| Русский |
Explosives are classified as low or high explosives according to their rates of decomposition: low explosives burn rapidly (or deflagrate), while high explosives undergo detonations. No sharp distinction exists between low and high explosives, because of the difficulties inherent in precisely observing and measuring rapid decomposition.
The chemical decomposition of an explosive may take years, days, hours, or a fraction of a second. The slower processes of decomposition take place in storage and are of interest only from a stability standpoint. Of more interest are the two rapid forms of decomposition, deflagration and detonation.
The term "detonation" is used to describe an explosive phenomenon whereby the decomposition is propagated by the explosive shockwave traversing the explosive material. The shockwave front is capable of passing through the high explosive material at great speeds, typically thousands of meters per second.
Explosives usually have less potential energy than, say, petroleum fuels, but their high rate of energy release produces the great blast pressure. TNT has a detonation velocity of 6,940 m/s compared to 1,680 m/s for the detonation of a pentane-air mixture, and the 0.34-m/s stoichiometric flame speed of gasoline combustion in air.
Explosive force is released in a direction perpendicular to the surface of the explosive. If the surface is cut or shaped, the explosive forces can be focused to produce a greater local effect; this is known as a shaped charge.
In a low explosive, the decomposition is propagated by a flame front which travels much more slowly through the explosive material.
The properties of the explosive indicate the class into which it falls. In some cases explosives can be made to fall into either class by the conditions under which they are initiated. In sufficiently massive quantities, almost all low explosives can undergo true detonation like high explosives. For convenience, low and high explosives may be differentiated by the shipping and storage classes. [edit] Explosive compatibility groupings
Explosives warning sign
Shipping tags will include UN & US DOT hazardous material class with compatibility letter as follows.
An article containing a primary explosive substance and not containing two or more effective protective features. Some articles, such as detonator assemblies for blasting and primers, cap-type, are included. (1.1B, 1.2B, 1.4B)
Propellant explosive substance or other deflagrating explosive substance or article containing such explosive substance (1.1C, 1.2C, 1.3C, 1.4C)
Secondary detonating explosive substance or black powder or article containing a secondary detonating explosive substance, in each case without means of initiation and without a propelling charge, or article containing a primary explosive substance and containing two or more effective protective features. (1.1D, 1.2D, 1.4D, 1.5D)
Article containing a secondary detonating explosive substance without means of initiation, with a propelling charge (other than one containing flammable liquid, gel or hypergolic liquid) (1.1E, 1.2E, 1.4E)
containing a secondary detonating explosive substance with its means of initiation, with a propelling charge (other than one containing flammable liquid, gel or hypergolic liquid) or without a propelling charge (1.1F, 1.2F, 1.3F, 1.4F)
Pyrotechnic substance or article containing a pyrotechnic substance, or article containing both an explosive substance and an illuminating, incendiary, tear-producing or smoke-producing substance (other than a water-activated article or one containing white phosphorus, phosphide or flammable liquid or gel or hypergolic liquid) (1.1G, 1.2G, 1.3G, 1.4G)
Article containing both an explosive substance and white phosphorus (1.2H, 1.3H)
Article containing both an explosive substance and flammable liquid or gel (1.1J, 1.2J, 1.3J)
Article containing both an explosive substance and a toxic chemical agent (1.2K, 1.3K)
Explosive substance or article containing an explosive substance and presenting a special risk (e.g., due to water-activation or presence of hypergolic liquids, phosphides or pyrophoric substances) needing isolation of each type (1.1L, 1.2L, 1.3L)
Articles containing only extremely insensitive detonating substances (1.6N)
Substance or article so packed or designed that any hazardous effects arising from accidental functioning are limited to the extent that they do not significantly hinder or prohibit fire fighting or other emergency response efforts in the immediate vicinity of the package (1.4S) [edit] Low explosives
A low explosive is usually a mixture of a combustible substance and an oxidant that decomposes rapidly (deflagration); unlike most high explosives, which are compounds.
Under normal conditions, low explosives undergo deflagration at rates that vary from a few centimeters per second to approximately 400 metres per second. However, it is possible for them to detonate, especially if the reaction occurs in a confined space. (See pipe bomb, flour bomb)
Low explosives are normally employed as propellants. Included in this group are gun powders and pyrotechnics such as flares and illumination devices. [edit] High explosives
High explosives are normally employed in mining, demolition, and military warheads. They undergo detonation at rates of 1,000 to 9,000 meters per second. High explosives are conventionally subdivided into two classes differentiated by sensitivity:
A sensitiser is a powdered or fine particulate material that is sometimes used to create voids that aid in the initiation or propagation of the detonation wave. [1] It may be as high-tech as glass beads (Glass Bubbles[2]) or as simple as black cumin seeds[3]. [edit] Military explosives
To determine the suitability of an explosive substance for military use, its physical properties must first be investigated. The usefulness of a military explosive can only be appreciated when these properties and the factors affecting them are fully understood. Many explosives have been studied in past years to determine their suitability for military use and most have been found wanting. Several of those found acceptable have displayed certain characteristics that are considered undesirable and, therefore, limit their usefulness in military applications. The requirements of a military explosive are stringent, and very few explosives display all of the characteristics necessary to make them acceptable for military standardization. Some of the more important characteristics are discussed below: [edit] Availability and cost In view of the enormous quantity demands of modern warfare, explosives must be produced from cheap raw materials that are nonstrategic and available in great quantity. In addition, manufacturing operations must be reasonably simple, cheap, and safe. [edit] Sensitivity
Main article: Oxygen balance
Oxygen balance is an expression that is used to indicate the degree to which an explosive can be oxidized. If an explosive molecule contains just enough oxygen to convert all of its carbon to carbon dioxide, all of its hydrogen to water, and all of its metal to metal oxide with no excess, the molecule is said to have a zero oxygen balance. The molecule is said to have a positive oxygen balance if it contains more oxygen than is needed and a negative oxygen balance if it contains less oxygen than is needed. The sensitivity, strength, and brisance of an explosive are all somewhat dependent upon oxygen balance and tend to approach their maximums as oxygen balance approaches zero. [edit] Heat of explosion
When a chemical compound is formed from its constituents, heat may either be absorbed or released. The quantity of heat absorbed or given off during transformation is called the heat of formation. Heats of formations for solids and gases found in explosive reactions have been determined for a temperature of 15 °C and atmospheric pressure, and are normally given in units of kilocalories per gram-molecule. (See table 12-1). A negative value indicates that heat is absorbed during the formation of the compound from its elements; such a reaction is called an endothermic reaction.
