EXPLOSIVES 355 CHIMIA 2004, 58, No. 6 Chimia 58 (2004) 355–362 © Schweizerische Chemische Gesellschaft ISSN 0009–4293 Explosive Substances and Their Applications: An Overview Hansruedi Bircher* Abstract: A systematic overview about the different types of energetic material is given. The chemical composi- tion, the reaction behavior, and the initiation mechanisms of explosives are discussed. Keywords: Explosives · Propellants · Pyrotechnics · Reactions 1. Introduction actions of explosive substances proceed made great progress [14]. High density exothermally. This means that explosives (>2.0 gcm–3) organic molecules with high The term ‘explosive material’defines a sub- need to have chemical groups in their mol- energy contents were developed [15]. A stance or a mixture of substances capable of ecules that increase their heats of formation whole series of new energetic plasticizers producing gas at such temperature and and produce gaseous products upon reac- and synthetic polymers exists, which are pressure by chemical reaction as to cause tion. This type of chemical groups is called the base for modern binder systems [16]. damage to the surroundings. This also in- explosophorics. The most famous ex- There are substances available with high ni- cludes pyrotechnic substances even though plosophoric groups are based on nitrogen trogen content for propulsion [17]. In the they may not evolve gases as they react [1]. and oxygen, such as the classical nitro, ni- field of pyrotechnics, where almost exclu- Similar definitions can be found in corre- trate, nitrate ester, nitramine, azide, and azi- sively inorganic substances are used to gen- sponding international standards [2] as well do groups. Also well known are perchlo- erate the intended effects, new reducing as in national laws and regulations, where- rates and chlorates. Due to these ex- agents, mostly introduced as fine grained as only those substances that are produced plosophoric groups, the oxygen needed for metal powders, and new oxidizers are under with the intention to be used as explosive the conversion of explosives into their close examination [18]. As the most recent material fall under these laws. Chemical gaseous reaction products, such as NOx, development in the field of explosive mate- by-products or intermediate products, such CO2,CO, and H2O, is available within the rials, the investigations using nano particles as peroxides, explosive clouds, or aerosols corresponding molecules. This prerequisite and the application of sol–gel chemistry formed by a fuel and the oxygen of air are becomes more and more important, the have to be mentioned [19]. In this paper an sometimes excluded, if they are not pro- faster a chemical reaction proceeds, be- overview is given of which explosive sub- duced to cause a damaging effect. This is al- cause the diffusion process in the air is no stances are used for different applications so the case for fertilizers containing ammo- longer efficient enough to deliver sufficient and what the future trends and tendencies nium nitrate, which can have the potential amounts of oxygen for the chemical con- are. Before going into more detail, different to be used as a component in explosive ma- version. The nitrogen of explosophoric reaction types and their initiation pathways terial [3]. groups strongly supports the gas production are introduced. From a chemical point of view, most ex- rate, as does the chlorine in chlorates and plosives are substances or mixtures thereof, perchlorates. So, the oxygen balance, i.e. which are capable of releasing large the percentage of oxygen chemically bound 2. Explosives and Reaction Types amounts of hot gaseous products over a in a molecule to oxidize it completely (Eqn. short period of time when they undergo the 1), as well as the nitrogen content of a mol- Four different reaction types are distin- intended chemical reactions. Almost all re- ecule are the key parameters for the identi- guished according to velocity. The slowest fication of explosive substances and their reaction type is the ageing reaction, which mixtures (Table) [4][5]. can proceed at ambient temperatures over Many books on explosives discuss ei- the timeframe of years. Ageing reactions re- ther in their introduction or in detail the sult in energy loss, change of mechanical most important properties of well-known properties, and gas evolution. Several or- traditional explosives, such as glycerine ders of magnitude faster is the burning re- trinitrate, nitrocellulose or trinitrotoluene action, which proceeds with linear burning *Correspondence: Dr. H. Bircher [6–9]. Some of them also cover recent de- velocities up to several meters per second. armasuisse Science and Technology velopments in the field [10–13]. For many A conversion of an explosive by deflagra- Feuerwerkerstrasse 39 applications however, traditional sub- tion is again faster and reaches velocities of CH–3602 Thun stances are still important and therefore are several hundred meters per second. And fi- Tel.: +41 33 228 30 03 Fax: +41 33 228 30 39 still being used in large quantities. Over the nally, the fastest reaction can be observed E-Mail: [email protected] last decades the chemistry of explosives has when explosives detonate. Detonation reac- EXPLOSIVES 356 CHIMIA 2004, 58, No. 6 by heat-flow calorimetry and then extrapo- lated back to ambient temperature using an appropriate kinetic model. Thereby either an Arrhenius approach based on an n-th order reaction [21] or a model free evalua- tion method is used, where for a tempera- ture raise of 10 °C an increase of reaction rate by a certain factor is assumed. Where- as most aromatic and aliphatic nitro com- pounds show only negligible chemical degradation over long time periods, ageing reactions are a critical problem for nitrate esters. The main factors for ageing reactions are temperature, humidity, and acid residues. Most nitrate esters need to be pro- tected with a stabilizer, which is capable of catching free NOx radicals originating from corresponding ageing reactions. Most of these stabilizers contain mono- or diphenyl- amine units, sometimes connected over a urea bridge. In the first step, these phenyl amino units are mainly converted to N-ni- troso and 2-nitro or 4-nitro derivatives [22]. The comparison of the actual stabilizer con- tent with the initial content gives a good in- dication about progress of ageing reactions. Also reducing agents of pyrotechnical re- dox systems, such as magnesium or titani- um can be affected by ageing processes, especially if humidity is present. The con- version of magnesium to magnesium hy- droxide under generation of hydrogen gas takes place quite easily. Such reactions can affect the functionality of corresponding pyrotechnic items fundamentally. A further important ageing reaction is the fissure of polymeric chains [23]. In the case of nitro- cellulose, the glycosidic bond is broken and the linear chain is split into polymeric units Table. Oxygen balance and nitrogen content of different substances in view of their explosiveness. of much lower molecular weight. The same ageing reaction can also be observed for other cellulose-based polymers, such as Substance Oxygen Balance Nitrogen Content Explosive cellulose-acetate-butyrate or poly-esters, which are quite often used in binders sys- Silicon dioxide 0.0% 0.0% no tems of solid rocket propellants. Polymer 1,2,3-Propanetriol trinitrate 3.5% 18.5% yes chain fissures change the mechanical prop- (Glycerine trinitrate) erties of corresponding charges and there- fore also their ballistic behavior. 1,3,5,7-Tetranitro- –21.6% 37.8% yes 1,3,5,7-tetraazacyclooctane 2.2. Burning Reaction (Octogen or HMX) The burning reaction of an explosive 2,4,6-Trinitrotoluene –73.9% 18.5% yes starts if the temperature is raised above its 2,4,6-Triamino- –114.2% 66.6% no ignition temperature. The burning process 1,3,5-triazine (Melamine) itself is a self-sustaining, exothermic redox reaction. Due to the heat, the corresponding hot gases, and the fine particles released in a first step, the reaction normally continues tions proceed with several thousand meters degree of conversion remains very low. in the gas phase under emission of light. For per second. Nevertheless, self heating due to exother- the transfer of heat generated by such reac- mal conversion and transition into a burning tions, a conductive and a convective mech- 2.1. Ageing Reactions reaction occurs according to the heat-flow anism is distinguished. The conductive heat As already mentioned ageing reactions equation as a function of heat production, transfer mechanism is influenced by the occur at ambient temperature and proceed specific heat, thermal conductivity, and the same parameters as already described for very slowly for most common explosives. dimension of an explosive charge (Eqn. 2) the heat transfer of ageing reactions (Eqn. In many cases no obvious changes can be [4][20]. Such heat flows are measured at 2). When modeling such phenomena based observed by visual inspection and also the slightly higher temperatures (40 °C–80 °C) on finite element simulations, the instant EXPLOSIVES 357 CHIMIA 2004, 58, No. 6 change of all material parameters behind achieves the magnitude of the material’s ters responsible for the acceleration of the the reaction zone has to be taken into ac- sound speed. First shockwaves passing the reaction are, among others, the energy re- count. The explosive charge itself burns material are generated. A deflagration reac- leased by the reaction and the rate of gas layer by layer and the temperature within tion is physically unstable and therefore generation. These two parameters are di- the charge decreases with distance to the re- hard to control. Whether the reaction is ac- rectly related to the composition of the ex- action zone. The velocity of the linear burn- celerated further to supersonic velocities plosive and the chemical structure of its ing reaction is given by the steady state of and so to a detonation, or the reaction de- components.