The chemistry of detonators - what's behind the bang.
For centuries, gunpowder, a mixture of potassium nitrate, charcoal and sulfur, was the only substance available for blasting. The discovery of nitroglycerine in 1846 by Sobrero opened the way to Alfred Nobel's development of dynamite in 1864. Dynamite was much more effective than gunpowder in breaking rock because it had a faster reaction rate capable of producing a shock wave. Dynamite was also safer than gunpowder which could be ignited by the slightest flame or spark, whereas dynamite required an explosive shock to cause effective initiation. The need for a safe and reliable means of initiating nitroglycerine and dynamite was the basis of another of Nobel's many inventions - the detonator. Nobel's first detonator used gunpowder, but by 1865 it was replaced by mercury fulminate, the first molecular initiating explosive put to commercial use.
The development of dynamite and detonators was a milestone in the history of the explosives industry. Dynamite is a powerful explosive that is stable under normal circumstances. This affords a high degree of safety while it is being transported and loaded into holes. The detonator is the key that makes it possible to unlock the chemical energy stored in the dynamite at the appropriate time.
Nowadays, dynamite has largely been replaced by other types of explosives. Refinements in detonators have resulted in many changes, including the replacement of mercury fulminate by safer alternatives. But the explosives industry still operates on the basic concept of using explosive charges that are safe to handle in combination with detonators that are able to reliably initiate the explosives.
The work done by an explosive in a blast is the result of the combined effects of a shockwave which travels very rapidly through the rock, resulting in fragmentation, and a slower gas pressure wave created by the gaseous combustion products which propels the fragmented material away from the centre of the blast. For the blast to be effective each hole containing explosives needs to have an open space or free face to break to. If a hole is too far from a free face, then it will be unable to move the large amount of rock between it and the face. This will result in most of the gas produced in the explosion being forced out of the top of the hole, greatly reducing the breaking effect on the rock, and producing a large amount of dangerous fly-rock (material thrown into the air and landing outside the blast area).
Since most blasts involve a large number of holes which are fired in one shot, there has to be some way to fire the individual holes in a sequence, such that a new free face is created by "completion" of the blast in one hole before the subsequent hole is fired. Use of a sequential blast will maximize blasting efficiency and minimize fly-rock. It will also significantly reduce concussion and ground vibration, since only a relatively small amount of the explosive is initiated at any one time. The most common method of inducing a time delay between successive holes is by using detonators that have a built-in delay. This type of initiation system allows all of the detonators in the blast to be ignited simultaneously, with each detonator then firing at a slightly different time based on the predetermined delay time of that detonator.
This article focuses on the nonelectric delay detonator, which has become the dominant type of detonator in the commercial explosives market, replacing other products such as electric detonators and safety fuse assemblies because it offers a higher degree of safety and greater ease of use.
Structure of a Nonelectric Delay Detonator
A sectional view of a typical nonelectric detonator is illustrated in Figure 1. The detonator itself is a metal tube, usually aluminum, containing a delay element and explosives. A nonelectric detonator uses a plastic shock tube, which contains a very fine coating of explosive on the tube's inner surface, to transfer the firing signal from the surface of the blast to the detonator. A surface connector containing a detonator with a small explosive charge is usually used to initiate the shock tube. When the detonator in the connector is fired, it initiates the explosive in the shock lube which rapidly propagates along the length of the tube to the detonator. The hot flash from the end of the shock tube then ignites the pyrotechnic composition in the delay element. An explosive priming charge, typically lead azide, is pressed into the detonator under the delay element. The flame from the delay element causes the lead azide to detonate which then initiates detonation of the base charge explosive, normally PETN. The combination of a base charge and priming charge is used because PETN is a more powerful explosive than lead azide and is therefore more effective at initiating the explosive charge in the blast hole, but PETN cannot be used as the only explosive in the detonator since it will not detonate directly from the flame of the delay element. The detonator delay time is determined by the time taken for the combustion front in the pyrotechnic composition to travel the length of delay element.
The Pyrotechnic Delay
A pyrotechnic delay composition is an intimate mixture of solid, powdered fuels and oxidants capable of a highly exothermic oxidation-reduction reaction. Examples of oxidants that are used in delay compositions include perchlorates, chromates, permanganates, sulphates and oxides. The oxidant needs to be stable to at least 100 [degrees] C, but should readily decompose to release oxygen at higher temperatures. If a fast burning delay mixture is desired, then an oxidant with a relatively low decomposition temperature should be selected. Oxidants with small endothermic (or even exothermic) heats of decomposition will give faster delay compositions than ones that require more energy for decomposition. Some of the fuels used in delay compositions are zirconium, titanium, molybdenum, selenium, boron, silicon, antimony and tungsten. The fuels with higher heats of combustion will give greater reactivity and faster burn rates. One of the common compositions used for delays ranging from 25 to 1000 ms is the mixture of silicon and red lead. The principal combustion reaction for this composition is:
2 Si + [Pb.sub.3][O.sub.4] - 2 Si[O.sub.2] + 3 Pb
There are other factors beside the choice of fuel and oxidizer that affect the burn rate of the composition. Burn rates vary greatly depending on the fuel/oxidizer ratio. The fastest burn rates generally occur at the stoichiometric point or when there is a slight excess of metallic fuel, presumably because the metal increases the thermal conductivity of the composition. The particle size of the components is also important, with finer particles generally giving faster burn rates because of the more intimate contact between fuel and oxidizer. Even the type of metal casing used for the delay element and the diameter of the powder core will affect timing because of the influence these factors have on the lateral heat loss into the casing.
