Explosives and Propellants

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Introduction

• Propellants and explosives are chemical compounds or theirmixture that rapidly produce large volume of hot gases whenproperly initiated.

OR

• An explosive is chemical compound, either solid, liquid orgas in an unstable state. When suitably initiated by shock,friction or heat, they undergo a rapid chemical change fromtheir original compound into one or more stablecompounds, usually gases.

• In the process of changing from an unstable to stablecompound, they give out energy in the form of heat, light orsound, which as a final result is violent explosion of gases.

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•Explosion is a process of internal burning.

• Instead of the O2 being supplied in the form thesurrounding air, it is supplied in the form of oxidizers oroxidants and is actually a solid material that is inintimate contact with fuel.

•An activation energy of 125-250kJ/mol is usuallyrequired to initiate the reaction, at which point the heatevolved is sufficient to cause the reaction to continueand become self-sustaining.

•Propellants burns at relatively low rates measured incentimetres per second, whereas explosives detonatesat rates of kilometres per second.

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Uses

• Commercial explosives are essential to industries such asmining, construction and geophysical exploration. The majorengineering jobs such as construction of tunnels and damswould be impossibly expensive without explosives.

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Types of Explosives

• Explosives differ widely in their sensitivity and power.

• Only those of comparatively insensitive nature, capable ofbeing controlled and having a high energy content are ofimportance industrially or militarily.

• There are three fundamental types of explosives

• Mechanical

• Atomic

• Chemical

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Mechanical Explosive:

A mechanical explosive is one that depends on a physicalaction, such as overloading a container with compressed air.

Atomic Explosives:

Atomic explosives or nuclear explosive is one in which asustained nuclear reaction can be made to take place withalmost instant rapidity, releasing large amounts energy.

Chemical Explosives:

• A chemical explosive is one that depends on a chemicalreaction.

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Chemical Explosives

•Chemical explosives are divided into two categoriesin accordance with their behaviour

1)Detonating or high explosives

a) Primary or initiating explosives (detonators)

b) booster and secondary explosives

2)Deflagrating or low explosives

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Difference between Detonating & Deflagrating

• High explosives detonate at very high rates, from 2 to 9 x 103

m/s and the reaction front is a shock phenomenon movingthroughout the material

• Low explosives burn in layers to the surface at low rates around10-2 m/s .

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Initiating or Primary Explosives• Primary explosives are materials that are quite shock and

heat sensitive and that can be made to explode by theapplication of the shock, flame, friction or heat source.

• They are very dangerous and are used in comparativelysmall quantities to start the explosion of larger quantities ofless sensitive explosive.

• They are usually inorganic salts while the otherconventional are organic.

• Some primary explosives are mercury fulminate[Hg(ONC)2] ,lead azide Pb(N3)2, basic lead styphnate ( tri nitroresorcinate), diazodinitrophenol, and tetrazine ( a complexconjugated nitrogen compound).

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•Most priming compositions consist of mixture ofprimary explosive, fuel, and oxidant.

•An example is

15% antimony sulfide, 20 % leadazide, 40% basic lead styphnate, 20% barium nitrateand 5 % tetrazine.

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Booster High Explosives

• Booster high explosive are materials that are insensitive tomechanical shock, and flame.

• But they explode with great violence when set off by anexplosive shock, such as obtained by detonating a smallamount of a primary explosive in contact with it.

• Thus they are energy amplifiers.

• Decomposition occurs, which is rapid rupturing of bondsprogressing directly through the mass of explosive.

• Detonating is thought to be a chain reaction and proceed atrates around 6000m/s.

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• High rate of energy release makes the product explosiverather than the total amount of energy release.

• As Nitro-glycerine has only one-eighth the energy of anequivalent mass of gasoline.

• Important booster are RDX ( cyclo-trimethylene-tri-nitramine), PENT ( pentarythritol-tetranitrate) , tetryl( 2,4,6,trinitrophenyl methyl nitramine) , HMX ( cyclotetramethylene tetranitramine)

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Blasting Agents:

blasting agents are power full explosive agents.

