I.

Introduction

Explosives, chemical compounds or mixtures that undergo rapid burning or decomposition with the generation of large amounts of gas and heat and the consequent production of sudden pressure effects. The chief use of explosives in peacetime is for blasting and quarrying (see Quarry and Quarrying), but explosives are also used in fireworks and signaling apparatus (see Fireworks) and for setting blind rivets and forming metals. Explosives are used as propellants for projectiles and rockets and as bursting charges for demolition purposes and for projectiles, bombs, and mines (see Bomb; Mine; Nuclear Weapons; Projectile; Rocket).

The first explosive known was gunpowder, also called black powder. In use by the 13th century, it was the only explosive known for several hundred years. Nitrocellulose (see Cellulose) and nitroglycerin, both discovered in 1846, were the first modern explosives. Since then nitrates, nitrocompounds, fulminates, and azides have been the chief explosive compounds used alone or in mixtures with fuels or other agents. Xenon trioxide, the first explosive oxide, was developed in 1962.

II.

Characteristics of Explosives

Explosives are grouped into two main classes, low explosives, which burn at rates of inches per second, and high explosives, which undergo detonation at rates of from 914 to 9140 m per sec (1000 to 10,000 yd per sec). Explosives vary in other important characteristics that influence their use in specific applications. Among these characteristics are the ease with which they can be detonated and their stability to conditions of heat, cold, and humidity. The shattering effect, or brisance, of an explosive depends upon the velocity of detonation. Some of the newer high explosives with a detonation rate of 9140 m per sec are extremely effective for military demolition and certain types of blasting. On the other hand, for quarrying and mining, when it is desirable to dislodge large pieces of rock or ore, explosives with a lower detonation velocity and lower brisance must be employed. Explosives used as propellants in rifles and cannon should burn still more slowly, as they are required to deliver a steadily increasing push to the projectile in the barrel of the gun rather than a sudden shock which, if strong enough, might break the gun. Special types of explosives that are sensitive to heat or shock and have a medium-high brisance are used to initiate the detonation of less sensitive high explosives. High explosives are often mixed with inert materials to reduce sensitivity and lower brisance, as in the case of dynamite.

III.

Propellants

Two types of explosive are in general use for the propulsion of projectiles in firearms and rockets, and both are commonly called by the generic name of smokeless powder. The term is properly applied to the low explosive, gelatinized nitrocellulose. The other type of smokeless powder, which consists of a mixture of nitrocellulose with a high explosive such as nitroglycerin, is known correctly as double-base powder or compound powder. A common double-base explosive is cordite, which contains 30 to 40 percent nitroglycerin and a small quantity of petroleum jelly as a stabilizer. The term smokeless powder applied to either type of explosive, however, is misleading, because neither is free from smoke when exploded, and neither takes the form of a true powder.

The rate of burning of either type of smokeless powder is controlled by the shaping of the powder grains. Because the powder grains burn from the surface inward, it is possible to produce grains that burn progressively more slowly, at an even rate, or progressively more quickly depending on the shape and dimensions of the grains. For example, spherical grains have progressively smaller surface areas as they burn, and therefore burn progressively more slowly. Such degressive burning powders are used in such short-barreled small arms as pistols.

IV.

High Explosives

A great number of explosives undergo detonation. Some of these, such as TNT, trinitrotoluene, have a high resistance to shock or friction and can be handled, stored, and used with comparative safety. Others, such as nitroglycerin, are so sensitive that they are almost invariably mixed with an inert desensitizer for practical use. To obtain desirable characteristics, explosives of different characteristics are often mixed.

During World War I, TNT was the high explosive most generally employed, but before and during World War II a number of extremely efficient new high explosives were developed. Among the most important are cyclonite and pentaerythritol tetranitrate.

Cyclonite, also called RDX, is used in detonators. A mixture with TNT and wax is called Composition B and is used in bombs. A similar mixture, containing aluminum and called torpex, has an underwater effect about 50 percent greater than that of TNT. A plastic composition containing cyclonite and an explosive plasticizer is used for demolition charges.

Pentaerythritol tetranitrate, also called PETN, has characteristics similar to those of cyclonite and is mixed with TNT to form the explosive pentolite. It also forms the core of the explosive primacord fuses used for detonating demolition charges and the booster charges used in blasting.

Two types of high explosives introduced since 1955 have largely replaced dynamite. A mixture of ammonium nitrate and fuel oil has explosive strength 25 percent greater than that of TNT. The so-called slurry explosives, which are also used for blasting, are mixtures containing sufficient water to form flowable material.

V.

Detonators

For detonating charges of comparatively insensitive high explosives, compounds are used that will themselves detonate under a moderate mechanical shock or heat with sufficient force to explode the main charge. For many years mercury fulminate, Hg(ONC)₂, was the compound chiefly employed for this purpose, either alone or mixed with other substances such as potassium chlorate. Its manufacture, however, is hazardous and it cannot be stored at high temperatures without decomposition. In addition, mercury may be difficult to obtain in time of war. As a result the fulminate has been replaced almost entirely in commercial and military detonators by lead azide, PbN6, diazodinitrophenol, and mannitol hexanitrate. These initiators are used in conjunction with a charge of cyclonite or PETN, which have largely replaced the tetryl (trinitrophenylmethylnitramine) used previously. These sensitive explosives have high brisance and explosive strength values. They are frequently used also as booster charges between the detonator and the major charge of high explosive in large shells and bombs. A blasting cap or exploder is a small charge of a detonator designed to be embedded in dynamite and ignited either by a burning fuse or a spark.

VI.

Safety Explosives

In coal mining the use of ordinary high explosives is hazardous because of the danger of igniting gases or suspended coal dust that may be present underground. For blasting under such conditions several special types of safety explosives have been developed that minimize the danger of fires or explosions by producing flames that last for a very short time and are relatively cool. The types of safety explosives approved for work in coal mines are chiefly mixtures of ammonium nitrate with other ingredients such as sodium nitrate, nitroglycerin, nitrocellulose, nitrostarch, carbonaceous material, sodium chloride, and calcium carbonate. Another kind of blasting charge for use in mining has grown in favor, because it produces no flame whatsoever. This charge is a cylinder of liquid carbon dioxide that can be converted into gas almost instantaneously by an internal chemical heating element. One end of the cylinder contains a breakable seal through which the gas can expand. The carbon dioxide charge is not a true explosive and absorbs heat rather than evolving it. It has the additional advantage that the force of the explosion can be directed at the base of the bore hole in which the charge is placed, thus lessening the shattering of the coal.

See also Nuclear Energy.