CHAPTER 1

INTRODUCTION

1

INTRODUCTION

1.1 General

Gun is a heat engine in which chemical energy of the propellant is

transformed into kinetic energy of the projectile. It plays predominant role in

warfare due to its effectiveness and versatile utility. Generally, solid gun

propellants are employed to propel the projectile from the gun barrel. The

propellants are required to burn rapidly in order to generate high pressure in

the gun, which propels the projectile from the gun barrel with desired muzzle

velocity. The scientists across the globe are striving hard to develop efficient

and powerful gun propulsion systems. The urge to achieve high muzzle

velocities much greater than 2 km/s has been the driving force of the gun

researchers since decades.1 High muzzle velocity directly results in decrease

in time of flight, increase in range, better target penetration, increase in hit

probability and magnification in terminal ballistic effect on the target.

The performance of gun propellants is measured in terms of force

constant, which is the thermal energy generated due to combustion of

propellant ingredients. Therefore, the propellant ingredients, to a large extent,

are responsible for the superior performance of gun propulsion. As the `force

constant’ is inversely proportional to the mean molecular weight of the

combustion products and directly proportional to the flame temperature, effort

of the researchers is to develop ingredients, which produce low molecular

weight gases like NH3, CO, CH4, N2, etc. The ingredients are so developed

that they impart high level of energy within the acceptable temperature level in

view of the barrel erosion.2

2

The conventional solid gun propellants consist of homogenous mixture

of one or more energetic materials with various additives. There are large

numbers of propellant compositions using nitrocellulose (NC) as basic

ingredient. On the basis of explosive ingredients, propellant is classified into

single base (NC as main ingredient), double base (NC and nitroglycerine-NG

as main ingredients) and triple base (NC, NG and nitroguanidine -NQ as main

ingredients). The propellant formulation based on NC and NG is known as

colloidal or homogenous propellant.3 In order to enhance the performance of

the gun propulsion, researchers incorporate other ingredients in the double

base matrix, which enhances the energy level of the formulation and/or

mechanical properties of the propellant grains. These ingredients may be

nitramines such as research and development explosives (RDX), high melting

explosives (HMX), etc. The high boiling liquid esters like dibutylphthalate

(DBP), dioctylphthalate (DOA), diethyl phthalate (DEP) and triacetin are

typically used as plasticizers in gun propellants.

Plasticizers play an important role in improving the flow characteristics

of propellant dough during processing and building up mechanical strength of

propellant grains after completion of curing cycle. The non-energetic

plasticizers modify the mechanical strength and fluid flow characteristics, but

do not contribute significantly to the energy of the system, whereas energetic

plasticizers improve upon their attribute as well as contribute to the energy of

the system. The azido linkage attached to the energetic plasticizers

contributes 85 kcal/mol of thermal energy to the system.4 A typical azido

based plasticizer, i.e., 1-azido 2,3-dihydroxy 2-azidomethylpropane (ADMP)

possesses heat of formation (∆Hf )+440 kcal/kg whereas the non energetic

3

plasticizer like DBP has negative heat of formation, i.e., (-) 723 kcal/kg.5

Therefore, the energetic plasticizers are the preferred choice over

conventional ones for the propellant formulations both for guns and rockets.

Energetic effect of plasticizer is basically due to presence of nitro (-NO2) &

azido (-N3) functionalities in the molecule. Researchers have shown interest in

few fluoro compounds also as energetic plasticizers.

