Flame retardant
Flame retardants are a diverse group of chemicals that are added to manufactured materials, such as plastics and textiles, and surface finishes and coatings. Flame retardants are activated by the presence of an ignition source and prevent or slow the further development of flames by a variety of different physical and chemical mechanisms. They may be added as a copolymer during the polymerisation process, or later added to the polymer at a moulding or extrusion process or applied as a topical finish. Mineral flame retardants are typically additive, while organohalogen and organophosphorus compounds can be either reactive or additive.
Classes
Both reactive and additive flame retardants types can be further separated into four distinct classes:- Minerals such as aluminium hydroxide, magnesium hydroxide, huntite and hydromagnesite, various hydrates, red phosphorus, and boron compounds, mostly borates.
- Organohalogen compounds. This class includes organochlorines such as chlorendic acid derivatives and chlorinated paraffins; organobromines such as decabromodiphenyl ether, decabromodiphenyl ethane, polymeric brominated compounds such as brominated polystyrenes, brominated carbonate oligomers, brominated epoxy oligomers, tetrabromophthalic anyhydride, tetrabromobisphenol A and hexabromocyclododecane. Most but not all halogenated flame retardants are used in conjunction with a synergist to enhance their efficiency. Antimony trioxide is widely used, but other forms of antimony such as the pentoxide and sodium antimonate are also used.
- Organophosphorus compounds. This class includes organophosphates such as triphenyl phosphate, resorcinol bis, bisphenol A diphenyl phosphate, and tricresyl phosphate ; phosphonates such as dimethyl methylphosphonate ; and phosphinates such as aluminium diethyl phosphinate. In one important class of flame retardants, compounds contain both phosphorus and a halogen. Such compounds include tris phosphate and chlorinated organophosphates such as trisphosphate and tetrakisdichloroisopentyldiphosphate.
- Organic compounds such as carboxylic acid and dicarboxylic acid
Retardation mechanisms
Endothermic degradation
Some compounds break down endothermically when subjected to high temperatures. Magnesium and aluminium hydroxides are an example, together with various carbonates and hydrates such as mixtures of huntite and hydromagnesite. The reaction removes heat from the substrate, thereby 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.Thermal shielding (solid phase)
A 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 surface into a char, which separates the flame from the material and slows the heat transfer to the unburned fuel. Non-halogenated inorganic and organic phosphate flame retardants typically act through this mechanism by generating a polymeric layer of charred phosphoric acid.Dilution of gas phase
Inert gases 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 quenching
Chlorinated 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 is much less reactive compared to H· or OH·, and therefore has much lower potential to propagate the radical oxidation reactions of combustion.Materials
Flame retardant cotton
Flame retardant cotton is cotton that has been treated to prevent or slow ignition by different treatments applied during the manufacturing process. Cotton is typically made flame-resistant by chemical applications of polymeric, nonpolymeric, and polymeric/nonpolymeric hybrids that are composed of one or more of the elements such as nitrogen, sodium, phosphorus, silicon, boron, or chlorine.Manufacturing
While non-organic fabrics are typically made flame-resistant by incorporating flame retardants into their matrices, surface modification is more convenient for organic fabrics like cotton.Use
Cotton fabrics have been frequently used worldwide because of their advantageous properties with regard to thermal insulation, biocompatibility and moisture absorption and breathability performance. These advantages indicate potential applications of cotton fabrics in protective clothing and human health. However, natural cotton fabric is flammable and will rapidly burn. This drawback limits the use of cotton fabrics. Therefore, treating cotton fabrics to obtain a fire-resistant fabric is important.Firefighters, or those exposed to flames on a regular basis, rely on flame-retardant cotton for both protection and comfort. Typically, their undergarments beneath the heavier fire-resistant gear are made of flame-retardant cotton or another breathable, organic fabric that has been treated to resist ignition.
