Explosive Substances and Their Applications: an Overview

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 composition, 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 temperature raise of 10 °C an increase of reaction rate by a certain factor is assumed. Where- as most aromatic and aliphatic nitro compounds 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 protected with a stabilizer, which is capable of catching free NOx radicals originating from corresponding ageing reactions. Most of these stabilizers contain mono- or diphenylamine 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 content with the initial content gives a good in- dication about progress of ageing reactions. Also reducing agents of pyrotechnical redox systems, such as magnesium or titanium can be affected by ageing processes, especially if humidity is present. The conversion 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 nitrocellulose, 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 therefore 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. The chemical structure de- heat production and the efficiency of heat creases again to burning, depends strongly fines also the general tendency to undergo transfer to reach ignition temperature with- on the type of explosive, its surface area in DDT. Nitrate esters with equalized or posi- in the material. If the pressure around a this moment and its confinement. It can be tive oxygen balance (Eqn. 1), such as glyc- charge rises and the hot gases have no pos- observed quite often that the reaction of a erine trinitrate or pentaerythriol tetranitrate sibility to escape, the heat transfer becomes deflagrating shell even stops, as soon as its (PETN) are known to have this behavior. A more efficient and the burning rate increas- casing has broken into parts and this way its large surface, related to the grain size of the es. These phenomena is described in the confinement was released. material and its porosity (or density), fur- law of Vieille (Eqn. 3), where A is a con- ther supports a burning or deflagrating ex- stant, depending on pressure units used, and 2.4. Detonation Reaction plosive to run into a detonation. Finally the n is the burning rate index, an experimen- The burning as well as the deflagration confinement of the material plays an im- tally determined parameter, which is char- reaction is based on heat transfer. This is in portant role. A heavy confinement prevents acteristic for a certain explosive. Typically contrast to the mechanism of a detonation, gases from a burning reaction from escap- n varies between 0.3 and 1.0 [11]. If the where the chemical reaction is triggered by ing. The pressure increases, this again ac- burning surface area and the density is a supersonic shock wave, traveling through celerates the reaction up to a deflagration known, this simple relationship allows the the explosive material. For a stable detona- and from there into a detonation [24]. The mass of explosive consumed per unit time tion, the velocity of this shock wave is giv- influence of confinement on the DDT be- to be calculated (Eqn. 4). en by a steady state, where the energy con- havior can be shown with trinitrotoluene Today thermo-chemical models are sumed by shock wave attenuation can be (TNT). If a pile of TNT grains is ignited available to calculate gas production rate, compensated by energy from chemical con- with a matchstick, it burns slowly, with a reaction temperatures and by integration version. In this case the shock wave veloci- sooty flame, until all TNT is consumed. If the pressure as function of time. As Eqn. 4 ty corresponds to the detonation velocity. one heats up TNT in a heavy confined tube, shows, the gas production rate and thereby The part of energy which is available to the reaction will end up in a detonation. the change of pressure as function of time maintain the shock wave, depends strongly This is a typical behavior for a main charge can be controlled by the burning surface on the reaction rate of the explosive. As lit- explosive. A different behavior is necessary area. erature shows, only the reaction energy, in ignition trains, where primary explosives The initial geometrical shape of a pro- which is released within 0.1 µs after the in unconfined conditions must show a DDT pellant grain or a solid rocket motor can be front of the shock wave has passed, con- characteristic. Therefore, primary explo- used to design the gas production rate in a tributes to the detonation velocity [24]. The sives are dangerous and should not be han- more progressive or more digressive way. reaction rate itself depends on the chemical dled without taking appropriate safety For example, a cylindrically shaped propel- homogeneity of the explosive. The reaction measures. lant grain shows a digressive burning char- of a pure organic explosive takes place in Another mechanism to initiate an ex- acteristic, because