A large number of energetic materials including trimethylol ethane

trinitrate (TMETN), triethylene glycol dinitrate (TEGDN), butanetriol trinitrate

(BTTN), glycidyl azide polymer (GAP), ethylene glycol bis- (azido acetate),

etc., have been reported by the reserchers.6-8 However, only scanty

information is available in the open literature pertaining to the effects on

ballistic parameters such as burning rate coefficient, force constant, and

mechanical properties, etc., with respect to extent of plasticizer in the

propellant composition. The aim and objective of the present study is to

synthesize the promising azido plasticizers such as 1,5-diazido-3-nitraza

pentane (DANPE); 1,3-diazido-2- propanol (DAPOL); ethylene glycol bis-

azidoacetate (EGBAA), methyl (-2-nitroxyethyl) - nitramine (Me-NENA), ethyl

(-2-nitroxyethyl)-nitramine (Et-NENA) and N-n-butyl-N-(2-nitroxyethyl)-

nitramine (Bu-NENA); and carry out systematic basic and applied studies on

their effect on ballistic parameters, in gun propellant formulations. The work

also includes comparative studies on synthesized azido plasticizers in various

gun propellant formulations with respect to their effect on propellant

performance parameters. An attempt has also been made to bring out the

theoretical aspects primarily responsible for variation in propellant

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performance parameters with change in kind of plasticizers and extent of their

presence in propellant composition.

1.2 Solid gun propellants

Conventional gun propellants consist of mixtures of one or more

energetic materials with various additives formulated and carefully processed

to burn smoothly without detonating under the conditions in which they are

normally employed. They are produced in characteristic shapes such as

flakes, ribbons, spheres, cylinders or tubes. The actual propellant charge in a

gun consists of an aggregate of such shapes, which are called grains or

elongated sticks. Even in small arms cartridges, the individual grains are large

enough for their characteristic shape to be discernible to the naked eye. The

volume of each grain rises in rough proportion to the size of the grain due to

the lengthening time scale over which they are required to burn.

Fig.1.1 shows some typical shapes of propellant grains. In addition to

the variation in size & shape, the range of gun propellants is considerably

extended by variation in ingredients so as to meet the ballistic requirements of

a given weapon system, viz., small arms, mortar, medium and high calibre

guns. The propellant must be carefully matched to its performance

requirements, to its limitations of mechanical strength and to its resistance to

erosion, so that it does not degrade the effectiveness of the gun or shorten its

useful life.

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1.3 Requirements of gun propellants

No single formulation can fulfil the user’s requirements of a gun

propellant.9 An ideal propellant needs to fulfil the following requirements:

a) It should have a high energy bulk ratio

b) It should have a predictable burning rate over a wide range of

pressure

c) It should have a low flame temperature

d) It should be easily & rapidly ignited

e) It should be low sensitive to all other possible causes of initiation

f) It should be easily manufactured with cost -effectiveness

g) It should have a long shelf life under all environmental conditions

Fig 1.1: Shapes of gun propellants

Rod

Tubular

Slotted

Ribbon

Multi-tubular

Rod

Tubular

Slotted

Ribbon

Multi-tubular

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h) It should not produce flash or smoke

i) It should not give toxic fumes on burning

1.4 Classification of gun propellants

Based on the main explosive ingredients, conventional gun propellants

are divided into three major classes:

(I) Single base propellants

(II) Double base propellants

(III) Triple base propellants

1.4.1 Single base propellants

The main explosive ingredient of single base propellants is

nitrocellulose (NC). All other ingredients present in single base propellants

are used primarily for stability & burning rate control. Single base propellants

are used in all kinds of guns from pistols to artillery weapons. They are not

used in rocketry.10 In single base propellants, nitrocellulose is not properly

plasticized with the addition of plasticizer. Only less webbed (less than 10mm)

charges are prepared as single base propellants. Such propellant grains

called as NC powders are felted in the cartridge cases for pistols, rifles, anti

aircraft guns, etc. The force constant of a single base propellant ranges from

940-1020 J/g. Force constant beyond 1050 J/g is not possible from single

base propellants due to acute deficiency of oxygen in nitrocellulose. Single

base propellants are manufactured by solvent process using ether-alcohol

mixture in a 60:40 ratio. The manufacture is comparatively safe. Single base

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propellants are cool burning formulations with low flame temperature. Hence,

they are less erosive than double base propellant. They produce less flash

and are less sensitive to impact and friction than double base propellants.