They are less sensitive (safe to handle) but more energetic and also used to detonate secondary explosive.

It is a mixture of fuel and oxidizer

An example is a mixture of NH4NO3 and carbonaceous combustibles as fuel oil or coal.

They can not be detonated by mean of blasting cap when unconfined.

Blasting Cap

A blasting cap is a small sensitive primary explosive device generally used to detonate a larger, more powerful and less sensitive secondary explosive such as TNT, dynamite.

Blasting caps come in a variety of types as electric caps and fuse caps.

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• The need for blasting caps arises from sensitivity issues of an explosivecompound. All explosive compounds require a certain amount of energyto detonate. If an explosive is too sensitive, it may go off unexpectedly,so most commercial explosives are formulated to be stable and safe tohandle and will not explode if accidentally dropped, mishandled, orexposed to fire. However, such explosives (called secondary explosives)are hard to detonate intentionally as well. A blasting cap contains aneasy-to-ignite explosive that provides the initial activation energy tostart a detonation in a more stable explosive.

• The blasting cap is stored separately

and not inserted into the main

explosive charge until just before use,

keeping the main charge safe

Slurry explosive

• slurry explosive are ammonium nitrate mixtures which frequentlycontain another oxidizer as well as fuel dispersed in a fluid medium.

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Explosives characteristics

Evaluation of the explosive characteristic of a material is mainly based on the assessment of following factors.

1. Test for sensitivity to impact and heat.

2. Stability on storage

3. Shattering ability (measure of the rapidity with which an explosive develops its maximum pressure)

4. Explosive strength per unit weight

5. Volatility

6. Solubility, density, hygroscopicity

7. Compatibility with other explosives

8. Additives and other possible metal used for inclosure

The above mentioned test must be run in standard manner.

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• Explosives use in mines and coal mine be of such a type that on explosion they evolve less amount of toxic gases and produce a minimum of flame. This requirement assures that the explosive is incapable of igniting mixture of air and coal dust or methane.

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Propellant for Rocket and MissilesPropellant:

• Propellants are mixtures of chemical compound that produce large volume of gas at controlled, predestined rates.

• Their chief application are in launching projectiles from guns, rockets, and for missiles system.

Propellant for Guns:

• Black powder is the oldest propellant known. but its today use is trivial.

• It burns far too fast for propellants use, if the ingredients are left in finely divided form. Hence large grains are used for better results.

• Black powder remains the best igniter available.

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Smokeless powder:

• Smokeless powder(nitrocellulose based) replaced black powder for several reasons.

• Its smoke less

• Its superior power and better storage characteristics.

• Principally because it can be formed into stable grains whose size and shape governs the burning characteristics.

• Smokeless powder is a colloidal cellulose nitrate, usually contains a plasticizer(such as nitro-glycerine or nitroglycol).

• Plasticizers increase the fluidity or plasticity.

• Some times non-explosive plasticizers such as dibutyl phthalate is added and usually about 1% of diphenylamine to improve its storage life.

• When only nitrocellulose of over 13% nitrogen content ,it is known as single base powder.

• When two or more explosive ingridients are used it is called double grain powder.

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MatchesThe manufacture of matches is an essential industry.

• They fall into two categories

1) Strike-anywhere matches 2) Safety matches

Strike-anywhere matches

The match head composition consist of Phosphorous sesquisulfide (P4S3) , an oxidizing agent such as Potassium or barium chlorate, ground glass and glue.

They can be ignited by striking on any suitable surface.

Safety matches:

Safety matches are ignited by the generation of heat on the striking surface of the box.

The striking surface on modern matchboxes is typically composed of 25% powdered glass or other abrasive material, 50% red phosphorus, 5% neutralizer(ZnO), 4% carbon black and 16% binder and the match head is typically composed of 45–55% potassium chlorate, with a little sulfur and starch, 20–40% of siliceous filler, diatomite and glue.

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