Flame retardant

This article is about chemical flame retardants used in textiles, thermoplastics and thermosets. For chemicals used to fight structure fires and wildfires, see fire retardant.

Flame retardants are chemicals used in thermoplastics, thermosets, textiles and coatings that inhibit or resist the spread of fire. These can be separated into several different classes of chemicals:

Minerals such as aluminium hydroxide ATH, magnesium hydroxide MDH, hydromagnesite, various hydrates, red phosphorus, and boron compounds, mostly borates.

Organohalogen Compounds. These include organochlorines such as, chlorendic acid derivatives and chlorinated paraffins; organobromines such as decabromodiphenyl ether (decaBDE), decabromodiphenyl ethane (a replacement for decaBDE), polymeric brominated compounds such as brominated polystyrenes, brominated carbonate oligomers (BCOs), brominated epoxy oligomers (BEOs), tetrabromophthalic anyhydride, tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCD). Most but not all halogenated flame retardants are used in conjunction with a synergist to enhance their effeciency. Antimony trioxide is widely used but other forms of antimony such as the pentoxide and sodium antimonate are also used.

Organophosphorus compounds such as organophosphates, tris(2,3-dibromopropyl) phosphate, TPP, RDP, BPADP, tri-o-cresyl phosphate, phosphonates such as DMMP and phosphinates. There is also an important class of flame retardants that contain both phosphorus and halogen, examples of such are the chlorophosphates like TMCP and TDCP.

Mineral flame retardants are typically additive while organohalogen and organophosphorus can be either reactive or additive. The basic mechanisms of flame retardancy vary depending on the specific flame retardant and the substrate. Additive and reactive flame-retardant chemicals can function in the vapor or condensed phase.

The annual consumption of flame retardants is currently over 1.5 million tonnes per year, which is the equivalent of a sales volume of approx. 1.9 billion Euro (2.4 billion US-$).

Endothermic degradationSome compounds break down endothermically when subjected to high temperatures. Magnesium and aluminium hydroxides are an example, together with various hydrates such as hydromagnesite. The reaction removes heat from the substrate thus cooling the material. The use of hydroxides and hydrates is limited by their relatively low decomposition temperature, which limits the maximum processing temperature of the polymers (typically used in polyolefins for wire and cable applications).

Thermal shieldingA way to stop spreading of the flame over the material is to create a thermal insulation barrier between the burning and unburned parts. Intumescent additives are often employed; their role is to turn the polymer into a char, which separates the flame from the material and slows the heat transfer to the unburned fuel.

Dilution of gas phaseInert gases (most often carbon dioxide and water) produced by thermal degradation of some materials act as diluents of the combustible gases, lowering their partial pressures and the partial pressure of oxygen, and slowing the reaction rate.

Gas phase radical quenchingChlorinated and brominated materials undergo thermal degradation and release hydrogen chloride and hydrogen bromide or if used in the presence of a synergist like antimony trioxide antimony halides. These react with the highly reactive H· and OH· radicals in the flame, resulting in an inactive molecule and a Cl· or Br· radical. The halogen radical has much lower energy than H· or OH·, and therefore has much lower potential to propagate the radical oxidation reactions of combustion.

 
       Environmental prevalenceIn 2009, the U.S. National Oceanic and Atmospheric Administration (NOAA) released a report on polybrominated diphenyl ethers (PBDEs) and found that, in contrast to earlier reports, they were found throughout the U.S. coastal zone:This nationwide survey found that New York’s Hudson Raritan Estuary had the highest overall concentrations of PBDEs, both in sediments and shellfish. Individual sites with the highest PBDE measurements were found in shellfish taken from Anaheim Bay, California, and four sites in the Hudson Raritan Estuary. Watersheds that include the Southern California Bight, Puget Sound, the central and eastern Gulf of Mexico off the Tampa-St. Petersburg, Fla. coast, and Lake Michigan waters near Chicago and Gary, Ind. also were found to have high PBDE concentrations.

