CHAPTER 1 INTRODUCTION TO HIGH ENERGY MATERIALS. Among exothermic reactions, some are extensively used in dayto-day

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1 CHAPTER 1 INTRODUCTION TO HIGH ENERGY MATERIALS Among exothermic reactions, some are extensively used in dayto-day life and in the scientific field. Mainy reactions involve release of large ajnounts of heat energy, usually accompanied with smoke ajid flajne, besides formation of gaseous products. For example, coal, wax, and other ordinary fuels which burn in air produce high heat energy and gases but in a relatively slow process. On the other hand, some materials are capable of burning even in the absence of air ajid these release comparatively less energy than the 'normal' fuels, but the energy release occurs in a very short span of time. This property of rapid release of energy and gas has brought about a wide range of specialised applications for these so-called "high energy" materials (HEM), which are conventionally called explosives and propellants. DEFINITION AND CRITERIA OF A HIGH ENERGY MATERIAL A "high energy" material is capable of releasing a fairly lange amount of energy (mostly as heat) in a self-sustained chemical reaction at a relatively rapid rate, sometimes in a fraction of a second, and sometimes in a dramatic and violent manner. High energy materials often (but not necessarily always) contain their own oxygen supply, which may or may not be sufficient for their complete combustion. It is quite likely that for complete combustion and for maximum energy release from its molecules, a high energy material requires an additional oxidizer or oxygen-

2 containing compound. Barring a few exceptions, therefore, a high energy material is a substance or mixture containing combustible (fuel) elements and oxidizing elements in a metastable system, capable of reacting under relatively minor provocation from a variety of stimuli, producing heat and gaseous products. The corollory of this is that, although high energy materials (which usually contain their own oxygen as well as fuel) release considerably less ajnount of energy than an equal weight of a straight-forward fuel, the former find extensive uses because of the high speed at which the energy is released and gaseous products are formed. Thus, the rate of energy-release, and not the total amount of energy-release from a given mass, in a chemical reaction, distinguishes a "high energy" material from other materials. High energy materials, when properly ignited or initiated, may release their energy through a relatively slow process (called burning or deflagration) or a much faster process (called detonation), depending upon their inherent chemical structure, input energy level, confinement, and queintity. Those explosives which normally detonate are popularly called "High Explosives", whereas those explosives which normally deflagrate are known as "Propellants". While the nature of materials and the mechanism of chemical reactions involved in detonation and deflagration processes may not be fundamentally different, the basic difference lies in the mode and rate of energy release and associated phenomena. For example, a detonation involves the formation and propagation of a detonation wave, which is an intense shock wave supported by chemical reaction energy. This shock wave causes compression heating effects which bring about a chemical reaction during its passage through the explosive medium at a very high pressure (about 10 bars) and a very high velocity (a few km/sec). In contrast, the

3 flame front in a deflagrating explosive moves at a low linear speed of a few mm/sec, which caxi accelerate to higher speeds with increase in ajnbient pressure (e.g. due to increase in degree of confinement). Under favourable circumstances, like large quantity, strong confinement, or powerful initiation, the deflagration of a propellajit may undergo trajisition to detonation (but the reverse transition is not known to occur). In fact, a deflagration phase, howsoever short, precedes the onset of detonation in a high explosive. Thus, aji explosive can be considered as a material capable of undergoing a self-sustained chemical reaction with rapid evolution of heat and large volumes of gases so as to produce pressure effects. It can be a solid, a liquid, or a gas. It can be a single compound capable of explosive reaction, or a mixture of substances which individually may or may not be explosive. Most of these compounds, which are capable of explosion all by themselves contain nitrogen and oxygen in their molecules-often in the carbon skeleton. The function of the nitrogen atoms is to "carry" the oxygen atoms and enable their reaction with the carbon and hydrogen atoms at the appropriate stress level. The disintegration of this molecular microcosm may be initiated by detonation, shock or rapid heating. The unstable oxygen-nitrogen linkages undergo sudden disruption and a chain of reactions ensues and accelerates to deflagration or detonation. CLASSIFICATION OF HIGH ENERGY MATERIALS HEMs or explosives can be conveniently classified on the basis of their (1) Chemical structure, and (2) Function.

