Fire Retardants and Flameproofing

The ideal fire retardant will not adversely affect the strength, feel, porosity, toxicity, colour, weight or indeed price of the material in which it is incorporated. 

With the exception of alumina hydroxides, fire-retardant phosphorus compounds are now used in greater quantities than any of the other main types of retardants, that is, antimony oxide, borates or halogenated hydrocarbons. Advantages claimed for organophosphorus-based fire retardants are relatively low toxicity and minimum harmful volatiles under burning conditions [37]. 

Ammonium orthophosphate was first suggested as a fireproofing agent for theatre curtains by Gay Lussac [38] in 1821. The relatively cheap ammonium orthophosphates (mono, di or a mixture) (Section 12.2) were the first compounds to be used and they are still widely employed as fire retardants in paper, fibre boards, wood and fabrics where water solubility and susceptibility to leaching during washing is not important. 

An early fire-retardant treatment for paper and cotton was to heat them with phosphoric acid and urea at 145-180°C to form insoluble ammonium polyphosphate, (NH4P03) . Ammonium polyphosphate is also used in intumescent paint formulations, where, like the orthophosphate, it releases ammonia and phosphoric acid on heating. The latter facilitates charring which, together with the release of ammonia, retards local combustion. A similar mechanism is believed to account, at least in part, for the fire-retardant action of many other phosphorus compounds. There is now evidence that impregnation of wood with phosphoric acid suppresses the formation of carcinogenic materials during pyrolysis [39]. 

Another inorganic commercial product is based on the ammonium salt of a cycloimidophosphate (12.206), which can be applied as an aqueous solution to paper and cotton. 

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