Health concernsFlame retardants have faced renewed attention in recent years. The earliest flame retardants, polychlorinated biphenyls (PCBs) were banned in 1977 when it was discovered that they were toxic.[3] Industries shifted to using brominated flame retardants instead, but these are now receiving closer scrutiny. The EU has banned several types of polybrominated diphenyl ethers (PBDEs) as of 2008, 10 years after Sweden discovered that they were accumulating in breast milk. As of December 2009, negotiations between EPA and the two U.S. producers of DecaBDE (a flame retardant which has been used in electronics, wire and cable insulation, textiles, automobiles and airplanes, and other applications), Albemarle Corporation and Chemtura Corporation, and the largest U.S. importer, ICL Industrial Products, Inc., resulted in commitments by these companies to phase out decaBDE for most uses in the United States by December 31, 2012, and to end all uses by the end of 2013.

Nearly all Americans tested have trace levels of flame retardants in their body. Recent research links some of this exposure to dust on television sets, which may have been generated from the TV heating up the flame retardants in the TV. Careless disposal of TVs and other appliances such as microwaves or old computers may greatly increase the amount of environmental contamination.A recent study conducted by Harley et al. 2010[7] on pregnant women, living in a low-income, predominantly Mexican-immigrant community in California showed a significant decrease in fecundability associated with PBDE exposure in women.

Another study conducted by Chevrier et al. 2010[8] measured the concentration of 10 PBDE congeners, free thyroxine (T4), total T4, and thyroid-stimulating hormone (TSH) in 270 pregnant women around the 27th week of gestation. Associations between PBDEs and free and total T4 were found to be statistically insignificant. However, authors did find a significant association amongst exposure to PBDEs and lower TSH during pregnancy, which may have implications for maternal health and fetal development.

A prospective, longitudinal cohort study initiated after 11 September 2001, including 329 mothers who delivered in one of three hospitals in lower Manhattan, New York, was conducted by Herbstman et al. 2010. Authors of this study analyzed 210 cord blood specimens for selected PBDE congeners and assessed neurodevelopmental effects in the children at 12–48 and 72 months of age. Results showed that children who had higher cord blood concentrations of polybrominated diphenyl ethers (PBDEs) scored lower on tests of mental and motor development at 1–4 and 6 years of age. This was the first study to report any such associations in humans.

A similar study was conducted by Roze et al. 2009in Netherlands on 62 mothers and children to estimate associations between 12 Organohalogen compounds (OHCs), including polychlorinated biphenyls (PCBs) and brominated diphenyl ether (PBDE) flame retardants, measured in maternal serum during the 35th week of pregnancy and motor performance (coordination, fine motor skills), cognition (intelligence, visual perception, visuomotor integration, inhibitory control, verbal memory, and attention), and behavior scores at 5–6 years of age. Authors demonstrated for the first time that transplacental transfer of polybrominated flame retardants was associated with the development of children at school age.

Another interesting study was conducted by Rose et al. 2010 to measure circulating PBDE levels in 100 children between 2 to 5 years of age from California. The PBDE levels according to this study, in 2- to 5-year-old California children was 10 to 1,000 fold higher than European children, 5 times higher than other U.S. children and 2 to 10 times higher than U.S. adults. They also found that diet, indoor environment and social factors influenced children’s body burden levels. Eating poultry and pork contributed to elevated body burdens for nearly all types of flame retardants. Study also found that lower maternal education was independently and significantly associated with higher levels of most flame retardant congeners in the children.

San Antonio Statement on Brominated and Chlorinated Flame Retardants 2010:A group of 145 prominent scientists from 22 countries signed the first-ever consensus statement documenting health hazards from flame retardant chemicals found at high levels in home furniture, electronics, insulation, and other products. This statement documents that with limited fire safety benefit, these flame retardants can cause serious health issues and as types of flame retardants are banned, the alternatives should be proven safe before being used. The group also wants to change widespread policies that require use of flame retardants.

Sudden infant death syndromeMain article: Sudden infant death syndrome#Toxic_gases UK scientist Barry Richardson claimed in 1989 that a fungus in bedding broke down the antimony, phosphorus, and arsenic flame retardants in infant bedding to form toxic gases. This research was taken up by New Zealand scientist Jim Sprott, who published a book on it, and eventually aired on The Cook Report in 1994. A 1998 UK government-sponsored study called the Limerick Report found that toxic gases were not created.Based on the Limerick report, position papers publicized by US SIDS organizationssay there is not enough evidence to support the toxic gas theory, and that parents should continue to put their babies to sleep on vinyl-covered crib mattresses. However, Sprott maintains that his findings were not refuted.