its surface area decreas- this time frame. The more heterogeneous a plosive charge is based on the impact of a es during the burning process. If the same high explosive composition is, the slower is shock wave. For this type of initiation, grain contains seven cylindrical perfora- the reaction rate and the more energy is lost known as shock-to-detonation-transition tions, the surface area and the gas produc- to maintain shock wave velocity. This effect (SDT), a critical energy can be defined tion rate will increase and the grain burns is obvious for some commercial or under- which describes the limit for initiation progressively [9]. water explosives, which can have detona- (Eqn. 5). This limit is a typical value for a The situation is completely different if tion velocities below 3000 ms–1 even at specific explosive at a certain density [4]. the convective heat transfer mechanism is very high reaction energies. The sound ve- Experimentally such critical energies can the main factor for heat transfer. Hot gases locity of a material depends on its density as be determined with so-called gap tests, and glowing particles distribute heat by does supersonic attenuation. So, the deto- where the shock wave pressure for 50% ini- convection. This type of reaction is much nation velocity also depends on the density tiation probability is determined by Bruce- harder to control, because it takes place be- of the corresponding explosive. Therefore, ton statistics [26]. In contrast to the DDT tween explosive particles or in porous ma- explosives with high detonation velocities mechanism, it is quite often observed that a terial, where the surface area is normally have not only to be chemically as homoge- shock wave impact on explosive material unknown and the convection is hard to de- neous as possible, but also their densities either results in a detonation or on the oth- scribe. If the energetic material is a fine and their reaction energies have to be opti- er hand in mechanical damage of a charge, powder, the surface area becomes very mized. The fastest detonation velocity without any visible traces of a chemical re- large and therefore the risk that the burning measured so far is 9280 ms–1 [25]. action. Apart from shock wave pressure, the reaction turns into a deflagration or detona- time of pressure interaction on the explo- tion is enhanced. sive is an important factor. These interac- 3. Initiation of Explosives tion times are very short (sub µs range) in 2.3. Deflagration Reaction the case of shock waves generated by deto- Deflagration is an intermediate reaction A detonation reaction can be initiated nations, but can be much longer in the case state between burning and detonation. As by two different energy transfer mecha- of large fragments. Further important pa- already mentioned convective burning of nisms: In the case of deflagration-to-deto- rameters for SDT initiation are the density energetic material with a large surface area nation-transition (DDT), a burning reaction and the sound speed of the explosive mate- can develop into deflagration. The same is is accelerated to a deflagration and by cor- rial, whereas these two parameters are not true for heavily confined explosive materi- responding interference of shock waves independent. Low density charges are much al. The linear velocity of the reaction further to a detonation. The main parame- easier to initiate by SDT compared to EXPLOSIVES 358 CHIMIA 2004, 58, No. 6

explosive welding

Fig. 1. Classification of explosives charges which approach the theoretically chamber, they react and ignite. Kerosene or primary explosives are sought. One exam- maximum density. Due to this fact, many asymmetric dimethylhydrazine are often ple of these new substances for use in lead booster charges have lower densities. used as fuel. Typical oxidizers are nitric free priming compositions is 1,3,5-triazido- acid or dinitrogen tetroxide [11][17]. 2,4,6-trinitrobenzene (TATNB) [27]. Un- High explosives (Fig. 1) can be grouped fortunately, over a timeframe of years, 4. Chemistry and Application into primary and secondary explosives. Pri- TATNB undergoes a decomposition reac- of Explosives mary explosives are most frequently based tion which results in a loss of initiating on heavy metal salts. They are used in low power. Another approach to obtain lead- Depending on their purpose, explosives quantities in detonators to convert a certain free primers was to substitute lead by an- are divided into three main types: high ex- input stimulus, such as fire, friction, impact other less toxic metal. Development work plosives, pyrotechnics, and propellants or electricity, into a detonation. This detona- with silver azide or silver fulminate was un- (Fig. 1). Whereas the output of high explo- tion is taken up by booster explosives, which dertaken. Both have a good initiation capa- sives is a detonation, propellants serve to