2.2.2.1 Inorganic flame retardants [20]
The main flame retardants in this group are aluminium trihydoxide (Al(OH) 3], magnesium hydr oxide [Mg(OH)2], ammonium polyphosphate and red phosphorus. This group represents about 50% by volume of the worldwide flame retardant production. Metal hydroxides are empl oyed alone or in combination with other flame retardants to achieve necessary improvements in flame retardancy. Antimony compounds (e.g. antimony trioxide) are used as synergistic co-additives in combination with halogen compounds to
enhance fire character istics such as smoke reduction or after glow suppression [23].
Ionic compounds have a very long history as flame retardants for wool or cellulose based products. Inorganic phosphorus compounds are primarily used in polymides and phenolic resins, or as comp onents in intumescent formulations.
Aluminum hydroxide (alumina/trihydrate) ATH decomposes when exposed to temperatures over 200 OC, which limits the polymers in which it can be incorporated. Magnesium hydroxide is stable at temperature above 300 OC and can be processed into several polymers. In the recent time, scientists have been using combination of the metal hydroxides in the concept of nanotechnology to improve the flame retardancy of polymer composites. This approach involves the dispensing of inorgan ic filler in a nanoscale as flame-retardants into a polymer matrix [24]. Usually, layered silicates and various nanoparticules (like Mg O, Mg(OH)2 Al(OH)3] undergo suitable pre treatment, and are used for this purpose. One of such flame retardants is the magnesium aluminum layered double hydroxide (Mg -Al- LDH). Like in Mg -Al-LDH, Mg(OH) 2 undergoes endothermic decomposition releasing bound water and p roducing a metal oxide residue [25-27]. Therefore, Mg-Al-LDH has potential as flame retardant which can act thr ough physical effects.
2.2.2.1.1 Antimony Compounds
Antimony trioxide is used as a synergist. It is utilized in plastics, rubber, textiles, pap er and paints [ 28]. Antimony oxides and antimonates must be converted to volatile species. This is accomplished by the reaction of halogen acids and antimony containing materials at fire temperatures to form antimony trihalide or antimony halide oxide which suppress flame propagation. Other antimony compounds include antimony pentoxide, available primarily as a stable colloid or as a redispersible powder. It is used in fibre and fabric. Sodium antimonate is recommended for formulations in which deep tone colours are required.
2.2.2.1.2 Boron Compounds
The most widely used in this class are boric acid (H 3BO 3) and sodium borate (borax) [Na 2B 4O7.10H2O]. They are primarily used for cellulosic material such as cot ton, paper, and roofing thatch [ 29]. They are effective in decreasing after glow time and lengthening of ignition time; although their use is limited to products for which non -durable flame retardancy is acceptable since both are very water soluble.
Zinc borate is however water insoluble and is mostly used in plastics and rubber products. It is used as a complete or partial replacement for antimony oxide in PVC, nylon polyolefin, epoxy, etc. It functions as a flame retar dant and smoke suppressant in condensed phase.
2.2.2.1.3 Other metal compounds
Molybdeum compounds have been used as flame retardants for cellulosic materials and in other polymers as smoke suppressants [17]. Titanium and Zirconium compounds are used for textiles, especially wool [ 30].
2.2.2.1.4 Phosphorus Compounds
Red phosphorus is effective as a flame retardant in polyurethane foams, polyamides and phenolic applications.
Ammonium polyphosphate is mainly applied in intumescent coatings and paints. Intumescent systems are material s that puff up to produce foams [ 17]. Owing to these characteristics they are used to protect materials such as wood, plastics and steel from high temperature. Other inorganic flame retardants like ammonium sulfamate (NH4SONH 2) and ammonium bromide (NH 4Br) are mainly for cellulose-based products and in front forest fighting.
2.2.2.2 Halogenated Organic Flame Retardants
The halogenated flame retardants are divided int o three classes, namely aromatic, aliphatic and cycloaliphatic [16].
Among the halogens, the compounds bromine and chlorine are significant as flame retardants. Fluorine compounds
though expensive are ineffective because the C – F bond is too strong. Iodine compounds though effective are expensive and too unstable to be useful [21].
The thermal stability of halogenated flame retardants vary in the order of; aromatic brominated flame retardants gr eater than the aliphatic chlorin ated flame retardant which in turn are more stable than aliphatic brominated flame retardants. The aromatic bromated flame retardants can be used in thermoplastics at fairly high temperatures without stabilizers but at very high temperatures, they must be used with stabilizers like tin compounds [16,21].
2.2.2.2.1 Brominated flame retardants.
Bromine-based flame retardants are highly brominated organic compounds with a relative molecular mass ranging from 200 to that of large molecule polymers. They usually contain 50 to 85% (by weig ht) of bromine [31]. Tetrabromo bisphenol A (TBBPA) and decabromo diphenyl
ether (DeBDE) are the two brominated flame retardants that have the highest usage by volume today [32,33]. TBBPA is an example of a flame retardant that can be used as an additive as well as a reactive. It is used in the production of flame retarded epoxy resins used in printed circuit boards [32]. DeBDE is used in high impact polystyrene which is in the production of television cabinets. Other uses of DeBDE are in ABS, engineering thermoplastics, polyolefins, thermosets PVC and elastomers and textiles[33]. Hexabromocyclo dodecane (HBCD), is a major cycloaliphatic flame retardant and it s primary used is in polystyrene foam and textiles.
2.2.2.2.2 Chlorinated flame retardants
Chlorine-containing flame retardants are divided into three classes: aliphatic, cycloaliphatic and aromatics. Chlorinated parafins are the most widely used aliphatic chlorime containing flame retardants. The have applications in plastics, fabrics, paints, and c oatings [34]. Bis (hexachlorocyclopentadi eno) cyclo-octane is a flame retardant having good thermal stability for chlorinated cycloaliphatic, with thermal stability comparable with brominated aromatics. It is used in several polymers especially polyamides and polyolyfins for wire and cable applications [21].
2.2.2.3 Organophosphorus Flame Retardants.
The predominant phosphorus flame retardants in use in plastics and textiles are phosphorus, phosphorus -nitrogen, phosphorus halogen and phosphate esters with or without halogen. Phosphorus -containing flame retardants are the most important material that impart durable flame resistance to cellulose [22,26]. The major groups of additive , among organo phosphorus compounds are phosphate esters, polyols, phosphoniu m derivatives and phosphonates. The phosphate esters include trialkyl derivatives such as triethyl or triocyt yl phosphate, triaryl deravatives such as triphenyl phosphate and aryl -alkyl derivatives such as 2 - ethyl hexyl-diphenyl phosphate. Phosphonuim salt s are use to improve the flame retard ancy for cellulosic products. The esters formed by reaction of the three functional groups of phosphoric acid with alcohols or phenols are excellent