4 Depending upon chemical structure, explosives are classified into the following main groups : (a) Nitro explosives These compounds contain nitro (-N0_) groups in the cairbon skeleton, i.e. C-NO_. Examples are trinitro benzene, trinitro toluene, picric acid, etc. (b) Nitrate esters These compounds contain nitric ester (-ONO^) groups in the carbon skeleton, i.e C-ONOp. Exajnples are nitroglycerine (NG), diethylene glycol dinitrate (DEGDN), pentaerythritol tetranitate (PETN), nitrocellulose (NO, polyvinyl nitrate (PVN), etc. (c) Nitramines These compounds contain nitramine (N-NO ) groups in the carbon skeleton and may be considered the organic derivatives of the simplest inorganic nitramine (NHpNO ). Exajnples are cyclonite (RDX), octogen (HMX), tetryl, etc. Some salts of nitric, chloric ajnd perchloric acids are not explosive themselves but are used as oxygen carriers in several mixtures so that these mixtures may act as either propellants or pyrotechnics. Examples are ammonium nitrate, sodium nitrate, potassium nitrate, ammonium perchlorate, potassium chlorate and perchlorate, etc. Depending upon impact sensitivity and functions, explosives are classified as follows :

5 I High explosives (or Detonating explosives) These explosives on undergoing an impact release their energy through the detonation process, i.e.at an extremely rapid rate creating very high pressure for a very short time. These are further divided into two types: (i) Primary high explosives (also called Initiating explosives) These are more sensitive but less powerful. The main examples are murcury fulminate, lead azide, lead styphnate, diazodinitrophenol, etc. (ii) Secondary high explosives These are less sensitive than Primary explosives and, therefore, not readily initiated. However, they detonate powerfully under the influence of the shock explosion of a suitable primary explosive. The main examples are TNT, tetryl, hexogen (RDX) and octogen (HMX). // Propellants (or Low explosives) Normally, on impact they are less prone to detonation/explosion. On being ignited, these burn and do not explode, or detonate. These function by producing gas at moderate 2 pressure (upto 3000 kg/cm ). Important examples are NC powder, cordites, AP/polybutadiene, liquid oxygen/liquid hydrogen, etc. Further classification of propellants is described later. /// Pyrotechnics These, on being ignited, produce light, smoke or fire and incendiary effects. These are usually mixtures of finely divided combustibles (including binders) and oxidants. Examples are gun

6 powder, Mg powder/varnish/nano^, Zr/KClO^, Mg/Ba(C102)2/L.}nseed oil/ lactose, etc. Types of Propellants Depending upon whether a propellant is used for the propulsion of a gun projectile or a rocket vehicle, propellants can be divided into following two groups : (I) Gun Propellants : Gun propellajits are compositions based on one or more energetic sustajices along with various additives which perform specific roles. Gun propellants, which are used in a cartridge of an ammunition round and ignited by a primer cap/igniter assembly, tend to operate at internal (gun chamber) pressures upto 450 Mpa with a very short burn time (of the order of milliseconds). On the basis of chemical composition, gun propellants may be sub-divided into the following three types : (i) Single base propellants Here, nitrocellulose (NC) is the only energetic ("explosive") ingredient. In addition, some quantity of gelatinizer/ plasticizer ajid stabilizer are incorporated in the formulation. (ii) Double base propellants This contains nitrocellulose (NC) and nitroglycerine (NG) as the major and energetic ingredients. In addition, small quantities of plasticizer, stabilizer, etc. are present. These are more energetic than single base propellants.