amplify the detonation and transmit it fur- bility, but their sensitivity makes a com- accelerate either projectiles or missiles, and ther to the main charge. Main charge explo- mercial use impossible [10]. In spite of all pyrotechnics produce some sort of fire. This sives are responsible for the final effect. Tra- these efforts to substitute lead in primary production of fire is based on a redox reac- ditional booster explosives are based explosive compositions, a real break- tion of inorganic reducing agents and oxi- on trinitro-2,4,6-phenylmethylnitramine through has not been realized up to now. dizer compounds. (Tetryl), pressed TNT or PETN. More mod- In the field of main charge explosives, For propellants, the chemistry of the ern ones are based on RDX. Main charge ex- especially in view of military applications, compounds can be divided into three plosives can be very different and their com- many new energetic molecules have been groups. There are the nitrocellulose-based position depends strongly on their applica- synthesized [14]. The main objective in ones, which are most widely used for gun tion. High-performance metal accelerating high-performance applications is to put as applications. Single base, double base and applications are often based on cyclotetram- much energy as possible into a minimum semi-nitramine propellants belong to this ethylene tetranitramine (HMX) and a binder amount of space [28]. Based on computer group. In double-base propellants up to system. The amount of binder can be as low simulations [24][29], high energy and at the 40% of glycerine trinitrate is added. This as 5%. On the other hand, blasting explo- same time high density explosive target way the energy output of such charges can sives can be very heterogeneous, for exam- molecules have been defined. In organic be enhanced. In semi-nitramines the same ple ammonium nitrate (AN) is used as an chemistry high densities can be achieved if effect is established by adding a crystalline oxidizer for a liquid fuel oil. Between these the molecular structure contains fused ring nitramine compound, such as cyclo-1,3,5- two extremes nearly every combination can systems. Energy can be brought into the trimethylene-2,4,6-trinitramine (RDX), to be found [24]. system by strained ring systems and nitro or the nitrocellulose matrix. A further group of nitramine groups are responsible for an ap- propellants is based on a synthetic polymer 4.1. High Explosives propriate oxygen balance and nitrogen con- binder system. This polymer binder can be The most frequently used primary ex- tent. So, target molecules such as octani- either inert or energetic. If organic plosives are lead azide, lead trinitroresorci- trocubane or octaazacubane (Fig. 2) were nitramine compounds are added to such a nate, and tetrazolyl guanyltetrazene hydrate defined. Octaazacubane was predicted to polymer, they form the base composition of (tetracene). In earlier times also mercury have a density of 2.69 gcm–3 and a detona- nitramine propellants. When an inorganic fulminate was used. Lead azide has an ex- tion velocity of about 15 kms–1,far more salt is added as an oxidizer the formulation cellent capability to initiate booster explo- than the most powerful high explosives is called a composite propellant. Composite sives, already at quantities below 0.1 g. known today [12][13][30]. Even if octani- propellants are widely used for rocket Lead azide is very sensitive to friction, lead trocubane has been synthesized in the propulsion. Finally there are the liquid pro- trinitroresorcinate towards electrostatic dis- meantime, cubane chemistry is too compli- pellants, which are mainly used in space ex- charge and tetracene to impact [10]. Quite cated to produce quantities for use in corre- ploration and technology. Liquid propel- often, corresponding mixtures are used to sponding charges [31]. Another interesting lants are divided into mono- and bi-pro- design a primary explosive for a certain in- cage structure, which can be synthesized pellants. A typical mono-propellant is put stimulus and a sufficient output per- quite easy, is based on the isowurtzitan hydrazine, which burns in the absence of formance. Common to most primary explo- structure (Fig. 3). There are two molecules external oxygen. The fuel and oxidizer in sives is the presence of lead and the corre- representing this class of energetic high bi-propellant systems are stored in separate sponding environmental impact when they density substances: 2,4,6,8,10,12-(hexani- tanks. Upon injection into a combustion are brought to function. Therefore lead-free tro-hexaaza)-tetracyclododecane (HNIW) EXPLOSIVES 359 CHIMIA 2004, 58, No. 6

