Tuesday, 30 September 2014

Why we use Phosphorus in our fire-resistant coatings

Elizabeth Henderson
Product Development Manager

Phosphorus plays a dual rôle in the context of fire – it is used in matches, and at Itac we incorporate it in our fire-resistant coatings. The element was first isolated from animal urine, and this gives us a clue as to its ubiquity. Phosphorus is a key component of all living organisms, found in nucleic acids and playing a major part in cell biochemistry. The element itself is comparable to its neighbours in the periodic table, carbon and sulphur, in that it exists in more than one crystal form. White phosphorous is a tetrahedron of atoms in either a body-centred cubic (α-form) or triclinic (β-form) array. Both these forms will gradually decompose to amorphous red phosphorus with time. White phosphorus is very reactive and must be stored in water to prevent it bursting into flame
Itac’s inclusion of organic phosphate esters in fire-resistant coatings is effective because at high temperatures the organic part of the material burns away and when the residual phosphorus is further heated it will form a polymeric form of phosphoric acid. This acid causes a char layer, which shields the remaining material from oxygen, in that way preventing the formation of flammable gases.  Organic phosphate esters are liquid – this means they are readily incorporated in our mixes and compatible with the organic solvents we use.  They have the advantage of being halogen-free, so no volatile acids are formed as by-products in a fire.
 The only source of phosphorus for the modern chemical industry is phosphate rock, and large deposits of phosphate from igneous rock are found in Canada, Russia, and South Africa. ‘Coprolites’ discovered in 1842 in Suffolk were formerly mined for use in fertiliser due to their high phosphate content. They are fossilised animal dung and for some time they were a major raw material for fertilisers but their use diminished towards the end of the 19th century.
In addition to the uses we make of phosphorus at Itac and their application in fertilisers, its compounds are very effective surfactants. Phosphate end-groups on polymer chains allow a molecule to have an affinity for both hydrocarbons and polar surfaces, allowing their use as wetting agents for materials such as pigments. Phosphorus is also an important component of phosphor bronze – an alloy of copper, tin and phosphorus which has excellent mechanical and workability properties.

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