The arbitrary convention usually employed in simple thermochemical calculations is to take heat contents of all elements as zero in their standard states at all temperatures (standard state being defined as natural or ambient conditions). Since the heat of formation of a compound is the net difference between the heat content of the compound and that of its elements, and since the latter are taken as zero by convention, it follows that the heat content of a compound is equal to its heat of formation in such non-rigorous calculations. This leads to the principle of initial and final state, which may be expressed as follows: "The net quantity of heat liberated or absorbed in any chemical modification of a system depends solely upon the initial and final states of the system, provided the transformation takes place at constant volume or at constant pressure. It is completely independent of the intermediate transformations and of the time required for the reactions." From this it follows that the heat liberated in any transformation accomplished through successive reactions is the algebraic sum of the heats liberated or absorbed in the several reactions. Consider the formation of the original explosive from its elements as an intermediate reaction in the formation of the products of explosion. The net amount of heat liberated during an explosion is the sum of the heats of formation of the products of explosion, minus the heat of formation of the original explosive. The net difference between heats of formations of the reactants and products in a chemical reaction is termed the heat of reaction. For oxidation this heat of reaction may be termed heat of combustion.
In explosive technology only materials that are exothermic—that have a heat of reaction that causes net liberation of heat—are of interest. Hence, in this context, virtually all heats of reaction are positive. Reaction heat is measured under conditions either of constant pressure or constant volume. It is this heat of reaction that may be properly expressed as the "heat of explosion." [edit] Balancing chemical explosion equations
In order to assist in balancing chemical equations, an order of priorities is presented in table 12-1. Explosives containing C, H, O, and N and/or a metal will form the products of reaction in the priority sequence shown. Some observation you might want to make as you balance an equation:
Example, TNT:
C6H2(NO2)3CH3; constituents: 7C + 5H + 3N + 6O
Using the order of priorities in table 12-1, priority 4 gives the first reaction products:
7C + 6O > 6CO with one mol of carbon remaining
Next, since all the oxygen has been combined with the carbon to form CO, priority 7 results in:
3N > 1.5N2
Finally, priority 9 results in: 5H > 2.5H2
The balanced equation, showing the products of reaction resulting from the detonation of TNT is:
C6H2(NO2)3CH3 > 6CO + 2.5H2 + 1.5N2 + C
Notice that partial moles are permitted in these calculations. The number of moles of gas formed is 10. The product, carbon, is a solid. [edit] Volume of products of explosion
The law of Avogadro states that equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules, that is, the molar volume of one gas is equal to the molar volume of any other gas. The molar volume of any gas at 0°C and under normal atmospheric pressure is very nearly 22.4 liters. Thus, considering the nitroglycerin reaction, C3H5(NO3)3 > 3CO2 + 2.5H2O + 1.5N2 + 0.25O2 the explosion of one mole of nitroglycerin produces 3 moles of CO2, 2.5 moles of H2O, 1.5 moles of N2, and 0.25 mole of O2, all in the gaseous state. Since a molar volume is the volume of one mole of gas, one mole of nitroglycerin produces 3 + 2.5 + 1.5 + 0.25 = 7.25 molar volumes of gas; and these molar volumes at 0°C and atmospheric pressure form an actual volume of 7.25 ? 22.4 = 162.4 liters of gas.
Based upon this simple beginning, it can be seen that the volume of the products of explosion can be predicted for any quantity of the explosive. Further, by employing Charles' Law for perfect gases, the volume of the products of explosion may also be calculated for any given temperature. This law states that at a constant pressure a perfect gas expands 1/273.15 of its volume at 0°C, for each degree Celsius of rise in temperature.
Therefore, at 15°C (288.15 Kelvins) the molar volume of an ideal gas is, V15 = 22.414 (288.15/273.15) = 23.64 liters per mole
Thus, at 15°C the volume of gas produced by the explosive decomposition of one mole of nitroglycerin becomes V = (23.64 l/mol)(7.25 mol) = 171.4 l [edit] Explosive strength
Main article: Strength (explosive)
The potential of an explosive is the total work that can be performed by the gas resulting from its explosion, when expanded adiabatically from its original volume, until its pressure is reduced to atmospheric pressure and its temperature to 15 °C. The potential is therefore the total quantity of heat given off at constant volume when expressed in equivalent work units and is a measure of the strength of the explosive. [edit] Example of thermochemical calculations The PETN reaction will be examined as an example of thermo-chemical calculations. PETN: C(CH2ONO2)4
Molecular weight = 316.15 g/mol