uniform distribution, the homogeneity of corresponding high explosive compositions should be enhanced and also the detonation velocity should increase. None of these effects could be observed so far. Because nano particles tend to form agglomerates, it is not clear at the moment whether a less favorable ratio of aluminum to aluminum oxide for small particles or insufficient dis- persion technology is responsible for the so far disappointing results. Fig. 2. High explosive Octanitrocubane Octaazacubane molecules based on the 4.2. Propellants structure of cubane The nitration of nitrocellulose is a well- known traditional process, which is not very exciting for most chemists [7]. The main development in the field of nitrocellulose based gun propellants can be identified in the application of new plasticizers, new surface-burning moderants and new crystalline nitramine compounds [17][40]. En- vironmental aspects have to be taken in- creasingly into account when new propellants are developed. Dinitrotoluene for example, which acts as coolant to reduce Fig. 3. High energy or high density compounds flame temperature, has to be substituted because its use was (or will be) forbidden in many countries. Lead- or tin-containing compounds used as burning rate modifiers have to be substituted also. The complete substitution of nitrocellulose was targeted when the first nitramine propellants were developed. The main problem with this sort of propellants is a completely different burning behavior and a lack of performance, especially if inert binder systems were used. Even if new energetic polymers and plasticizers are available today, for gun Fig. 4. Insensitive, low performance energetic molecules applications nitrocellulose will remain the first choice, because of its well-known favorable properties and its highly competi- [15] and 4,10-dinitro-2,6,8,12-tetraoxa- cules. The chemical base structures of these tive price. In the area of solid rocket pro- 4,10-diaza-tetracyclododecane (TEX) [32]. molecules are very different. Whereas the pellants, new energetic polymers and plas- Both of these molecules can be produced in most traditional insensitive high explosive, ticizers are available in large scale [16]. no more than three steps and in yields 1,3,5-triamino-2,4,6-trinitrobenzene Also energetic compounds with high gas which have allowed the synthesis to be (TATB) has an aromatic base structure production rate are entering in new deve- scaled up. A density of 2.04 gcm–3 was [7][34], there are newer ones based on het- lopment programs [41]. achieved for the most dense crystal modifi- erocyclic ring systems [14] or even on an cation of HNIW and in case of TEX we olefinic unit (Fig. 4) [35], e.g. 3-nitro-1,2,4- 4.2.1. Solid Gun Propellants measured a value of 1.99 gcm–3. The theo- triazole-5-one (NTO), which can be synthe- Single-base propellants consist of 90% retically calculated detonation velocities sized in two steps [36], is used in many new or more of nitrocellulose, which is gelled by are 9600 ms–1 for HNIW and 8665 ms–1 for high explosive applications, designed to be adding a plasticizer. Quite frequently TEX. Also quite promising for high per- less sensitive [37]. The same potential have dibutyl phthalate is used for this purpose. It formance applications seems to be 1,3,3- 2,6-diamino-3,5-dinitropyrazin-1-oxide acts at the same time as a surface moderant trinitro-azetidine (TNAZ) [33], which is (LLM-105) [38] or 1,1-diamino-2,2-dini- and as a coolant [11]. Surface moderants based on a strained heterocyclic ring sys- troethene (FOX-7) [35]. For most of them, and their diffusion characteristics are im- tem. Its synthesis could be simplified from the reduced sensitivity is attributed to their portant factors for the design of the burning initially up to 10 steps down to 5 steps. The capability to form hydrogen bridges [11]. behavior of propellants. Especially their density of TNAZ is around 1.84 gcm–3 and Less powerful high explosives are quite of- diffusion from the surface into the interior the detonation velocity approaches the ten used to generate blast effects or to pro- of the grain during the ageing process in- range of HMX. TNAZ has a low melting duce large quantities of gases. Therefore, fluences the ballistic stability significantly. point (101 °C) and can therefore be cast to additional components such as aluminum In double-base propellants glycerine charges without energy-diluting binder to enhance blast and ammonium nitrate or trinitrate acts as an additional source of en- components [10]. perchlorate as oxidizer for gas production, ergy as well as a plasticizer. Small mole- There is a second area of interest for are added [24]. First papers have been pub- cules such as dibutyl phthalate or camphor chemists over the last two decades: insensi- lished about the use of nano-scaled alu- cannot be used as surface moderants in such tive, less powerful high explosive mole- minum particles [39]. Due to their more propellants. Due to the plasticizing charac- EXPLOSIVES 360 CHIMIA 2004, 58, No. 6 teristic of glycerine trinitrate, they migrate much too fast into the propellant grain and the