Primary and secondary explosives

Explosives are classed as primary or secondary. Typically, a small quantity of a primary explosive would be used in a detonator (known colloquially as a cap ), whereas larger quantities of secondary explosives are used in the booster and the main charge of a device. This collection of explosives is known as an explosive train in which a signal (mechanical, thermal, or electrical) from the control system is converted first into a small explosive shock from the detonator, which in turn initiates a more powerful explosion in the booster, which amplifies the shock into the main charge. 

Primary explosives are sensitive to modest stimuli such as heat, spark, or friction application of the correct stimulus will lead to a detonation. The primary explosives used in detonators are typically extremely sensitive but not particularly powerful common examples are mercury fulminate, lead azide, and lead styphnate. In principle, the heavy metals present in most primary explosives should be a good cue for detection however, there are primary explosives that do not contain such elements. 

It should be noted that there are modem detonators that are designed to function without primary explosives. These usually rely on an electrically generated shock to produce detonation in a small charge of a specially prepared and sensitive charge of a secondary explosive. 

The requirement for an explosive train, that is, a primary explosive to initiate the secondary explosive, is a safety feature. In the past, people wishing to illegally use explosives usually had to steal the detonators (e.g., Timothy McVey). Consequently, the effective control of access to detonators has been widely regarded as a key pubhc safety measure by many governments and law enforcement agencies. However, recently, triacetone triperoxide (TATP) has been used as the primary explosive (e.g., Richard Reid s shoe bomb) and TATP is readily, although hazardously, synthesized from acetone, hydrogen peroxide, and acid. 

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Figure A.26 Anhydrous ammonia ingredient used to make primary and secondary explosives.

A major difference between primary and secondary explosives arises from the fact that primary explosives are initiated to detonate by burning whereas secondary explosives are initiated to detonate by shock waves. Therefore, the most important property of a primary explosive is its ability to undergo a fast deflagration-to-detonation transition (DDT). Thus, fast DDT is the strength of primary explosives as well as their weakness. All other parameters being equal, the faster the DDT, the better the primary explosive. At the same time,... 

Values of power index for some primary and secondary explosives are given in the Table 1.7 which shows that the values for power index of secondary explosives are more than the values for primary explosives. 

Tablel.7 Power index values of some primary and secondary explosives (standard-picric acid).

A comparison of the velocities of detonation for some primary and secondary explosives is presented in Table 3.2. 

The Figure of Insensitiveness for the standard explosive sample RDX is 80. Figure of Insensitiveness values for some primary and secondary explosives are presented in Table 4.3. 

Figure 4.2 Classification of primary and secondary explosives by their Figure of Insensitiveness (F of I)...

The thermodynamic path presented in Figure 5.1 will most likely not be the same as the kinetic path . For instance, the reaction may take place in several stages involving complex systems of reaction chains, etc. Nevertheless, the energy evolved depends only on the initial and final states and not on the intermediate ones. Once the reaction is completed, the net heat evolved is exactly the same as if the reactant molecules were first dissociated into their atoms, and then reacted directly to form the final products (Hess s Law). The heats of formation of some primary and secondary explosive substances are presented in Table 5.11. 

Table 5.11 Heats of formation of some primary and secondary explosive substances...

Table 5.12 Heat of explosion and heat of detonation at constant volume for some primary and secondary explosive substances using the K-Wand modified K-W rules. AHf (H20) is in the gaseous state...

The higher the value of Q for an explosive, the more the heat generated when an explosion occurs. Table 5.12 presents calculated heats of explosion together with their calculated heats of detonation for some primary and secondary explosive substances. The negative sign for the heat of explosion is generally omitted since it only denotes an exothermic reaction. 

This book outlines the basic principles needed to understand the mechanism of explosions by chemical explosives. The history, theory and chemical types of explosives are introduced, providing the reader with information on the physical parameters of primary and secondary explosives. Thermodynamics, enthalpy, free energy and gas equations are covered together with examples of calculations, leading to the power and temperature of explosions. A very brief introduction to propellants and pyrotechnics is given, more information on these types of explosives should be found from other sources. This second edition introduces the subject of Insensitive Munitions (IM) and the concept of explosive waste recovery. Developments in explosive crystals and formulations have also been updated. This book is aimed primarily at A level students and new graduates who have not previously studied explosive materials, but it should prove useful to others as well. I hope that the more experienced chemist in the explosives industry looking for concise information on the subject will also find this book useful. 

Tab. 2.1 Typical sensitivity and performance data of primary and secondary explosives.

The trend toward separate apparatus for primary and secondary explosives has become more apparent in recent years. While this approach may permit greater precision in the test measurements, it will attribute little to the basic understanding of initiation by impact unless improved diagnostics of the thermomechanical behavior of the substances are also introduced. 

Table 4.1 Temperatures for the initiation of some primary and. secondary explosives hy friction (via hotspots) and thermal tnechnnisms...

Don t be misled by the technical title of this book — Explosive Principles is a readable, easy-to-understand primer on the nature of detonations, as well as a concise look at the basic but often overlooked techniques that professionals use to maximize the performance of primary and secondary explosives to get any job done right. A must for demolition personnel, amateur powder monkeys, or anybody interested in things that go bang,... 

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