Explosives

Explosives

An explosive is a substance or a mixture, which when subjected to thermal or mechanical shock, gets very rapidly oxidized exothermically into products of greatly increased volume with sudden release of potential energy.

  In explosive reactions the products get heated up to very high temperature and exert very high temperature on surroundings which can be used for constructive or destructive purposes. When an explosion occurs in a confined space, the high pressure conditions developed within the system can be instrumental to shatter the confining walls.

The quantity of power realizable from given weight(or volume) of an explosive is called “power to weight(or volume) ratio”-which is quite small in the case of gases but large in solids or liquids when used as explosives. Explosives are mainly used for defense purposes, for blasting mines for obtaining ores, drilling holes in mountains for constructing tunnels, quarrying purposes, excavating earth(for dams), seismic prospecting etc.

Classification of Explosive

Explosives may be in the form of solids TNT(trinitrotoluene), liquids or gases (H2 and O2). They may exist as a single component (TNT), a mixture of two or more substances none of which is an explosives by itself (gun powder), a mixture of one or more explosives compounds (TNT + NH4NO3) or an explosives compound mixed with a non- explosive material (dynamite).

A variety of inorganic or organic compounds can serve as explosives. They are generally in the form of nitrates, chlorates, perchlorates, peroxides or compounds containing nitro (-NO), nitroso (-NO), azide (-N3) or fulminate (-ONC) groups.

 All known explosives can be categorized under three heads: Atomic, mechanical and chemical explosives. Besides this Explosives are broadly classified into three broad groups.

Primary Explosives or Detonators These are highly sensitive explosives which can explode under slightest shock or blow, by ignition, and have to be very carefully handled. They are used in comparative small quantities in blasting caps and cartridges.

Lead Azide (PbN6) – It is low cost, excellent initiating action and stable in storage. It reacts with brass and caps loaded with it are made of aluminum.

Mercury Fulminate (Hg(CNO)2) – More sensitive as well as more expensive lead azide but it is slightly toxic.

Tetracene (C2H7N7O) – it is low initiating primary explosive. It is mainly used as detonator.

Diazodinitro phenol (DDNP) – it is quite sensitive and has high brisance value. It is used in blasting caps.

Low Explosives or Propellants

They do not explode suddenly but only burn and, their rate of combustion rarely exceed 0.25 ms-1. the chemical reaction taking place in such explosives are fairly slow.

Colloidal cellulose nitrate(prepared by treating cellulose with nitric acid and sulphuric acid) and gun powder (mixture of 75% potassium nitrate, 15% charcoal, 10% sulphur) are examples of this category.

 Low explosives are used as propellants(to propel missiles) and in pyro-techniques (fireworks). The first category is mainly used for military purposes. Eg. RDX (cyclotrimethylene trinitroamine), TNT, Nitrocellulose, Picric acid (Trinitrophenol), PETN (Pentaerythritol tetranitrate), DDNP, Leadazide etc. whereas industrial explosives are GTN and Dynamite etc.  

High or Secondary Explosives

This type is quite insensitive to mechanical shock as well as to flame i.e. they do not explode on ignition. They explode with great violence when initiated with an aid of detonators. They possess higher energy contents then primary explosives and are stable. Secondary high explosives are mainly Nitro compounds, Nitrate and Nitramines used as such or with an initiators or as mixtures. Trinitrotoluene(TNT), Ammonium nitrate(AN), RDX, Picric acid, Dinitrotoluene, Pentaerythritotetranitrate(PETN), ethylene dinitramine etc. come under this category of explosives which are generally used in the main body of projectiles.

  Primary and secondary high explosives are generally referred to as high explosives.

High explosives are sub-divided into 1. Military high explosives, 2. Blasting or industrial purpose explosives.

Examples:

GTN: Glyceryl trinitrate

RDX: Cyclo trimethylene trinitroamine

PETN: Pentaerythetol tetranitrate

TNT: Trinitrotoluene

Ammonium nitrate: It is half as powerful as TNT (2,4,6 trinitrotoluene) and employed in

making binary explosives. It is dangerous to store near any inflammable material. It cannot be used in contact with brass, since it produces a detonator – tetramino cupric nitrate.

TNT:It is used in shell-firing and under-water explosives. It can be loaded in

containers because of its low melting point (81°C). Because of its (1) non-hygroscopic nature and (2) inertness to metals, TNT is used as safe explosive in the manufacture, storage and transportation.

RDX or cyclonite (cyclotrimethylene trinitroamine): It is a powerful high explosive. It is more sensitive and less toxic than TNT.