surface effect is lost. Therefore oligomeric and polymeric surface moderants have been developed [42]. Normally double-base propellants are chemically less stable than single-based ones. This reduced stability can be detected by a faster con- sumption of stabilizers and a more pro- nounced heat flow generated by corresponding ageing reactions [43]. The shelf life of a double-base propellant is Fig. 5. Modern ingredients for solid rocket propulsion therefore normally shorter than that of a single-base propellant. Due to the higher energy content also the flame temperatures observed. Therefore in some solid rocket methyl derivative of N-alkyl-N-(2-nitrox- of a double-base propellant are higher. propellant compositions an HCl-suppres- yethyl)nitramine (alkyl-NENA) [48] or the High flame temperatures are an impor- sant is added. The most commonly used in- mixture of bis-(2,2-dinitropropyl)acetale tant factor influencing erosion effects in gredients for this purpose are magnesium- and bis(2,2-dinitropropyl)formal (BDNPA- gun barrels [44] and should therefore be aluminum alloys [46] or sodium nitrate F) [49] are used as plasticizers (Fig. 5). The kept below a certain limit, which is for ex- [11]. AN on the other hand does not show main problem in the application of plasti- ample about 3000K for non chromium- the problem of HCl production. But the cizers is the migration stability in the binder plated medium caliber tubes [45]. Some- main drawback in the use of AN for rocket polymer. Therefore some chemical similar- times titanium dioxide or talc is used as a propulsion is its conversion into different ities are needed to avoid separation of poly- propellant ingredient to reduce barrel ero- crystal modifications already at low tem- mer and plasticizers. Sometimes oligomers sion. Often the surface of single- or double- peratures. Therefore AN has to be phase- of the same base type as the binder polymer base propellants is treated with graphite. stabilized with potassium nitrate. are added to obtain a plasticizer effect. This This treatment improves the flow character- A new and very powerful compound for approach was chosen for the energetic poly- istics of the grains and results in a higher solid rocket motors is ammonium mers poly-3-nitratomethyl-3-methylox- loading density. Furthermore graphite dinitramide (ADN). Originally developed ethane (poly-NIMMO) and glycidyl azide avoids electrostatic charging. Finally de- in the former Soviet Union, ADN has be- polymer (GAP) [50]. On the other hand coppering agents and muzzle flash in- come a key ingredient in research for rock- butyl-NENA shows also a good migration hibitors can belong to a classical single- or et propulsion all over the world. ADN has a stability in a poly-NIMMO matrix [51]. double-base propellant composition. Typi- positive oxygen balance (+25.8%), a high There are many other new energetic cal muzzle flash inhibitors are potassium nitrogen content (45.1%) and a high en- polymers under development. Some of nitrate or sulphate as well as sodium oxalate thalpy of formation. Its sensitivity towards them are very promising, especially be- [11]. In semi-nitramine propellants most friction or impact remains reasonable [10]. cause it seems to be possible to combine a frequently RDX is added to the nitrocellu- Also other nitramines, such as RDX, HMX high energy content with well-designed lose matrix. The main problems encoun- and HNIW are used in rocket motors for- mechanical properties. This design is based tered with these propellants are again very mulations. In contrast to their use in high on hard and soft block polymer elements, high flame temperatures with correspond- explosives, the content in rocket motors is which tend to form crystallite structures or ing barrel erosion problems. On the other in the range of about 60% (w/w) only. To random coils respectively. The number and hand, the so-called LOVA propellants, guarantee a uniform burning process, the sequence of these hard and soft block which are based on a nitramine and an inert ground material with a medium particle elements define the mechanical properties binder system, do not contain enough ener- size of a few microns, in combination with of the polymer [52]. gy to fulfill the performance requirements corresponding bonding agents and burn rate for most modern applications. Maybe new catalysts, are used [17][47]. In classical 4.3. Pyrotechnics energetic polymers and corresponding plas- composite rocket motors, inert hydroxy- Most pyrotechnic compositions are ticizers, which are also under development terminated poly-butadienes (HTPB) or based on reducing agents, also called fuels, for solid rocket propellant applications, will polyesters are used as binder prepolymers. oxidizer components, binders and further solve this problem in future. Using diisocyanates, these binders are ingredients to generate certain effects. The cured to polyurethanes