1.4.2 Double base propellants

Double base propellants contain two main energetic ingredients, viz.,

nitrocellulose and nitroglycerine (NG) along with other additives.

Nitroglycerine is used to plasticize the high molecular weight nitrocellulose to

yield a thermoplastic material. Both nitrocellulose and nitroglycerine are high

explosives and capable of undergoing detonation with detonation velocity of

6-7 km per second. These two materials when compounded properly together

form strong material capable of undergoing well controlled deflagration with a

burning rate of few mm per second under standard conditions of evaluation

(70 ksc pressure in the combustion chamber). The double base propellants

have density close to 1.6 g/cm3 and is translucent (unless darkening agents

are added) and have smooth surface. The burning rate of commonly used

double base propellants vary between 5 to 20 mm per second under standard

conditions and the heat of combustion generally varies from 900 to 1200

cal/g. Double base propellants have high force constant as nitroglycerine has

high content of oxygen. NC to NG ratio of 6:8 gives highest force constant.

The percentage of NG should not exceed beyond 50% in the propellant

composition. The double base propellants are more energetic than single

base propellants. There are two disadvantages in the use of double base

propellants – higher barrel erosion as a result of higher flame temperature and

presence of muzzle flash which discloses the location of the gun. Double base

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propellants are manufactured by solvent extrusion process. Double base

propellants are used in pistols, mortars, rockets and missiles. Table 1.1 gives

a comparison of different properties of single base and double base

propellants.11

Table- 1.1: Comparison of Properties of Single and Double Base Propellants

Details Single base propellants Double base propellants

Colour Amber brown or black Grey-green to black

Controlled burning

Can be controlled to maximum efficiency

Can be controlled as single base

Ignition temperature

Ignites at around 3150C Ignites at around 150-1600C

Sensitivity Ignition is difficult; may detonate if burned in large quantities

Detonates more readily than single base; higher potential and more heat

Stability Can be made stable with addition of stabilizing ingredients

Can be made stable as single base

Residue Some residue and smoke Little residue as there is less inert material

Manufacture Complicated but safe; involves a no. of raw materials

Complicated & more hazardous; involves a no. of raw materials

Erosive action Erosion of bore. Adiabatic flame temperature is 2600-3600 K

More erosive than single base propellants because of higher flame temperature and heat of explosion

Flash Caused by hot gases igniting with oxygen at muzzle, however controllable

Increase in flame temperature increases tendency to flash as compared to single base propellants.

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1.4.3 Triple base propellants

Along with other additives such as plasticizer, stabilizer, etc., the main

energetic ingredients of triple base propellants are nitrocellulose,

nitroglycerine and nitroguanidine (NQ). NC and NG belong to the same class

of chemical compounds. They are esters of nitric acid with characteristic

O-NO2 group. Nitroguanidine is an aliphatic nitramine with characteristic N-

NO2 group. It cannot be fully homogenised due to its poor solubility, however,

incorporation of nitroguanidine into fully colloidal double base matrix is

possible, for which NQ of ultra fine grade is used. The triple base propellant

contains up to 40% nitroguanidine. The triple base composition can be

processed into grains with reasonably good mechanical properties. They are

also ballistically more stable and safe to use. These propellants are

smokeless in nature and less erosive because of their low flame temperature.

They give larger number of moles of gaseous product and higher ratio of

specific heat than double base propellants. These propellants are processed

by solvent extrusion process. They are exclusively used in tank gun

ammunition. Table 1.2 gives the comparison of some characteristics of single

base, double base and triple base gun propellants.

