Introduction to Explosive
Explosive, a substance that will burn up or decompose almost instantly, producing intense heat and a large volume of expanding gas. This rapid change of state is called an explosion. Depending on its nature, an explosive may be detonated (set off, or exploded) by applying heat (such as a flame or spark), by shaking, or by striking. The sharp sound, or report, of an explosion is due to air waves set in motion by the explosion's force. This article discusses chemical explosives. Fuels such as gasoline and diesel oil are not considered explosives, even though they can be caused to explode.
Explosives have both constructive uses, as in building roads, railways, and tunnels, and destructive uses, as in war. Ammunition and blasting make use of the expanding gases that result from an explosion. Fireworks use the light and noise.
Explosives may be chemical substances mixed together by mechanical means (gunpowder is an example) or they may be chemical compounds (such as TNT). The explosion is caused by a chemical reaction set off by shock or heat. In explosives such as gunpowder (a low explosive) this reaction takes the form of rapid combustion (burning). In an explosive such as TNT (a high explosive) the reaction is a rapid decomposition, or breaking down, of the compound into its parts.
Low explosives can be set off by impact, heat, a spark, or an electric charge; high explosives generally require an explosion to set them off. When ignited, a low explosive is converted into gas at a relatively slow rate, a high explosive at a much more rapid rate. An explosion will move through a low explosive at a speed less than that of sound. In a high explosive, an explosion will move at a speed up to several times that of sound, creating a shock wave that produces a shattering effect. Some explosives, in particular low explosives, generally do not explode unless they are confined within a casing or within a hole.
Propellants
A propellant is an explosive—generally a low explosive—used to fire a bullet or artillery shell. The speed at which a bullet or shell travels depends on the size of the particles that make up the propellant. The smaller the particles, the more rapidly they burn and, therefore, the more rapid the explosion. Rocket fuels are also called propellants.
The first propellant developed was gunpowder, a mixture of finely divided charcoal, sulfur, and saltpeter. It has been replaced by more powerful explosives called smokeless powders. Despite the name, these explosives are not in powder form (they are usually pellets or cords) and are not entirely smokeless. However, the smoke is not as heavy as, and is lighter-colored than, that of gunpowder. Smokeless powders include ballistite and cordite.
Smokeless powders are made from nitrocellulose, or guncotton. It is prepared by treating cellulose with a combination of nitric and sulfuric acids. The nitrocellulose is usually mixed with acetone to produce a material that can be made into cords, flakes, or other forms. For some kinds of smokeless powders, called double-base propellants, nitroglycerin is added to the nitrocellulose.
Demolition and Blasting Charges
A demolition charge is a high explosive used for destructive purposes—in bombs, mines, shells, missiles, and torpedoes. A blasting charge is a high explosive used for such purposes as mining and building roads. Demolition and blasting charges include:
TNT(trinitrotoluene), the most important military chemical explosive. It is a pale yellow crystalline substance that can be safely melted and cast into charges for artillery shells and bombs. TNT is often combined with other explosives. It is made by treating toluene (a chemical obtained from the refining of petroleum or coal tar) with a mixture of nitric and sulfuric acids.
Nitroglycerina yellow, oily liquid produced by mixing glycerol with nitric and sulfuric acids. It is a very powerful explosive and very sensitive to shock, making it dangerous to handle. It is almost always combined with more stable materials.
Dynamitean explosive made by combining nitro-glycerin with an absorbent material (such as wood pulp) and other ingredients (such as ammonium nitrate or sodium nitrate). Dynamite is relatively safe to handle and was long the most important explosive for blasting. It has been largely replaced by less expensive and less sensitive materials composed primarily of ammonium nitrate.
Ammonium Nitratea colorless crystalline substance made by treating nitric acid with ammonia vapor. It does not readily explode, but in combination with fuels and more sensitive explosives (such as TNT) it is very effective in blasting operations. ANFO, a mixture typically consisting of 94 per cent ammonium nitrate and 6 per cent fuel oil, is a relatively inexpensive material widely used for blasting. Slurry, or water gel, explosives consist essentially of thickened solutions of ammonium nitrate. They have a high resistance to water and are suitable for use in wet conditions. Slurries typically contain a number of ingredients, including sodium nitrate, calcium nitrate, aluminum, ethylene glycol, and TNT, whose proportions depend on the explosive characteristics desired. Amatol is a mixture of roughly equal amounts of TNT and ammonium nitrate.
Picric Acid(trinitrophenol), a yellow crystalline solid made by heating phenol with concentrated sulfuric acid and then treating the mixture with nitric acid. Picric acid is the chief component of the British explosive lyddite. Picric acid was an important military explosive in World War I.
Ammonium Picrate, or Explosive Da powerful high explosive. It is produced through the action of picric acid upon ammonia. It is extremely insensitive to shock or friction, making it suitable for armor-piercing shells (shells that penetrate armor plate and explode inside a warship or tank by means of a delayed fuze).
Permissible Explosivesspecial high explosives used for blasting in underground coal mines. They typically contain ammonium nitrate and nitroglycerin and are formulated to explode at a relatively low temperature, so that that will not ignite mine gases or coal dust.
Detonators
Most demolition and blasting charges are so powerful that they would not be safe to handle if they could be set off directly. For this reason they are designed to explode only when subjected to the shock of an explosion produced by a small charge of a relatively sensitive explosive. The charge is called a detonator, or initiator. Although these explosives are usually used to detonate less sensitive explosives, they are also sometimes used alone. Explosives used as detonators include:
Lead Azidea colorless crystalline compound of lead and nitrogen. It is widely used as a detonator for military explosives and has largely replaced mercury fulminate, a less stable explosive.
Mercury Fulminatea gray crystalline solid made by treating mercury with strong nitric acid and alcohol. It is readily ignited by spark, flame, friction, or shock.
PETN(pentaerythrite tetranitrate), a white crystalline solid made by mixing acetaldehyde with formaldehyde to form pentaerythritol, which is then treated with nitric acid. PETN is very sensitive to impact. It is commonly used in Primacord and other detonating fuses used in blasting. Pentolite is a mixture of PETN and TNT.
RDX, or Cyclonitea white crystalline solid produced by mixing ammonia with formaldehyde and adding nitric acid. It is a sensitive explosive similar to PETN. Composition B is a mixture of RDX and TNT.
Tetryla yellow crystalline compound prepared from aniline. It is commonly used as a booster charge, an explosive charge that augments the explosion produced by the detonator in order to set off insensitive explosives.
History of Explosives
Gunpowder was known in China as early as the 10th century and was probably first used in Europe in firearms during the 14th century. The use of gunpowder in blasting for construction and mining began in the 17th century. Gunpowder was the only practical explosive available until the middle of the 19th century, when nitrocellulose (gun-cotton) and other explosives were invented. Nitroglycerin, invented in 1847, was too dangerous for practical use until Alfred Nobel, a Swedish chemist, invented dynamite in 1866. Nobel also developed the use of mercury fulminate as a detonator. Several kinds of smokeless powders, including ballistite and cordite, were introduced in the late 1800's. Picric acid and TNT were important military explosives in World War I. PETN and RDX did not come into general use as military explosives until World War II. During the early 1900's dynamite became the most important explosive for blasting, but since the 1950's it has been largely-replaced by ANFO and slurry explosives.