after the motor has understanding of a pyrotechnic reaction can 4.2.2. Solid Rocket Propellants been cast. A typical plasticizer for these sometimes be very difficult, because in Solid rocket propellants are either of the types of polymers, such as di-(2-ethyl- solid-state reaction chemistry many factors, type of double base, elastomer modified hexyl) adipate (DOA) helps to design the fi- such as particle size, impurities and oxida- double base (EMDB) or composite. Com- nal mechanical properties, especially at low tion layers on the surface of fuels can influ- posite propellants consist of a binder sys- temperatures. ence the reaction behavior significantly. In tem that acts as a fuel, a crystalline oxidiz- Due to this high amount of inert materi- many cases when the manufacturer of a py- er and further additives, such as bonding al in classical composite propellants, inves- rotechnic compound was changed, also the agents or burn rate catalysts. The classical tigations have been started for the synthesis pyrotechnic composition had to be adjust- and most commonly used oxidizers are am- of energetic polymers and energetic plasti- ed. So many pyrotechnic compositions up monium perchlorate (AP) and ammonium cizers. Today a whole series of energetic to now are based on trial and error or, more nitrate (AN). During the burning process of plasticizers are available in large scale [16]. scientifically, have an empirical back- an AP-based rocket motor a lot of hy- To decrease the freezing point and finally ground. This is the main reason why only a drochloric acid is developed. Together with also the glass transition point of the poly- few common compositions exist, which are the moisture of the air it forms a white mer, where these plasticizers are applied used exactly the same way all over the smoke, so that the trajectory and also the also, quite frequently eutectic mixtures world. In military as well as in civic starting point of the missile can easily be are used. This is the case if the ethyl and pyrotechnics, the generation of effects has EXPLOSIVES 361 CHIMIA 2004, 58, No. 6 been published in a general way, but the In the future pyrotechnics will make an predicted in detail, a great potential for new details for a certain application have to be important step forward by applying the pos- explosives with new properties is expected adjusted from case to case. sibilities of nano technology. A series of to become reality. Pyrotechnic effects can be generation of metal powders in nano scale is available to- heat, hot particles or a flame, generation of day, but some have a highly pyrophoric Received: March 30, 2004 gas or smoke, generation of light in differ- characteristic. On the other hand also some ent colors, generation of noise, such as a oxidizers can be produced as nano powders. [1] ‘Manual of Data Requirements and Tests loud bang or a whistling sound. Further- By means of sol-gel chemistry fuels and ox- for the Qualification of Explosive Materi- more, very slowly reacting pyrotechnic idizers can be put together to mixtures with als for Military Use’, Allied Ordnance compositions are used to delay an initiation homogeneities that were not achievable be- Publication Nr. 7, second edition, North train, e.g. in a hand grenade. fore [19]. Atlantic Treaty Organization, Brussels, Generation of hot particles can be used for 2003. ignition of propellants. In many traditional ap- [2] ‘Recommendations on the Transport of plications black powder was used for this pur- 5. Conclusions Dangerous Goods, Manual of Tests and pose [11]. In rocket propulsion boron/potassi- Criteria’, third revised edition, United Na- um nitrate is used quite frequently. People working with explosives are said tions, New York and Geneva, 1999. Generation of heat and light is the ba- to be conservative with respect to the use of [3] a) Bundesgesetz über explosions- sic concept of flares, which serve for the new energetic compounds. Many old tradi- gefährliche Stoffe (Sprengstoffgesetz), protection of aircrafts towards hostile tional explosives, such as black powder, ni- 941.41, Bern, 1977; Verordnung über missiles attacks. Traditional flares are trocellulose or TNT are still in use today. explosionsgefährliche Stoffe (Spreng- based on Magnesium Teflon and Viton Nevertheless, new energetic material has stoffverordnung, SprstV), 941.411, Bern, (MTV) compositions. To overcome new been developed and introduced in modern 2000; Gesetz über explosionsgefährliche sensor technologies, modern flare compo- applications. Stoffe (Sprengstoffgesetz, SprengG), sitions try to imitate the emission spec- In the field of high explosives, high den- Berlin, BGBI I S. 3970, 2002; Ers- trum of a jet plume [53]. sity and high energy compounds based on te Verordnung zum Sprengstoffgesetz Many smoke-generating compositions fused ring systems or even cages are avail- (1. SpengV), Berlin, BGBI I S. 4013, 2002. not only