explosives
Explosives
A material when subjected to sudden shock releases energy in a brief period causing a violent outburst. Such explosions are characterized by oxidation reactions and release of heat and large volume of gases. The exothermic nature of the explosive reaction results in heating up of the products to a very high temperature and exerts very high pressure on the surroundings which can be utilized for either constructive or destructive purposes. When the chemical reactions are carried out in slower and controlled conditions, the energy may be used to propel projectiles.
Explosive are mainly used for defense purposes. In the industry they are used for blasting mines to obtain ores, drilling holes in mountains for construction of tunnels, evacuating the earth constructions of dams, quarrying purposes etc.
Explosives may be in the form of solids, liquids or gases. They may be a single component or a mixture of explosive materials or with a mixture of non-explosive materials. A variety of organic and inorganic compounds can serve as explosives. All explosives can be categorized as atomic, mechanical and chemical explosives.
PRIMARY OR INITIATING EXPLOSIVES OR DETONATORS
Detonators are very sensitive which explode even under slight mechanical shock or ignition. They initiate the main explosion. They are used in small quantities in cartridges and blasting caps. Examples of this type of explosives are lead or silver azide, silver acetylide, mercury fulminate, diazo dinitro phenol (DDNP) and tetracene.
1. Lead azide: this is prepared by the reaction of ammonia with sodium forming sodium amide, which on treatment with nitrous oxide gives sodium forming sodium azide. Sodium azide when treated with a solution of lead acetate gives lead azide. Lead azide is used as blasting caps, mainly in military operations.
2. Mercury fulminate: It is prepared by dissolving mercury in excess of concentrated ammonia and then adding this solution to ethyl alcohol. The mixture starts boiling immediately and mercury fulminate is precipitated. Mercury fulminate is more sensitive to detonation, slightly toxic and hence relatively less used. It gives more sensitive to detonation, slightly toxic and hence relatively less used. It gives nitrogen carbon monoxide and mercury on decomposition.
3. Diazo dinitro phenol (DDNP) (DINOL) : This is made by diazotizing acid. It is quite sensitive and can initiate explosion even in less sensitive high explosives. It is highly used in commercial blasting caps.
SECONDARY HIGHER EXPLOSIVES
These explode only when initiated by detonators. They are stable and not affected by mechanical shock and flame. They have high energy content than primary explosives. Secondary explosives are mainly nitro compounds, nitrates and nitramines. Trinitro toluene (TNT), ammonium nitrate, GTN, cyclonite (RDX), picric acid, dinitrotolune, tetryl etc., come under this category. High explosives are further classified as
(i)Single compound explosives
(ii)Binary compound explosives
(iii)Plastic explosives, and
(iv)Dynamite
(i) Single compound explosives: They exist as a single component e.g. TNT, ammonium nitrate, penta erythritol tetranitrate etc.
a) Trinitro toluene (TNT): It is obtained by nitration of toluene in these stages- mononitro toluene, dinitrotoluene and trinitrotoluene. It is the most commonly used explosives in military operations. It is popular due to its economic and safe preparation. It is highly stable and least sensitive to shock. It has low melting point (81°C).
b) Ammonium nitrate: It is prepared by passing ammonia through 40-60°Cammonium solution. It is very stable, non toxic and cheap. It can be detonated by a suitable blasting cap. It decomposes on heating to give nitrogen, nitrogen oxide and nitrogen dioxide.
C) Penta Erythritol Tetra nitrate (PETN): It is made by formaldehyde and acetaldehyde. It is extremely sensitive and powerful. It is used as a standard military explosive.
(ii)Binary explosives:
(iii)Plastic explosives: These are explosives which are in plastic state and can be handmpoulded. Wax or oil is mixed with PETN a highly sensitive and powerful explosive and is hand moulded to different shapes without any serious risk of detonation.
(iv)Dynamics: The principle component of this compound is GTN-Trinitro Glycerin which is only liquid and detonated spontaneously above 50°C. it is mixed with inert absorbents such as wood pulp, starch meal, saw dust etc. as it is highly sensitive. It is used for blasting rocks and pulverizing it.
a) Straight dynamics: It contains 15-60% GTN in wood pulp along with sodium nitrate. It is used for blasting rocks, coal and other minerals.
b) Blasting dynamites (Gelatin dynamics): This is obtained when GTN is partly gelatinized by nitrocotton or collodian cotton. It is gel like and highly water proof and can be used under wet conditions and cold weather. It is used for submarine blasting, tunnel driving, deep well shooting etc.
b) Gun cotton: It is prepared by treating purified, bleached and dried cotton linters with a mixture of ammonium acid, sulphuric acid and water. Dry gun cotton explodes very rapidly when exploded. It is used as a propellant in rifles, artillery shells and torpedoes and submarine.
d) Cordite: This is mixture of GTN, cellulose nitrate and petroleum jelly in acetone. It is used as smokeless powder. The petroleum jelly present in its acts as a stabilizer or cooling agent which has a tendency to reduce the temperature of the explosion. This makes it an excellent propellant for large caliber naval guns.
LOW EXPLOSIVES OR PROPELLENTS (DEFLAGRATING EXPLOSIVES)
These do not explode suddenly but only burn. The chemical reactions taking place and slow and burning process proceeds from surface inwards in layers. The gases evolved from the reaction disperse readily and hence do not exert pressure. Gun powder, black powder, colloidal cellulose nitrate etc. belong to this category. They are used as propellants to propel missiles and in pyrotechniques. Black powder is a combination of 75% potassium nitrate, 15% charcoal and 10% sulphur.
GUN POWDER: It is used as blasting of coal and in primer assemblers for propellants. The explosion generates a lot of smoke. The oxidation rate is very fast and is responsible for the explosion.
SMOKELESS POWDER: It is prepared by allowing a solution of cellulose nitrate in alcohol and ether to evaporate. It is mixed with glycerol trinitrate and the paste is forced out to a machine in long cylinder resembling cords and hence is known as cordite. It does not produce smoke but carbon monoxide, carbon dioxide, nitrogen and water vapors are evolved from the explosion. Following are the requisites of explosives.
The explosive substance must necessary have at least one chemical bond which can be easily broken. The explosions must be fast, exothermic accompanied by release of large volume of gas. It must be stable under normal conditions. It must be cheap. It should have a positive oxygen balance. This gives the quantity of oxygen available for the oxidation carbon to carbon dioxide and hydrogen to water.
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