Engineering Chemistry S Datta

16
Explosives and Propellants
Explosives
Explosives are pure substances or mixtures which on being subjected to specific stimulus like
mechanical impact, frictional forces, electrical discharge and other sources of heat, high pressure
shock waves initiate an oxidation reaction producing large volume of hot gases.
Once initiated, the heat evolved is sufficient to cause the reaction to continue and become
self-sustaining. The minimum quantity of energy required for initiation has a characteristic
value that depends on the chemical and physical properties of the material.
The reactions of explosives are exothermic. The products at hot and high pressure and
when confined to a closed space lead to explosion but if the reaction is controlled at slower rate,
this energy can be utilized to propel a projectile. The amount of power available from a given
weight of explosive is known as “power to weight ratio”.
Characteristics of Explosives
Following are the characteristics required for an explosive.
1. The decomposition rate of an explosive should be very fast and large volume of hot
gases should be liberated exothermally leading to large increase in volume.
2. The explosive molecule should have a low energy of dissociation.
3. The oxygen-balance of an explosive molecule should be higher which indicates that
more oxygen is available in the molecule to oxidize C and H to CO 2
and H 2
O.
4. The explosive should be sensitive towards specific impacts. It should explode immediately
after the application of the stimulus. The lesser the stimulus required better
is the sensitivity of the explosion.
5. Explosives should not be volatile or hygroscopic but should be stable chemically so
that they do not decompose on keeping or react with the storage container.
6. Lastly, explosives should be cheap.
Uses of Explosives
The principal non-military use of explosives is in mining. In coal mines, only very safe
explosives with lowest possible detonation temperature are used. These explosives usually
contain a large amount of NH 4
NO 3
and NaCl. In industry explosives are used for blasting ores
of different metals, quarrying limestone to be used in road construction, for blasting holes in
mountains for tunnels and roads, digging earth, seismic prospecting, blasting rocks and other
social and useful works.
The other main area is in military and ammunition for war as well as tackling regular
social problems, as well as in the preparation of torpedos, grenades, bombs etc. Another area is
in the launching of rockets, spacecrafts etc.
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