act in the visible area of the elec- able. But a real breakthrough, where deto- [4] P.W. Cooper, ‘Explosives Engineering’, tromagnetic spectrum, but also in the in- nation velocities in a new order of magni- Wiley-VCH, New York, 1997. frared range [54]. Traditional smoke gener- tude become true, has not been achieved. As [5] G.F. Kinney, K.J. Graham, ‘Explosive ators are based either on red phosphorus long as traditional organic nitration chem- Shocks in Air’, Springer-Verlag, New and corresponding oxidizers or zinc, hexa- istry is the basis for the synthesis of new en- York, 1985. chloroethane, aluminum mixtures. ergetic molecules, this situation will not [6] S. Fordham, ‘High Explosives and Propel- Light-generating compositions are used change. Maybe there will be a potential in lants’, Pergamon Press, Oxford, 1980. in tracers, for illumination, but also for art nitrogen cage chemistry or in silicon nano [7] T. Urba´nski, ‘Chemistry and Technology fireworks. Many of them contain nitrates technology [56]. of Explosives’, Pergamon Press, Oxford, and perchlorates as oxidizers and alkaline Big steps forward were made in the field 1986. [8] P.R. Lee, in ‘Explosive Effects and Appli- earth metals, magnesium, aluminum, zirco- of intrinsically less sensitive, less powerful cations’, Ed. J.A. Zukas, W.P. Walters, nium and titanium as fuels [11]. To produce high energetic molecules. Some new sub- Springer-Verlag, New York, 1997, p. 23. a light of a certain color, they contain some stances have already been introduced into [9] A. Bailey, S.G. Murray, ‘Explosives, Pro- metal compounds that produce spectral applications or are on the way to it. This pellants & Pyrotechnics’, Brassey’s, Lon- emissions at characteristic frequencies. So new energetic material is one of the main factors in the improvement of safety in cor- don, 2000. red light is produced if a strontium salt is [10] R. Meyer, J. Köhler, A. Homburg, ‘Explo- added to a pyrotechnic system. Corre- responding applications. In the area of propellants, especially sives’, Wiley-VCH, Weinheim, 2002. sponding mixtures of perchlorates with a [11] J. Akhavan, ‘The Chemistry of Explosives’, sodium salt gives a yellow, with a barium for guns, nitrocellulose will remain the key ingredient, also in future. There are RSC Paperbacks, Cambridge, 1998. salt a green and with a copper (II) salt a blue [12] J.C. Oxley in ‘Explosive Effects and Ap- new energetic polymers in discussion, but light [55]. In these reactions the decompo- plications’, Ed. J.A. Zukas, W.P. Walters, at the moment it is doubtful whether they sition of perchlorate and the formation of Springer-Verlag, New York, 1997, p. 137. can ever be produced for a competitive excited metal chloride cations play the key [13] N.C. Paul in ‘Explosives in the Service of price. role in light emission. Man’, Ed. J.E. Dolan, S.S. Langer, The Loud bangs are produced by gas-generat- In the field of solid rocket propellants a Royal Society of Chemistry, Cambridge, ing pyrotechnic mixtures filled in a sealed development of new energetic binder sys- 1997. cardboard tube. After ignition a lot of gas is tems and new high performance fuels can [14] P.F. Pagoria, G.S. Lee, A.R. Mitchell, R.D. produced that eventually bursts the tube re- be observed. Especially ADN, HNIW and Schmidt, Thermochim. Acta 2002, 384, sulting in a big bang. Quite frequently black GAP have to be mentioned in this context. 187. powder is used for this purpose [11]. The application of these new ingredients [15] G.P. Sollott, J. Alster, E.E. Gilbert, O. San- Other gas generators are needed for air will enhance the performance and mini- dus, N. Slagg, J. Energ. Mater. 1986, 4,5; bags in car safety systems. In this case sodi- mize the signature of solid rocket propul- J. Boileau, J.-M. L. Emeury, J.-P. Kehren, um azide, which develops nitrogen in large sion significantly. ‘Tetranitroglycoluril and method of prepa- quantities when heated up with a correspon- In pyrotechnics the development of ration thereof’, United States Patent ding ignition charge, is used. spectral flares to mislead modern missile 4,487,938, 1984; A.T. Nielsen, ‘Caged Delay mixtures on the other hand contain sensors, IR tight smokes or compositions Polynitramin Compound’, United States frequently zirconium or nickel as a fuel and with illumination capability in the infra red Patent 5,693,794, 1997. lead oxide, barium chromate or potassium range is of high interest. [16] A. Provatas, ‘Energetic Polymers and perchlorate as oxidizers. In case of chromates The future will show the impact of nano Plasticisers for Explosive Formulations – no or only low quantities of gases are pro- technology on pyrotechnic applications. A review of recent advances’, DSTO duced during the reaction. Today, even if the consequences cannot be Technical Report Nr. TR-0966, 2000. EXPLOSIVES 362 CHIMIA 2004, 58, No. 6

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