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Table- 1.2: Comparison of Characteristics of Single Base, Double Base and Triple Base Propellants

Propellant Density (g/cm3)

Flame temperature

(K)

Force Constant

(J/g)

Cal val (cal/g)

Single base 1.58 2500-3000 940-1020 700-800

Double base 1.61 2600-3600 940-1180 720-1200

Triple base 1.62 2400-3300 950-1140 780-900

Non conventional gun propellants

Double and triple base compositions particularly for high energy

applications, suffer from the disadvantages as they are highly vulnerable to

unwanted ignition, when subjected to a hostile environment to attack by an

energetic projectile, e.g., a projectile comprising of shaped warhead charge.12

1.4.4 LOVA gun propellants

The conventional gun propellants are prone to accidental initiation due

to external stimuli like fire, impact or electric spark, friction and shock wave.

Therefore, keeping in view the above factor, a new class of propellant was

developed for insensitive munitions. They are low vulnerable ammunition

(LOVA) propellants. These propellants are designed to mitigate

predetermined threats under specific conditions of weapon storage and use.

Conventional propellant ingredients (NC, NG or NQ) are replaced by other

materials, whose combinations yield the desired performance & vulnerability

protection. Ideally LOVA propellants are designed to work synergistically with

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armour and stowage design concepts to enhance the survivability of a

weapon system. These propellants contain mixture of inert binders mixed with

energetic ingredient or oxidizer along with plasticizer, stabilizer, energetic

binder and processing aids. The inert binders mainly used are cellulose

acetate, cellulose acetate butyrate, ethyl cellulose, etc. The high energy

oxidizers used are RDX, HMX, triamino guanidine nitrate (TAGN), etc. The

plasticizers used are triacetin, dioctyl adipate, dioctyl phthalate, triethyl citrate,

polyvinyl chloride, tributyl citrate & acetyl triethyl citrate. Carbamite is used as

stabilizer and nitrocellulose as an energetic binder to enhance mechanical

properties and improve processability.

1.4.5 High energy propellants

The typical requirements of high energy propellants are

a) Higher Force Constant (>1180 J/g)

b) Large no. of moles of combustion gases per gram of propellant or

low molecular weight of combustion gases per gram of propellant

c) Isochoric flame temperature preferably less than 3200 K in order to

minimize the gun erosion for longer life of gun

d) Linear burning rate coefficient (β1 ) less than 0.15 cm/s/ MPa

e) Pressure exponent (α) should be less than unity

f) Greater thermal and chemical stability

g) Lower impact and friction sensitivity

These propellants are widely used in high performance kinetic energy

projectiles, where higher energy is needed mainly for FSAPDS projectiles.13

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The basic criteria for design of high energy propellants for tank gun in general

was to achieve maximum possible muzzle velocity for FSAPDS (Fin Stabilized

Armour Piercing Discarding Sabot) projectiles to ensure successful

penetration through the toughest armour plate by the kinetic energy of the

projectiles. Also, it should have a comparatively low flame temperature to

minimize the gun barrel erosion. These propellants exhibit a force constant of

1150 – 1200 J/g. The flame temperature of such propellants typically

ranges from 3200 to 3300 K. The desired burning rate constant is

0.135 - 0.150 cm/s/MPa, the density of the propellant is >1.65 g/cm3 and

storage life is expected to be more than 15 years.

1.5 Ingredients of gun propellants

Solid gun propellants are employed in the form of dense cylindrical or

spherical grains elongated hollow or split sticks or as sheets of plasticized

nitrocellulose. Gun propellants are mostly based on NC to provide mechanical

strength. They also contain inert or energetic liquid plasticizer to improve

physical and processing characteristics, high explosives to increase available

energy, stabilizers to prolong storage life and small amount of inorganic

additives to facilitate handling, improve ignitibility and decrease muzzle

flash.14 The additives used in gun propellants can be classified according to

their functions as shown in Table 1.3.2, 15

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Table- 1.3: Additives used in Gun Propellants

