Class 5.1 - Oxidising Agents
Because of their high oxygen content, these are often reactive materials. They may react with other flammable or combustible materials, and the heat generated may start the latter burning. Then the agents supply the oxygen to keep them burning without any help from oxygen in the air, as is the case with normal combustion.
Such fires may therefore break out and continue in confined spaces, e.g. inside cargo holds. And once started, they may be difficult to extinguish. Blanketing with powder or foam is useless, as the oxygen is already present in the agent underneath.
The only method is to use a large amount of cold water, but if the fire is in an enclosed space, it may be difficult to reach, and the heat generated is such that a very large amount of water may be needed.
Some oxidisers can be explosive if heated strongly, particularly in the presence of carbon. Ammonium nitrate mixed with hydrocarbon oil, e.g. diesel, becomes a powerful explosive, much used in the extractive industries, and by terrorists.
Class 5.2 – Organic peroxides
The molecule contains structures containing carbon (organic) linked by a double oxygen bond (peroxide). Thus the fuel and the oxygen are together in the same molecule, making them even more liable to ignition than a separate combustible material.
They are designed to be reactive for a number of industrial purposes, and may consequently be unstable, and sometimes explosive. When they are first developed, they may be classified as either Class 1 or Class 5.2 generally depending on the intended end-use. On the basis of their chemical structure alone, they could be considered as either.
They often have to be maintained under refrigeration to keep them inactive, and then the temperature must be carefully controlled. Otherwise, if they exceed a certain temperature specific to the material, they will start to decompose rapidly, similar to the self-reactive materials in Class 4.1, resulting in uncontrollable progress towards fire or explosion.
Because of their reactive nature, they can be very damaging to the human body, particularly the eyes.