The flame retardancy of polybrominated diphenyl ethers (PBDEs) increases with the number of bromine atoms in the molecule. Therefore only the higher brominated BDEs like Penta, Octa or Deca are of commercial interest.
The general chemical formula of polybrominated diphenyl ethers is:
PBDEs have many congeners depending on the number and position of the bromine atoms on the two phenyl rings. The total possible number of congeners is 209, and the number of isomers for mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona- and deca BDE are: 3, 12, 24, 42, 46, 42, 24, 3 and 1, respectively.
PBDEs are produced by the bromination of diphenyl ether. Technical OBDE may be produced by the reaction of diphenyl ether with eight equivalents of bromine in the presence of Al2Cl6/Al2Br6 first at 35 oC and then at 120 ¡C. Technical PeBDE is synthesized by treating diphenyl ether with five equivalents of bromine at 30-65 ¡C in the presence of powdered iron.
Commercial or technical grade PBDEs are generally mixtures which are in part a result of the production process (impurities). For example, HexaBDE is not purposefully produced, but almost always as a by-product of the Penta or Octa BDE production. The compositions of commercial BDEs are given in table 1 below.Footnote 15
a Unknown structure.
b No longer commercially produced. Analysis of one single sample.
However there are many variations likeFootnote 16:
|PBDEa||TrBDE||TeBDE||PeBDE and HeBDE||HpBDE||OBDE||NBDE||DeBDE|
Commercial DeBDE was the most widely used flame retardant in the world in 20012with OBDE being the second most widely used (largely in acrylinitrile-butadiene-styrene (ABS) polymer (12 – 18% by weight) in computer and other business machine cases / cabinets. However, OBDE is being phased out and replaced by tetrabromobisphenol (TBBP-A) as this is stable in UV light, while OBDE is not.
Another advantage is that TBBP-A is chemically bonded to the polymers while PBDEs are just physical blends. During abrasion or corrosion of the polymers PBDEs escape more readily to the environment while TBBP-A will still be bonded to the polymers. It is mainly used in printed circuit boards and in ABS which is used in TV’s.
Another ‘more modern’ brominated flame retardant is hexabromocyclododecane (HBCD) which has 16 stereo isomers of which the most common are:
HBCD is mostly used in Extruded (XPS) and expanded (EPS) polysterene at resp 0.7 and 2.5%Footnote 17
Pa at 25°C
|Water solubility µg/L at 21°C||
Octanol/water distribution coefficient
* for upper trophic level
A few physical properties of BDEs are presented below in the form of graphs, called homologe series, to show the relationship between the various compounds and their dependancy on their molecular make up.
In Figure B.1 the octanol – water partitioning coefficient is shown in relation to the number of bromine atoms. Clearly the higher the bromination the higher the affinity of the compound to reside in the octanol phase. Among other things this explains the very low solubility of BDEs in water.
This is shown in Figure B.2 where clearly the higher brominated compounds display a lower solubility. Note the scale is logaritmic. The much higher solubility underlies the higher toxicity of the lower brominated coumpounds. Unfortunately, as is described later in this report, the higher brominated coumpounds degrade in the environment into the lower brominated compounds.
Figure 7 is a graph that illustrates the reverse of Figure 6. This time, it is water solubility that is shown on the y-axis (nanograms BDE dissolved per litre of water at 21 degrees centigrade) in logarithmic scale versus the number of bromine atoms in BDE molecules on the x-axis (ranging from 1 to 10). A BDE with one bromine in the molecule has a much higher level of solubility (between 1.E+06 and 1.E+07) than deca-BDE that has a solubility of less than 1.E+02. The toxicity of the BDEs is believed to be associated with its solubility, the lower brominated BDEs being progressively more toxic.
Figure B.3 shows the vapour pressure, or the ease of evaporation. At bromination of 5 bromine atoms and higher the vapour pressures are very low, at 10-5 to 10-6 Pascal at 21 oC. This means the BDE’s used as flame retardants are unlikely to evaporate quickly at room temperature and equally unlikely to evaporate out of landfills. However at higher temperatures (during a fire) they do evaporate and fall apart releasing free bromine which binds with the combustible materials before oxygen can and thereby extinguishing the fire.
Figure 8 is a graph that illustrates the inverse relationship between vapour pressure (y-axis) and the number of bromine atoms in BDE molecules (x-axis) i.e. the heavier the molecule the lower its vapour pressure, and in other words, the lower its propensity to evaporate. Vapour pressure, is set out logarithmically on the y-axis in Pascals relative to 10 -6 (10 to the minus 6) Pascals at 25 degrees centigrade. Vapour pressure drops steeply with each additional bromine atom but the steepness of the curve tails off with the BDEs that contain more than 5 bromine atoms. The significance of this is explained in the text.
The commercial BDEs have their own CAS (Chemical Abstract Services) identification number which sets them apart from the individual congeners. In time, due to changing production processes the composition has changed. Some manufacturers also produce different blends.
* These poly-BDE’s are not produced as individual flame retardants, but are by-products of the fabrication process
b Older deca-BDE formulation no longer commercialised
Back to footnote reference 15 IPCS InChem, International Programme on Chemical Safety (UNEP-WHO), Environmental Health Criteria no. 162, by Dr. G.J van Esch, Bilthoven, Netherlands, published Geneva 1994, Brominated Diphenyl Ethers, http://www.inchem.org/documents/ehc/ehc/ehc162.htm#SectionNumber:2.2
Back to footnote reference 16 TOXICOLOGICAL SUMMARY FOR SELECTED POLYBROMINATED DIPHENYL ETHERS Integrated Laboratory Systems B.L.Carson, 03-2001 , http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPdf/PBDEs.pdf