, 1994), and 162 (EHC-162, 1994), respectively. Recent reviews by de Wit (2002)
Acknowledgments
The authors acknowledge editorial assistance provide by Mrs. Sue Quade from the University of Guelph and Mr. Richard Jones from CDC.
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