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1,2,5,6,9,10-Hexabromocyclododecane (HBCD or HBCDD) is a brominated flame retardant primarily used in expanded and extruded polystyrene foam insulation.
Listed under the Stockholm Convention as a persistent organic pollutant, its global production and use are restricted due to its toxicity and environmental persistence.
Chemical Identity & Properties
CAS Number: 3194-55-6
Formula: C₁₂H₁₈Br₆
Molecular Weight: 641.70 g/mol
EC Number: 221-695-9
Key Isomers: Alpha (α-), beta (β-), and gamma (γ-) stereoisomers
Synonyms: Hexabromocyclododecane, HBCDD, HBCD, 1,2,5,6,9,10-Hexabromocyclododecane, 1,2,5,6,9,10-HBCD, Hexabromocyclododecan, Hexabromcyclododecan, Cyclododecane, 1,2,5,6,9,10-hexabromo-, 1,2,5,6,9,10-Hexabromocyclododecane, CAS No. 3194-55-6, HBCDDs, Hexabromocyclododecane mixture, Hexabromocyclododecane technical mixture, HBCDD technical mixture
Hexabromocyclododecane (HBCD or HBCDD) is a brominated flame retardant.
1,2,5,6,9,10-hexabromocyclodecane consists of twelve carbon, eighteen hydrogen, and six bromine atoms tied to the ring.
Its primary application is in extruded (XPS) and expanded (EPS) polystyrene foam used as thermal insulation in construction. Other uses are upholstered furniture, automobile interior textiles, car cushions and insulation blocks in trucks, packaging material, video cassette recorder housing, and electric and electronic equipment.
According to UNEP, "HBCD is produced in China, Europe, Japan, and the USA.
The last known current annual production is approximately 28,000 tonnes per year.
The main share of the market volume is used in Europe and China" (figures from 2009 to 2010).
Due to hexabromocyclododecane's persistence, long-range environmental transport, toxicity and ecotoxicity, the Stockholm Convention on Persistent Organic Pollutants lists 1,2,5,6,9,10-hexabromocyclodecane in Annex A to the convention with exemptions for production and use in expanded polystyrene and extruded polystyrene in buildings.
1,2,5,6,9,10-hexabromocyclodecane is synthesized via the bromination of cyclododecatriene.
1,2,5,6,9,10-Hexabromocyclododecane is a brominated flame retardant and a specific structural form of hexabromocyclododecane (HBCD).
1,2,5,6,9,10-Hexabromocyclododecane is identified by CAS No. 3194-55-6 and is included within the substance definition of hexabromocyclododecane under the Stockholm Convention.
1,2,5,6,9,10-Hexabromocyclododecane is a brominated flame retardant belonging to the hexabromocyclododecane (HBCD or HBCDD) group.
It is a highly brominated cyclic organic compound with the molecular formula C12H18Br6 and CAS Number 3194-55-6.
1,2,5,6,9,10-Hexabromocyclododecane has historically been used primarily as an additive flame retardant in expanded polystyrene (EPS) and extruded polystyrene (XPS) insulation materials used in buildings.
1,2,5,6,9,10-Hexabromocyclododecane has also been used in certain textile applications and electrical and electronic equipment.
The compound can exist in different stereoisomeric forms, including alpha-, beta- and gamma-HBCD.
Commercial HBCD has historically consisted of mixtures of these stereoisomers.
1,2,5,6,9,10-Hexabromocyclododecane is characterized by its high bromine content, which enables 1,2,5,6,9,10-Hexabromocyclododecane to interfere with combustion processes and improve the flame resistance of polymeric materials.
HBCD is environmentally significant because of its persistence, bioaccumulation potential, biomagnification potential and toxicity to aquatic organisms.
For these reasons, hexabromocyclododecane has been listed under Annex A of the Stockholm Convention on Persistent Organic Pollutants, with specific exemptions for certain applications.
Chemical Structure
1,2,5,6,9,10-Hexabromocyclododecane consists of a twelve-membered cyclododecane ring containing six bromine atoms attached to the carbon framework.
Commercial HBCD is not necessarily a single stereoisomer and may contain different diastereoisomers, including alpha-, beta- and gamma-HBCD.
Physical Properties
Commercially available HBCD is described as a white solid substance.
Its structure consists of a cyclic ring with bromine atoms attached.
The CAS number 3194-55-6 specifically identifies 1,2,5,6,9,10-Hexabromocyclododecane, while CAS No. 25637-99-4 is used for hexabromocyclododecane more generally in the Stockholm Convention documentation.
Properties
Melting point: 186 °C (367 °F; 459 K) (175–195 °C, depending upon isomer)
Solubility in water: 3.4 µg/L in water
ChEBI: CHEBI:134063 check
ChEMBL: ChEMBL375298
InChI: 1S/C12H18Br6/c13-7-1-2-8(14)10(16)5-6-12(18)11(17)4-3-9(7)15/h7-12H,1-6H2
InChIKey: DEIGXXQKDWULML-UHFFFAOYSA-N
Flame Retardant Properties
HBCD functions primarily as an additive flame retardant.
1,2,5,6,9,10-Hexabromocyclododecane has been incorporated into materials to provide fire protection during the service life of products and buildings and also during storage.