Ingredient Additive Function

Energetic binder Nitrocellulose Provides mechanical strength

Stabilizer Carbamite, methyl centralite, chalk, diphenyl amine, 2-NDPA

Increases shelf life of propellant

Plasticizer Diethyl phthalate dibutyl phthalate, dioctyl phthalate, triacetin

For gelatinisation of NC

Coolant Dibutyl phthalate , carbamite, methyl centralite, DNT

Reduces the flame temperature

Surface moderant Dibutyl phthalate, carbamite, methyl centralite and DNT

Reduces burning rate of grain surface

Surface lubricant Graphite, lead stearate Improves flow characteristics

Flash inhibitors Potassium sulphate, potassium nitrate, potassium aluminium fluoride and sodium cryolite

Reduces muzzle flash

Decoppering agents

Lead or tin foil, compounds containing lead or tin

Removes deposits of copper left by driving band

Anti wear agents Titanium dioxide and talc Reduces erosion of gun barrel

1.6 Plasticizers

Plasticizers are substances which when added to plastic materials

improve the flow properties of the materials and increase softness and

flexibility. In 1927, Manfred & Obriet16 described plasticization as the

separation or disaggregation of the polymer molecules followed by oriental

aggregation. Between 1944 and 1947 Doolittle17 published work in support of

14

this theory developed from studies on the mechanism of solvent action

whereby polymer molecules in solution are attracted to each other by forces

originating from active centres along the polymer chains. The Council of the

International Union of Pure & Applied Chemistry adopted the definition of

plasticizers on 15th Sep 1951 as a substance or a material incorporated in the

plastic or elastomeric material to increase its flexibility, workability or

dispensability. A plasticizer may reduce the melt viscosity, lower the flame

temperature of second order transition or lower the elastic modulus of the

product. In general, plasticizers are high boiling liquid esters such as dioctyl

phthalate, tricresyl phosphate and dioctyl sebacate. Materials with boiling

points much below 2500C are not normally classed as plasticizer due to their

high volatile loss at processing temperature.

A large number of applications of plasticizers are driven by even large

no. of expectation of improvement of original properties of polymers and

products into which these polymers are formulated with the use of plasticizers.

In general, the important expectations of plasticizer are in the development of

the following properties:

a) Decrease in the glass transition temperature (Tg) of the polymer

matrix.

b) Making material mix flexible – the influence related to the

charge in polymer structure which is frequently measured by

decrease in glass transition temperature.

c) Increased elongation and decrease tensile strength are typical

results from glass transition decrease on addition of plasticizer,

although in some polymer specific results are obtained.

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d) Low temperature properties of many materials are improved by

different types and concentration of plasticizer.

e) Viscosity is controlled by plasticizers as they are typically low

viscous liquids. In some systems, they work as gelatinizing

component which enhances the viscosity.

f) Modification of rheological properties.

g) In addition to lowering fusion and glass transition temperature,

plasticizers lower melting temperature. Addition of plasticizers

offers new possibilities of material processing. Mixing time is

reduced in presence of plasticizers.

h) Assist dispersion of liquid and solid addition.

i) Resistance to biodegradation or otherwise.

j) Increased compatibility between additives and core material.

Plasticizers conventionally used in propellant formulations are low

molecular weight non volatile, non energetic esters or hydrocarbons, which

are compatible with other ingredients. Plasticizers with molecular weight (Mw)

above 2000 tend to be viscous, with properties more akin to polymer matrix.

Those with Mw below 200 may be more effective in reducing glass transition

temperature (Tg), but they are highly volatile and tend to migrate out of

formulation matrix. Number average molecular weight (Mn) of plasticizer in

the range of 400-1000 may be appropriate to give optimum plasticizing effect.

Properties of some inert plasticizers are given in Table 1.4.18

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Table- 1.4: Properties of inert plasticizers

Plasticizer Mol. Wt.

