Investigation Of Effects Of Two Flame Retardants On The Fire Characterisit Ics Of Flexible Poly Ether Foam

retardants also appear to involve char enhancement [ 22]. Salts that dissociate to form acids or bases are usually strong flame retardants . Salts of strong acids or weak bases such as ammonium and amine salts are most effective either by themselves or when formed in combustion [16].
2.4    Some Suppressants
Smoke suppressants can act physically o r chemically in the condensed phase. Presently, no uniform theory on the mode of action of smoke suppressants has been established [17].
Additives can act physically in a similar fashion to flame retardants ( i.e. by coating or by dilution). Thus limiting the formation of pyrolysis products and hence of smoke, for example, chalk (CaC O3) frequently used as filler, acts in some cases physically as a diluents and also chemically in PVC, by absorbing hydrogen chloride or by effecting cross - linking so that the smoke density is re duced(16). Smoke can also be suppressed by the formation of a charred layer on the surface of the substrate , e.g. when organic phosphates is used on unsaturated polyester resins. Again, in halogen  containing polymers, such as PVC, iron (iii) chloride causes charring by formation of a strong Lewis acids. Compounds such as ferrocene complexes that suppres s smoke development are formed [16].
In the gas phase smoke can be suppressed through chemically and physical actions. Physical effects take place mainly by shielding the substrate with heavy gases against thermal attack and by diluting smoke gases, thereby reducing smoke density. Two ways are used to suppress smoke chemically; these are by elimination of either smoke precursors or the soot itself. Removal o f soot precursors occurs by oxidation of the aromatic spices with the help of transition metal complexes and flocculation of soot parti cles by some transition metal oxides [16].
2.5    Melamine as a flame retardant
Melamine is a German word coined from the combination of two other chemical products melam (a distillation derivati ve of ammonium thiocyanate and amine [39].
Melamine (2,4,6 – triamino – 1,3,5 trianzine, is a white crystalline powder with a melting point of approximately 354OC and a density of 1.573 grams/cc. Melamine (C 3N6H6) is a stable carbon and nitrogen based compound far below its melting point at 200OC melamine is known to vapourize or sublime (diluting the fuel gases and oxygen near the combustion source).
When sublimating it absorbs an amount of heat of 470 kcal/mole, thus a cting as a heat sinks in fire situations . Additionally, melamine exhibits low solubility in water and most other solvents. It has an excellent UV absorption above 250nm and a pH of 8.1 .

2.5.1    Synth esis of Melamine
Melamine was first synthesized by the German Chemist J ustus Von Liebig in 1834 . In early production, first , calcium cyanamide is converted into dicyan odiamide, then heated above its melting temperature to produce melamine. In the modern t ime, most industrial manufactures use urea in the following reaction to produce melamine [40,41].
6(NH2)2   CO    C3H6N6 +    6NH3 + 3CO2
Urea
It can be understood as two step reaction; first, urea decomposes into cyanic acid and ammonia in an endothermic reaction
(NH2)2   CO    HCNO    +    NH 3
Urea    cyanic acid
Then, cyanic acid polymer izes to form melamine and carbon dioxide.
6 HCNO    C3H6N6  + 3CO2
The second reaction is exothermic but the overall process is endothermic. The above reaction can be carried out either of two    methods.        Catalyzed    gas -phase    production        or    high pressure liquid-phase production. In any of the methods, molten urea is introduced onto a flu idized bed with catalyst for reaction. Hot ammonia gas is also present to flu idize the bed and inhibits    deammonization. The    effluents then is cooled. Ammonia and carbon dioxide in the off -gas are separated from the melamine containing slurry. The slurry is further concentrated and crystallized to yield melamine [40].        The    off–gas        contains    large    amounts        of    ammonia. Therefore melamine production is often integrated into urea production which uses ammoni a as feedstock. The waste water        from    the        washing        of    melamine    may        contain approximately 70% melamine, 23% oxytriaz ines (ammeline, ammelide and cyanuric acid) 0.7% poly conde nsates (Melem melam amd melon) [41].
2.5.2    Mechanism of reaction of melamine as flame retardants [42].
Flame retardants function by interference with one of the three components that initiate and/or support combustion: heat, fuel, and oxygen. Melamine acts both in condensed phase mechanism and gas phase. It has the ability to interfere with the combustion process in all stages through various ways.
During the initial stage of combustion melamine retards ignition by causing a heat sink through endothermic sublimation at roughly 350 OC. Again a larger heat sink effect is generated by the subsequent decomposition of the melamine vapours.
Melamine is regarded as a “poor fuel” having a heat of combustion of only 40% of that of hydrocarbons. Furthermore , melamine produced nitrogen during combustion which acts as inert diluents. Another inert diluents that is produced by melamine is ammonia gas which is released during break -down of the melamine or self