7 (iii) Triple base propellants These contain nitroguanidine (major constituent), nitrocellulose and nitroglycerine, plus small amounts of additives. In addition to the above, certain nitramine compounds, like RDX and HMX, are also used as energetic ingredients in some modern gun propellants, especially for tank gun ammunition. In recent years, liquid propellants have also been investigated for use in gun propulsion. The most promising liquid gun propellant is an aqueous mono-propelleint consisting of hydroxyl ammonium nitrate (HAN), alkyl ajnine nitrate (AAN) and water. (II) Rocket Propellants Rocket propulsion systems are devices in which a propellant system is ignited aoid burnt in the combustion chamber producing hot gases (the working fluids) which are expanded and accelerated through a convergent-divergent shaped nozzle to produce a reaction force or thrust in the opposite direction. Thus, a rocket engine operates on the basis of application of Newton's third law of motion. The propellants may be in different physical states prior to combustion, for example, solid or liquid or a combination of solid and liquid (called hybrid). The operating pressure inside the combustion chamber may range from about 2 to 20 MPa, while the burn time may vary from about one second to several minutes, depending upon the application. Rocket propellants may be conveniently divided into the following classes : (A) Solid propellants These are pre-mixed compositions which may be further sub-divided into the following types :

8 (i) Double base or homogeneous propellants These are formulated from a mixture of nitrocellulose and nitroglycerine along with some additives. (ii) Composite or heterogeneous propellants These contain aji oxidiser dispersed uniformly in a binder matrix, along with some additives. The oxidiser is usually a finely divided inorganic oxy-salt, for example, ammonium perchlorate (most widely used), ajnmonium nitrate, sodium nitrate etc. The binder, in most of the current propellants, is an organic polymer, as for example, Polybutadiene (Hydroxy-terminated polybutadiene, and Carboxy-terminated polybutadiene), Polyurethajves, Polysulphides, etc. Occasionally, a metal fuel, like finely divided aluminium powder, is added to increase the performance. (iii) Composite modified double base propellants These are usually made from fine ammonium perchlorate and aluminium particles dispersed in a NC/NG matrix. (B) Liquid propellants following types : Liquid propellants can be sub-divided into the (i) Monopropellants A monopropellant combines the properties of an oxidizer and a fuel in a single substance. These are either neat compounds, as for example, hydrazine, ethylene oxide, isopropyl nitrate, or homogeneous solutions, for example, nitromethane-methanol (myrol). These find limited, mostly auxiliary, applications in rockets.

9 (ii) Blpropellants A bipropellant system consists of an oxidiser and a fuel as two separate entities, which are stored in separate tanks 8Lnd injected into the combustion chamber at the time of launching the rocket. Examples of oxidiser are Red fuming nitric acid (RFNA), nitrogen tetroxide, hydrogen peroxide, chlorine trifluoride, bromine pentatluoride (all of which are storable), and liquid oxygen, liquid fluorine (both of which are cryogenic). There are very l8u:'ge number of fuels which have been (or can be) used in bipropelleoit systems. Among the better known examples are hydrazine, unsymmetricaldimethylhydrazine (UDMH), orthotoluidine, triethyl amine, diethylene triamine, a mixture of triethyl amine and xylidines, RP kerosine (all of which are storable) and liquid hydrogen (which is cryogenic). Several fuel-oxidizer pairs are self-igniting or 'hypergolic' eg. UDMH/RFNA, hydrazine/h O^. For space launch applications, cryogenic or nonstorable propellant combinations are usually preferred (due to higher performance), whereas for military applications, the storable oxidizer-fuel combinations are usually preferred (because of their easy handling). Hypergolic storables have led to the development of "pre-packaged" liquid rocket engines. (C) Hybrid Propel 1ants This class of propellants involves a combination of a solid fuel and a liquid oxidiser (called 'standard hybrid') or a liquid fuel and solid oxidiser (called 'reverse hybrid'). An example of a standard hybrid propellant is aniline formaldehyde/rfna which is capable of spontaneous or hypergolic ignition.

10 Polyvinyl nitrate as a High energy material Polyvinyl nitrate (PVN)- the subject of the present investigation - is a nitrated polymer capable of giving rapid decomposition with evolution of considerable heat and gaseous products. Therefore, polyvinyl nitrate generally meets the criteria for a high energy material. As per the available published literature (which will be serveyed in the next chapter), PVN has not yet been used on a significant scale in ajiy gun or rocket propellant compxjsition. It was, therefore, considered desirable to investigate the properties of PVN which would be relevant to its applications as a high energy material. The results of these investigations would be given in subsequent chapters.