Boiling range (0C)

Specific gravity

(/0C)

Refractive index (/0C)

Melting point (0C)

Viscosity

(cP /0C)

DMP 194 282 1.190/25 1.568/20 5.5 17.1/20

DEP 222 298 1.119/20 1.501/20 -40 10.0/25

DBP 278 339-340 1.040/20 1.040/20 -35 20.0/25

DBS 314 175-180 0.935/20 1.442/20 -11 7.9/25

DMP: Dimethyl phthalate DBP: Dibutyl phthalate DBS: Dibutyl sebacate DEP: Diethyl phthalate

1.7 Mechanism of plasticization

Plasticizers are the key ingredients of gun propellants. It is imperative

to know the mechanism of plasticization. The individual molecular chains in a

mass of polymeric materials are held together by combination of various types

of attractive forces (Vander Waals forces). These forces are electrical in

nature and their effect decreases rapidly with increasing distance between the

molecules. The effect of intermolecular attractive forces is to give the mass of

polymeric material a three dimensional structure in which the polymer

molecules are mutually attracted at various points along their length. Under

these conditions the molecules will take upon positions in which energy of the

system is at a minimum, i.e., positions such that the points of attraction are

closest together. The presence of polar groups in the polymer molecule

greatly increases the strength of the intermolecular attractive forces. Thus,

low density polyethylene which is non polar is a soft wax like material,

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whereas PVC which contains in its molecule the polar C-C group is basically a

much harder & more rigid material.

The function of a plasticizer is to decrease the effectiveness of the

polymer intermolecular forces and reduce the rigidity of the three dimensional

structure. In order to function, a plasticizer must penetrate between

molecules. It can then reduce the polymer intermolecular forces by

neutralizing the polymer polar groups with its own polar groups or merely by

increasing the distance between the polymer molecules and so reducing the

strength of the intermolecular forces.

1.8 Selection of azido plasticizers

Energetic plasticizers mainly contain the explosophoric groups such

as nitro, nitrato, fluoroamino, fluoronitro, azido, etc., in the carbon backbone of

the molecule and thus they significantly contribute to the energy of the

propellant. Large number of plasticizers are used in gun propellants like

nitroglycerine (NG), butane triol trinitrate (BTTN), trimethylol ethane trinitrate

(TMETN), diethylene glycol dinitrate (DEGDN) and bis(2,2-dinitropropyl)-

acetal/formal (BDNPA/F). However, these plasticizers are associated with

some disadvantages. Due to the presence of secondary hydroxyl group, NG

and BTTN show low thermal stability. DEGDN and NG have high vapour

pressure. BDNPA/F, METN, DEGDN have low energy content. Ethylene

glycol dinitrate (EGDN) possesses lower density and greater volatility than

nitroglycerine. It was reported by researchers like Flanagan et al19 that 1,5-

diazido -3-nitraza pentane (DANPE) can be used as energetic plasticizer in

gun propellant formulations. It has been reported to be used in single base

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propellant and LOVA propellant. It has two explosophoric groups, viz., azido

and nitramine. It has high heat of formation (+554 kJ/mol) and high oxygen

balance (-79%) and low molecular weight combustion gases. It is also

reported to be compatible with NC and RDX. It exists in liquid state with

freezing point 3.5-4.2 0C.

1.9 Selection of nitramine plasticizers

Nitramines offer superior energy due to positive heat of formation,

typically stoichiometry with higher decomposition temperatures and also

possess negative oxygen balance. They are less sensitive than

stoichiometrically balanced nitroglycerine. Alkyl NENAs ( Me NENA, Et NENA

and Bu NENA) include a nitrate ester as well as nitramine group and as a

consequence thereof, NENA compounds are of high interest to gun

propulsion systems. Alkyl NENAs have various advantages as energetic

materials. A well known property is to readily plasticize cellulosic polymers

(such as nitrocellulose) to yield a new type of double base propellants. The

double base propellants offer very low molecular weight combustion gases

(<20) which in turn provides high driving force (impetus) at any given flame

temperature than conventional gun propellants or alternatively a lower flame

temperature at any given impetus level. Alkyl NENAs have also been

demonstrated to be successful as ingredients in more modern and explosive

compositions particularly as plasticizers in polymeric materials such as poly 3-

nitratomethyl-3-methyl oxetane (poly NIMMO), hydroxy terminated poly ether

(HTPE) and others. The need for increased muzzle velocity within acceptable

pressure limits while retaining good barrel life, can be met with nitramines

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containing formulations. However, the current interest is towards developing

propellant formulations with lower vulnerable properties.