Polymers containing nitrogen, sodium, and phosphorus atoms can work as materials for fire-resistant cellulosic textiles, such as cotton or rayon. Specifically, organic polymers can work as a flame retardant due to the presence of one or all three types of these elements. These atoms can be in the original polymers, or they can be incorporated by chemical modification. Flame retardant materials and coatings are being developed that are phosphorus and bio-based.
Use and effectiveness
Fire safety standards
Flame retardants are typically added to industrial and consumer products to meet flammability standards for furniture, textiles, electronics, and building products like insulation.U.S. state of California
In 1975, California began implementing Technical Bulletin 117, which requires that materials such as polyurethane foam used to fill furniture be able to withstand a small open flame, equivalent to a candle, for at least 12 seconds. In polyurethane foam, furniture manufacturers typically meet TB 117 with additive halogenated organic flame retardants. Although no other US states have a similar standard, because California has such a large market many manufacturers meet TB 117 in products that they distribute across the United States. The proliferation of flame retardants, and especially halogenated organic flame retardants, in furniture across the United States is strongly linked to TB 117.In response to concerns about the health impacts of flame retardants in upholstered furniture, in February 2013 California proposed modifying TB 117 to require that fabric covering upholstered furniture meet a smolder test and to eliminate the foam flammability standards. Gov. Jerry Brown signed the modified TB117-2013 in November and it became effective in 2014. The modified regulation does not mandate a reduction in flame retardants.
EU
In Europe, flame retardant standards for furnishings vary, and are their most stringent in the UK and Ireland. Generally the ranking of the various common flame retardant tests worldwide for furniture and soft furnishings would indicate that the California test Cal TB117 - 2013 test is the most straightforward to pass, there is increasing difficulty in passing Cal TB117 -1975 followed by the British test BS 5852 and followed by Cal TB133. One of the most demanding flammability tests worldwide is probably the US Federal Aviation Authority test for aircraft seating which involves the use of a kerosene burner which blasts flame at the test piece.The 2009 Greenstreet Berman study, carried out by the UK government, showed that in the period between 2002 and 2007 the UK Furniture and Furnishings Fire Safety Regulations accounted for 54 fewer deaths per year, 780 fewer non-fatal casualties per year and 1065 fewer fires each year following the introduction of the UK furniture safety regulations in 1988.
Effectiveness
The effectiveness of flame retardant chemicals at reducing the flammability of consumer products in house fires is disputed. Advocates for the flame retardant industry, such as the American Chemistry Council's North American Flame Retardant Alliance, cite a study from the National Bureau of Standards indicating that a room filled with flame-retarded products offered a 15-fold greater time window for occupants to escape the room than a similar room free of flame retardants. However, critics of this position, including the lead study author, argue that the levels of flame retardant used in the 1988 study, while found commercially, are much higher than the levels required by TB 117 and used broadly in the United States in upholstered furniture.Another study concluded flame retardants are an effective tool to reduce fire risks without creating toxic emissions.
Several studies in the 1980s tested ignition in whole pieces of furniture with different upholstery and filling types, including different flame retardant formulations. In particular, they looked at maximum heat release and time to maximum heat release, two key indicators of fire danger. These studies found that the type of fabric covering had a large influence on ease of ignition, that cotton fillings were much less flammable than polyurethane foam fillings, and that an interliner material substantially reduced the ease of ignition. They also found that although some flame retardant formulations decreased the ease of ignition, the most basic formulation that met TB 117 had very little effect. In one of the studies, foam fillings that met TB 117 had equivalent ignition times as the same foam fillings without flame retardants. A report from the Proceedings of the Polyurethane Foam Association also showed no benefit in open-flame and cigarette tests with foam cushions treated with flame retardants to meet TB 117. However, other scientists support this open-flame test.
Compared with cotton, flame retardants increase fire toxicity. They have a large effect on bench-scale flammability tests, but a negligible effect on large scale fire tests. Furniture of naturally flame-retardant materials is much safer than foam with fire retardants.