The delay time of a detonator is determined by the burn rate of the composition used in the delay element and the length of that element. A series of different delay compositions with progressively slower burn rates is required to produce detonators over a wide range of delay times from about nine milliseconds to nine seconds. Each composition type is used to cover a specific range of delay times, with consecutive delay intervals in that range being achieved by using various element lengths ranging from 0.5 to 6 cm. Element lengths beyond 6 cm are not used because the resulting detonator would be excessively long.
The Explosive Components
In the previous section, the point was made that the reactivity of a pyrotechnic mixture could be increased by using finer particle sizes, thereby creating a more intimate mixture and reducing the diffusion required for reaction to occur. Explosives contain both electron donors and electron acceptors within the same molecule. This gives the most intimate contact Possible, with no need for diffusion. When sufficient activation energy is applied to the explosive, the electron transfer reaction occurs at the intramolecular level. The energy released from the reaction of the first molecule is sufficient to break neighbouring bonds, starting a violent reaction that propagates through the explosive at supersonic velocity. Table 1 lists reaction velocity and pressure for the explosives discussed in this article. The chemical structure of each of these explosives is shown in Figure 2.
[TABULAR DATA FOR TABLE 1 OMITTED]
Pentaerythritol tetranitrate (PETN) is the most commonly used base charge in detonators. The nitrogen atom in the nitrate ion is electron deficient and needs to accept electrons to relieve bonding stress. The carbon and hydrogen atoms in the same molecule are good electron donors. The products formed from this reaction are gaseous [N.sub.2], CO, C[O.sub.2] and [H.sub.2]O.
Lead azide, Pb[([N.sub.3]).sub.2], which is commonly used as the priming charge, contains no carbon, hydrogen or oxygen. The basis of its energetic nature is the azide ion ([[N.sub.3].sup.-]). Without external stimulus, lead azide can maintain its stable condition indefinitely. But if sufficient stimulus is applied, the bonds of the triatomic azide ion will be broken and energetically favoured [N.sub.2] molecules will be formed. The products formed following the detonation of lead azide are nitrogen gas and metallic lead. Lead azide is not used in its absolutely pure crystalline form because it is too sensitive to accidental initiation by friction, impact or electrostatic charges. Co-precipitation with a crystal habit modifier such as dextrin produces lead azide which can be safely pressed into the detonators.
The explosive used in the shock tube is cyclotetramethlylenetetranitramine or more simply HMX or octagen. The nitro group is the electron deficient component, but in this explosive it is bonded to carbon through a nitrogen atom instead of oxygen as with PETN. The major reaction products from HMX are gaseous [N.sub.2], CO, and [H.sub.2]O. HMX is an acronym for High Melting explosive. The relatively high thermal stability of HMX is one of the main reasons it is used in shock tube where the explosive coating is applied to the inner wall of the plastic tube as it is being extruded at a high temperature. It was mentioned earlier that only a fine layer of explosive is used in shock tube. The explosive charge is less than 20 mg/m, so one kilometre of shock tube contains less than 20 grams of explosive. The low detonation pressure and velocity reported for shock tube in Table 1 are the result of this minimal amount of explosive. The small explosive charge also allows the shock wave to propagate through the shock tube without destroying it.
I hope that this article has managed to convey some of the interesting aspects of the energetic materials used in a detonator. There are many other fascinating uses of energetic materials. For example, the airbags in automobiles use sodium azide as their source of nitrogen gas for inflation. Sodium azide is less reactive than lead azide and does not detonate, but it still reacts rapidly enough to quickly inflate the airbag. The solid-fuel rockets used on the space shuttle are another well known use of energetic materials. These rockets contain a mixture of 70 percent ammonium perchlorate, 16 percent aluminum and 14 percent organic binder, Approximately 900,000 kg of this mixture are used in each shuttle launch.
Brent Ball, MCIC is a Development Officer for Orica Canada Inc. (formerly ICI Explosives) based in Brownsburg, QC. Over the past 16 years, he has been involved in various areas of development and technical support related to detonator manufacture. He is also Chair of the Hawkesbury Local Section of the CIC.
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|Publication:||Canadian Chemical News|
|Date:||Jul 1, 1998|
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