1.10 Theories of plasticization

1.10.1 The viscosity theory

It was Leilich20 who first observed that the viscosity of a plasticizer was

an important criterion in determining the behaviour of a plasticized polymer.

The viscosity theory involved the principle that since plasticizers function by

modifying the rheological properties of polymer and their reaction with

polymers is physical rather than chemical, the viscosity of miscible plasticizers

and in particular their viscosity-temperature behaviour are factors of prime

importance. In general, plasticizers of lower viscosity give softer plastics than

that of higher viscosity. If polymers are plasticized by liquids having high

temperature coefficients of viscosity, they become very hard at low

temperatures and very soft at high temperatures; in other words, their

mechanical and physical properties are very sensitive to temperature change.

Another basic principle of the plasticizer viscosity theory concerns the

plasticizer-to polymer ratio and postulates that the temperature –dependence

of the viscosity of plasticizers is greater than that of polymers. This means

that compounds of low plasticizer-to-polymer ratios are less temperature

sensitive than if the ratio is high. Therefore, to prepare compounds whose

properties show a minimum variation with temperature, low viscosity

plasticizers having low temperature coefficients (e.g. diesters of some

aliphatic dibasic acids) should be used at low concentration.

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1.10.2 The lubricity theory

This theory was elaborated by a group of workers in the period 1940-

43.21-23 The function of the plasticizer was considered to be that of a lubricant

in reducing intermolecular friction between the polymer molecules. Different

workers proposed slight variations in the lubricating mechanism, but the basic

idea was the same. When the plastic specimen is flexed, the polymer

molecules must slide backward and forward over each other. Intermolecular

attractions will impede such movements and the plasticizer reduces the

internal resistance by lubricating action.

1.11 Energetic Plasticizers

Energetic plasticizers invariably contain the explosophoric group such

as nitro, nitroso, fluronitro, fluroamino, azido, etc. in the carbon backbone of

the molecule and thus significantly contribute to the energy of the propellant

as they have positive heat of explosion. Heat of explosion is defined as energy

released by burning the propellant or ingredient in an inert atmosphere and

then cooling to ambient temperature in a fixed volume. A large no. of

plasticizers have been patented for application in both rocket and gun

propellants and explosive formulations.24

Plasticizers are usually incorporated into energetic compositions as

processing aids to improve the workability and flexibility. These improvements

are accomplished by altering the mechanical properties such as glass

transition temperature or formulation viscosity.25 One of the best known

energetic plasticizer is nitroglycerine (NG). NG is most known energetic

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plasticizer both for double and triple base propellants. A significant drawback

shared by nitroglycerine and other nitrate esters containing molecules and

polyol polynitrates, such as diethylene glycol dinitrate and triethylene glycol

dinitrate is their poor thermal stability and high shock sensitivity which make

compositions containing such plasticizers dangerous to handle and prone to

accidental detonation.26 Organic compounds such as adiponitrile, triacetin,

dibutyl phthalate are very good plasticizers but are inert and actually lower

the energy content of the nitropolymer. On the other hand, compounds such

as diethylene glycol dinitrate, 1, 1, 1- trimethylol ethane trinitrate, nitroisobutyl

trinitrate and nitroglycerine contribute energy. However, they have the

undesirable characteristics associated with nitrate esters; toxicity (headache

potential) and volatility.

The primary role of energetic plasticizers is to modify the mechanical

properties of the charge to improve the safety characteristics. This is achieved

by softening the polymer matrix and making it more flexible. In addition to

improvement of properties such as tensile strength, elongation, toughness

and softening point, i.e, glass transition temperature (Tg), the plasticizers can

have secondary roles. These secondary roles include reduction of mix

viscosity to ease processing, modification of oxygen balance and energy

content and as in case of propellants, burn rate modification to tailor ballistics.