It is used both in military and industrial purpose explosive. Picric acid (or trinitrophenol): It is replaced largely by TNT since it forms shock-sensitive picrates with

metals.Explosives, based on their state of aggregation are classified as solid (e.g.,TNT), liquid (e.g., nitroglycerine) and gaseous (e.g., mixture of oxygen and acetylene).

Precautions During Storage of ExplosivesFollowing precautions should be taken while storing the explosives 1. All wiring and electrical fittings should be properly insulated and

checked regularly. 2. Jerks or drops of explosive should not be allowed to take place. 3. Detonators and explosives should be stored separately. 4. Different explosives should be stored in separate boxes. 5. Fire or smoking should be strictly prohibited within the radius of

50 m from the explosive store (magazine). 6. Only authorized persons with magazine shoes should be allowed

to the explosive store. 7. The explosive store should not be within 500m from any working

kiln or furnace. 8. Lighting conductors should be provided to safeguard the

magazine.

Blasting Fuses

Safety FuseSafety Fuse consists of a small diameter core of black powder enclosed in water-proof fabrics. It has an approximate burning speed of 30-40 seconds per foot. While blasting, a sufficient length of the fuse is used so as to allow sufficient time for the shot firer to reach a point of safety (analogous to the wick present in fire crackers).

Detonating FuseDetonating Fuse consists of a charge of high velocity explosives (6000 meters per second) e.g. TNT contained in a small-diameter bent tube. The line of fuse is in contact with the charge throughout its length and this is responsible for instantaneous detonation. These are mainly used for exploding charges of explosives in deep-holes.

Application of Explosives

Objects at a smaller distance can be shattered with high brisance explosives Eg. shells, bombs and mines etc. Low brisance explosives can be used as propellants.

Explosives are burst a few feet below the land so that subsoil is brought up to the surface and gases like SO2, CO2, etc are liberated, which can kill insects. Nitrate based explosives incorporates certain fertilizers into the soil.

Explosive are used to shape the metal to required shapes and this process is of recent origin.

Explosives used for dislodging the coal seam in a coal mines must have long-lasting flame. The addition of NaCl and AN to GTN not only helps to lower the flame temperature but also the duration of the flame.

Rocket Propellants• A rocket engine is a tube-like missile which carries

both the fuel and the oxidant, collectively referred to as propellant. The propellant is burnt in a combustion chambers and a hot jet of gases at a pressure of 300 kg cm–1 and a temperature of 3000oC comes out through a small nozzle at a supersonic velocity. This act of pushing of the gases downwards produces an equal and opposite reaction (Newton’s third law of motion) causing the rocket to move upward .

Classification of PropellantsThe chemical propellants are classified into solid and liquid categories.

Solid PropellantsDepending on their physical structure, solid propellants are divided into two main groups: homogeneous and composite (or heterogeneous).

A solid propellant mixture in which the propellant mixture is intimately mixed in a colloidal state is known as homogeneous solid propellant.

Examples: Nitroglycerine and nitro cellulose (gun cotton or smokeless powder combination). It is an example of single-base propellant. When a single propellant is employed, it is called a single-base propellant. In a double-based propellant, 50–55% nitrocellulose and 40–45% nitroglycerine are present. In order to make it a homogeneous plastic mass, diethyl phthalate is added as a solvent cum plasticizer. These propellants give a flame temperature of 800–1650oC and the volume of gases is about 1500 times the original volume of charge

Liquid PropellantThey have several advantages over solid propellants. They are more versatile and possess high specific impulse. But the engine is more delicate and is less suitable for rough handling than the engine for solid propellant. Liquid propellants may be mono-propellants or bi-propellants. Mono-propellants: They require single storage tank and one fuel injection and control system. The fuel as well as oxidizer are in the same solution. For example hydrogen peroxide, nitro methane, hydrazine, ethylene oxide, 21.4% methanol and 78.6% hydrogen peroxide is highly reactive, metal oxides catalyze the decomposition, hence storage tanks must be made of special materials.

Bi-propellants: Bi-propellants are widely used. Liquid oxygen (LOX), hydrogen peroxide, ozone, fuming nitric acid, liquid fluorine are the common oxidizers used in bi-propellants. In bi-propellants, liquid fuel and oxidizers are kept separately and injected separately into the combustion chamber. Ethyl alcohol and 25% water is a good fuel. Addition of water reduces flame temperature and molecular mass of combustion gases, which compensates for reduction in performance. Liquid oxygen is safe, non-toxic and good oxidant, but it has to be stored under pressure in insulated containers. Ozone though a powerful oxidant but explodes at high concentration. Liquid fluorine is toxic, corrosive, volatile, very reactive, but a good oxidant. It is difficult to store and handle.