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Flame retardants are materials that inhibit or resist the spread of fire. The common classes include aluminium hydroxide, magnesium hydroxide and various hydrates; organobromines and organochlorines; phosphorus compounds; antimony trioxide; and boron compounds. Different retardants work by different mechanisms, from cooling the material to interrupting the chemistry of the flame itself.
Flame retardants are materials that inhibit or resist the spread of fire. Naturally occurring substances such as asbestos as well as synthetic materials, usually halocarbons such as polybrominated diphenyl ether (PBDEs), polychlorinated biphenyls (PCBs) and chlorendic acid derivates, most often dibutyl chlorendate and dimethyl chlorendate, have been used in this capacity. Generally, these classes of flame retardant compounds are the most common: aluminium hydroxide, magnesium hydroxide, and various hydrates; organobromines and organochlorines; phosphorus, in the form of organophosphates, halogenated phosphorus compounds, and red phosphorus; antimony trioxide; and boron compounds, mostly borates.
Tetrakis(hydroxymethyl) phosphonium salts, made by passing phosphine gas through a solution of formaldehyde and a mineral acid such as hydrochloric acid, are used as flame retardants for textiles.
Other flame retardants include chlorinated paraffins, polybrominated biphenyls (PBB), pentabromodiphenyl ether (pentaBDE), octabromodiphenyl ether (octaBDE), decabromodiphenyl ether (decaBDE), hexabromocyclododecane (HBCD), tri-o-cresyl phosphate, tris(2,3-dibromopropyl) phosphate (TRIS), bis(2,3-dibromopropyl) phosphate, tris(1-aziridinyl)-phosphine oxide (TEPA), and others.
Mechanisms of function
Some compounds break down endothermically when subjected to high temperatures. Magnesium and aluminium hydroxides are an example, together with various hydrates. The reaction removes heat from the surrounding, 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.
Inert fillers, eg. talc or calcium carbonate, act as diluents, lowering the combustible portion of the material, thus lowering the amount of heat per volume of material it can produce while burning.
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 into a carbonized foam, which separates the flame from the material and slows the heat transfer to the unburned fuel.
Inert 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.
Chlorinated and brominated materials undergo thermal degradation and release hydrogen chloride and hydrogen bromide. 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 reaction the burning is. Antimony compounds tend to act in synergy with halogenated flame retardants. The HCl and HBr released during burning are highly corrosive, which has reliability implications for objects (especially fine electronics) subjected to the released smoke.
| The above article is licensed under the GNU Free Documentation License. From Wikipedia, the free encyclopedia (https://en.wikipedia.org/wiki/flame_retardant). Article modified by Apparel Search 11/27/07 We suggest you do further research for the most current definition. |
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Retardants in Textiles: Applied and Built InIn fabrics, flame retardancy is achieved in two broad ways. A finish can be applied to a finished cloth — phosphorus-based treatments for cellulosic fabrics, including the tetrakis(hydroxymethyl) phosphonium salt treatments used on cotton, bond reactive chemicals to the fiber so the fabric chars instead of sustaining flame, and durability to washing is the central engineering problem: a treatment that washes out protects only until the first laundering. Alternatively, resistance can be built into the material itself, through inherently resistant fibers or through retardant additives compounded into a polymer before it is spun or molded — the route used for much contract upholstery, protective workwear, sleepwear regulated for flammability, and the foams and plastics of transport interiors. The choice among the chemical classes listed in the definition is a matching exercise: the retardant must survive the processing temperature of its host material, work at the temperature the host begins to decompose, and suit the way the finished article is expected to burn — dripping, charring or spreading flame across a surface. Why No Single Mechanism Is EnoughThe five mechanisms — cooling, fuel dilution, shielding, gas dilution and radical quenching — attack different stages of combustion, and real formulations often combine them. A burning polymer must be heated, must decompose into fuel gases, and those gases must sustain a radical chain reaction in the flame; interrupt any stage and burning slows, interrupt several and it may stop. That is why inert fillers such as talc or calcium carbonate, which simply reduce the combustible fraction, appear alongside sophisticated halogen and phosphorus chemistry, and why antimony trioxide, weak on its own, is valuable as a synergist that multiplies the effect of halogens. The trade-offs are equally real: the hydrogen chloride and bromide released by halogenated systems are corrosive and have reliability implications for electronics exposed to the smoke, several older compounds named in the definition have been restricted as their environmental behavior became understood, and every additive changes the hand, strength or cost of the host material. Flame retardancy is therefore specified by test performance for a defined use — a fabric, foam or molding passes a stated flame test — rather than by chemistry alone. Common QuestionsHow do aluminium and magnesium hydroxide flame retardants work?By endothermic degradation: subjected to high temperatures they break down in a reaction that absorbs heat, cooling the material below the point that sustains burning, and releasing water. Their low decomposition temperature limits the processing temperature of polymers they can be used in. What is an intumescent flame retardant?An additive that, under heat, turns the surface of the polymer into a carbonized foam — a char layer. The foam is a thermal insulation barrier between the burning and unburned parts of the material, separating the flame from the fuel and slowing heat transfer to unburned material beneath. How do halogenated flame retardants stop a flame?Chlorinated and brominated materials release hydrogen chloride and hydrogen bromide when they degrade in heat. Those gases react with the highly reactive H and OH radicals that propagate burning, replacing them with halogen radicals of much lower energy, which slows the oxidation reaction. Antimony compounds act in synergy with halogenated retardants. More to ExploreMore from Apparel Search: Fabric Definitions | Clothing Industry Glossary | Fashion Terms | Apparel Search Home
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