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Class 5 - Oxidising substances
Class 5 materials tend to be difficult to explain. The immediate danger they pose is not obvious from the hazard symbol, and usually only becomes real after witnessing a practical demonstration of their power. Class 5 has two subdivisions; Oxidising Agents and Organic Peroxides, which are basically more unstable, more reactive oxidisers.
Class 5.1, Oxidising Agents
The basic property of any oxidiser is that it has Oxygen as part of its chemical composition that can easily be released. Consider the fire triangle Below. This demonstrates the minimum requirements to start a fire. Remove any one element from a side and burning is impossible.
But, In the event of an oxidiser fire, the fuel and the oxygen are mixed and the only way to extinguish them is to remove the heat, usually with large quantities of water. The heat involved need not be a flame, the air temperature can be quite sufficient. It is this reaction that is used to produce underwater flares and spaceship rocket motors that can burn where there is no oxygen. As a general rule of thumb, if a substance name ends in "ate", it may be an oxidiser. Particularly strong oxidisers can be identified with the names Nitrate, Chromate, Chlorate and Bromate. By far the commonest oxidiser offered for transport is Ammonium Nitrate based fertilizers and Chlorate based weedkillers.
On Monday 24th May
2004 a truck carrying Ammonium Nitrate fertilizer exploded at Mihailesti, on
the Buzau-Bucharest highway, Romania. Class 5.2, Organic Peroxides Peroxides get their name from their chemical composition-they contain carbon (a fuel) and two oxygen atoms joined together. So, a peroxide already has two of the components to start a fire, and need only the application of heat to trigger them. All peroxides are unstable. Basically, the two oxygen atoms do not like to be joined together and will split themselves wherever possible. This instability is especially marked in some materials that can only be stored or transported under strictly controlled temperatures. As the temperature rises the peroxide begins to decay. This decay raises the temperature (exotherm) and leads to faster decay. This chain reaction continues until the Spontaneous Auto-Decomposition Temperature (SADT) is reached at which point it will explode. The commonest peroxide offered for transport is Hydrogen Peroxide which has a wide variety of uses from rocket fuel, swimming pool and contact lens sterilizer to making baking soda toothpaste frothy. An example of the potential reactivity of this material occurred in Alabama, USA in May 2004. Fire fighters were called to 4000 litre tank at a trailer park which was steadily reacting and exotherming, finally peaking at 68oC. The subsequent investigation showed that a mesh grille in one of the vent pipes had been chewed through by a rodent, probably a squirrel. The unfortunate creature had fallen through into the tank, and it's body provided sufficient organic material and heat energy to start the decomposition reaction. Large users of peroxides are the water treatment, plastics, rubber and adhesive industries.
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