Flame retardant Flame retardant

FLAME RETARDANT

Flame retardant Flame retardant

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FLAME RETARDANT

Flame retardants are chemicals used in textiles and leather goods that inhibit or resist the spread of fire.
These can be separated into several categories:

  • Minerals such as asbestos, compounds such as aluminium hydroxide, magnesium hydroxide, hydromagnesite, antimony trioxide, various hydrates, red phosphorus, 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.
  • Synthetic materials, usually halocarbons. These include organochlorines such as polychlorinated biphenyls (PCBs), chlorendic acid derivates (most often dibutyl chlorendate and dimethyl chlorendate), Diethyl Thio Phosphoryl Chloride (DETC) and chlorinated paraffins; organobromines such as polybrominated diphenyl ether (PBDEs), which be further broken down into pentabromodiphenyl ether (pentaBDE), octabromodiphenyl ether (octaBDE), decabromodiphenyl ether (decaBDE) and hexabromocyclododecane (HBCD); organophosphates in the form of halogenated phosphorus compounds such as tri-o-cresyl phosphate, tris(2,3-dibromopropyl) phosphate (TRIS), bis(2,3-dibromopropyl) phosphate, tris(1-aziridinyl)-phosphine oxide (TEPA), and others.

Additives: Additive flame-retardant chemicals can be added to a manufactured product without bonding or reacting with the product. They are incorporated and dispersed evenly throughout the product, but are not chemically bound to it. Additive types are used especially for thermoplastics. These include halogenated compounds, phosphorous compounds, metallic oxides and inorganic fillers.

Reactives: Reactive flame-retardant chemicals may be incorporated into the product during manufacture of the plastic raw materials. They are chemically bound to the raw materials that are used to make the final product and thus they tend to exert a much greater effect than additive flame retardants on the properties of the polymer. Reactives are typically used for thermosets.

Flame-Retardant Synergists: Many flame-retardant synergists do not have significant flame-retardant properties by themselves; however, their use increases the overall effectiveness of the flame-retardant system.

Example of flame retardant synergism is the addition of inorganic synergists to organo-phosphorous flame retardants. When used alone, organo-phosphorous flame-retardant concentrations may need to be extremely high which often adversely affect the properties of the product. Testing has shown that adding inorganic synergists can dramatically increase the flame-retardant efficiency thereby requiring a significantly smaller quantity of the flame retardant to achieve the desired effects.

The basic mechanisms of flame retardancy (as discussed below) will vary depending on the specific flame retardant and substrate. Additive and reactive flame-retardant chemicals can function in the vapor or condensed phase. Depending on the specific chemical, any of the mechanisms may be utilized. Due to specific physical and chemical properties of the flame retardant and its effects on the substrate, most are used exclusively as either reactive or additive.

 
In Advantages of wearing Muslin Dresses! (1802), James Gillray caricatured the hazard of muslin clothing not treated with a flame retardant.Many of these chemicals are considered harmful, having been linked to liver, thyroid, reproductive/developmental, and neurological effects. PCBs were banned in 1977 and the EU has banned several types of brominated flame retardants as of 2008, following evidence beginning in 1998 that the chemicals were accumulating in human breast milk. Currently some US states and various countries are investigating PBDEs as well; of the major ones only decaBDE remains on the North American market.

Aside from various conventional alternatives such as antimony or phosphorus-based retardants which have toxicological problems of their own, Environmental Health Perspectives surveys the halogen-free alternatives being explored. These include a technique to fuse flame retardants into products (so no chemicals leak), nanoclays incorporating montmorillonite, an entirely new plastic which produces water when burned called bishydroxydeoxybenzoin (BHDB), and possibly other nanomaterial solutions. Inherently flame-resistant products are ideal, and the aerospace industry uses such plastics, but they are too costly for widespread use.

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

Mechanisms of function:
Endothermic degradation
Some 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 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.

Dilution of fuel
Inert fillers, e.g. 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.

Thermal shielding
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.

Dilution of gas phase
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.

Gas phase radical quenching
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 reactions of combustion. 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.

 




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