The main historical global application of HBCD has been in expanded polystyrene (EPS) and extruded polystyrene (XPS) foam insulation used in buildings.
Smaller applications have included textiles and electrical and electronic equipment
Uses & Applications
Polystyrene Insulation: Added to thermal insulation foams in the construction industry.
Textiles: Formerly used in back-coating for residential and transportation textiles.
Electronics: High-impact polystyrene (HIPS) housing for electrical equipment.
Environmental & Health Concerns
Persistence: Resists natural degradation in air, water, and soil.
Bioaccumulation: Builds up in the fatty tissues of aquatic life, birds, and mammals.
Toxicity: Acts as an endocrine disruptor and affects liver and thyroid functions.
Properties
HBCD has 16 possible stereo-isomers, each with distinct biological activities.
The HBCD commercial mixture is composed of three main diastereomers denoted as alpha (α-HBCD), beta (β-HBCD), and gamma (γ-HBCD) with traces of others.
A series of four published in vivo mice studies were conducted between several federal and academic institutions to characterize the toxicokinetic profiles of individual HBCD stereoisomers. The predominant diastereomer in the HBCD mixture, γ-HBCD, undergoes rapid hepatic metabolism, fecal and urinary elimination, and biological conversion to other diastereomers with a short biological half-life of 1–4 days.
After oral exposure to the γ-HBCD diastereomer, β-HBCD was detected in the liver and brain, and α-HBCD and β-HBCD was detected in the fat and feces with multiple novel metabolites identified - monohydroxy-pentabromocyclododecane, monohydroxy-pentabromocyclododecene, dihydroxy-pentabromocyclododecene, and dihydroxy-pentabromocyclododecadiene.
In contrast, α-HBCD is more biologically persistent, resistant to metabolism, bioaccumulates in lipid-rich tissues after a 10-day repeated exposure study, and has a longer biological half-life of up to 21 days; only α-HBCD was detected in the liver, brain, fat and feces with no stereoisomerization to γ-HBCD or β-HBCD and low trace levels of four different hydroxylated metabolites were identified.
Developing mice had higher HBCD tissue levels than adult mice after exposure to either α-HBCD or γ-HBCD indicating the potential for increased susceptibility of the developing young to HBCD effects.
The reported toxicokinetic differences of individual HBCD diastereoisomers have important implications for the extrapolation of toxicological studies of the commercial HBCD mixture to the assessment of human risk.
Uses
1,2,5,6,9,10-Hexabromocyclododecane has been associated with flame-retardant applications in polystyrene insulation materials.
1,2,5,6,9,10-Hexabromocyclododecane has also been used in textile applications, including residential and commercial upholstered furniture, transportation seating, wall coverings and draperies.
The most important historical use was the incorporation of HBCD into EPS and XPS insulation to reduce the flammability of these materials.
Production
Commercial HBCD has been produced using a batch process.
The Stockholm Convention describes production involving the addition of elemental bromine to cyclododecatriene at approximately 20–70°C in the presence of a solvent in a closed system.
Environmental Persistence
HBCD is considered a persistent organic pollutant.
1,2,5,6,9,10-Hexabromocyclododecane has properties associated with environmental persistence, potential for long-range environmental transport, bioaccumulation and biomagnification.
HBCD is also considered very toxic to aquatic organisms.
These environmental properties were important factors in its evaluation under the Stockholm Convention.
Bioaccumulation
HBCD has a strong potential to bioaccumulate and biomagnify in organisms and food webs.
This characteristic contributes to its classification as a persistent organic pollutant.
Human Health
Information concerning the human toxicity of HBCD has historically been limited.
However, international assessments have identified potential concerns for vulnerable groups, particularly in relation to observed neuroendocrine and developmental effects.
Regulatory Status
Hexabromocyclododecane is listed under Annex A of the Stockholm Convention on Persistent Organic Pollutants.
The listing was adopted through decision SC-6/13.
The Stockholm Convention defines hexabromocyclododecane to include hexabromocyclododecane (CAS No. 25637-99-4), 1,2,5,6,9,10-Hexabromocyclododecane (CAS No. 3194-55-6), and its main diastereoisomers: alpha-HBCD, beta-HBCD and gamma-HBCD.
Specific exemptions were established for the production and use of HBCD as a flame retardant in expanded polystyrene and extruded polystyrene in buildings.
Alternatives
The production of HBCD has decreased, and chemical alternatives have become available for applications such as high-impact polystyrene and textile back-coating.
The Stockholm Convention also notes that replacement can require qualification and re-certification of polymer and foam products because of fire-rating requirements.
Environmental Classification
HBCD is particularly important from an environmental perspective because 1,2,5,6,9,10-Hexabromocyclododecane combines several characteristics associated with persistent organic pollutants: persistence, bioaccumulation and biomagnification, potential for long-range environmental transport and high toxicity to aquatic organisms.
Overall, 1,2,5,6,9,10-Hexabromocyclododecane is a brominated flame-retardant compound associated primarily with the historical use of HBCD in EPS and XPS insulation.
Its environmental persistence, bioaccumulation potential and aquatic toxicity led to its inclusion under Annex A of the Stockholm Convention, subject to specific exemptions.