To fulfil these roles plasticizers require certain characteristics such as positive

influence on safety, performance and mechanical properties. The energetic

plasticizers have chemical and physical compatibility with all ingredients. The

chemical stability and absence of toxicity are also important for plasticizer

selection. The other important features of energetic plasticizer include

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absence of volatility and exudation, low environmental impact, availability and

affordability.

1.11.1 Nitrate Ester Plasticizers

Nitroglycerine (NG) is most commonly employed plasticizer in several

energetic formulations. However, it is highly sensitive to impact and friction.

Nitroglycerine when heated above 200 0C, it explodes and upon storage it is

found to be unstable at temperatures exceeding 70-80 0C.27 In addition,

nitroglycerine exhibits significant physiological effects causing dilation of

arteries and severe headache. However, NG still remains an effective

plasticizer for many applications. In order to overcome this problem, structural

analogous molecules were developed to replace NG. Some of the major

nitrate esters in use today include trimethylol ethane trinitrate (TMETN),

triethyleneglycol dinitrate (TEGDN), ethyleneglycol dinitrate (EGDN or

nitroglycol) and butanetriol trinitrate (BTTN) 28 and their molecular structures

are shown in Fig.1.2. Being structurally similar to nitroglycerine, these were

developed to replace this material. Most of these molecules possess some of

nitroglycerine’s properties without severe hazards as exhibited by NG. Most

of the energetic nitrate esters possess high volatility and high sensitivity,

making them difficult to handle.

23

MTN or TMETN

CH2ONO2

CCH2ONO2

CH2ONO2

H3C

CH2ONO2

CH2ONO2

CH2

CH2

CH2

CH2

O

O

TEGDN

CH2ONO2

CH2ONO2

EGDN

CH2ONO2

CH2

CHONO2

CH2ONO2

BTTN

TMETN : Trimethylol ethane trinitrate TEGDN : Triethylene glycol dinitrate

EGDN : Ethylene glycol dinitrate BTTN : Butane triol trinitrate

Fig 1.2: Structures of some Nitrate Ester Plasticizers

TMETN is chemically stable, insoluble in water and has low volatility.

TEGDN is also chemically stable and has less impact sensitivity than NG. It

is less volatile than EGDN. EGDN is more effective plasticizer for

nitrocellulose than NG. It has more energy than NG but it is also sensitive to

impact. It possesses a lower density, improved stability and greater volatility

than NG. BTTN has lower density and lower volatility than NG but offers

improved stability. Hence, BTTN is often used in propellants as a

replacement for NG. Most of the energetic nitrate esters are HD 1.1

explosives that possess low critical diameters, high volatility and high

sensitivity making them difficult to handle. Some of the properties of nitrate

ester plasticizers are given in Table 1.5.

24

Table - 1.5 : Properties of Nitrate Ester Plasticizers

Property NG EGDN TEGDN TMETN BTTN

Molecular weight

227.11 152 240 255 242

Oxygen balance, %

+3.5 ±0 -66.6 -34.5 -16

Melt. point, 0C 13.5 -22.3 -23 15.7 -27

Density (20oC), g/cm3

1.596 1.49 1.33 1.46 1.52

NG : Nitroglycerine EGDN : Ethylene glycol dinitrate TEGDN: Triethylene glycol dinitrate TMETN : Trimethylol ethane trinitrate BTTN : Butane triol trinitrate

Rockwell International Corporation reported solid gun propellant

compositions based on triamino guanidine nitrate (TAGN) and plasticized

NC29 comprising of nitrate ester plasticizers, viz., TMETN, TEGDN along with

RDX. A typical composition (RDX / TAGN / TMETN / TEGDN / NC / EC /

Additives :12 / 55 / 23 / 2.5 / 6 / 1.3 / 0.2) found to exhibit force constant of

1196 J/g with 2892 K flame temperature.25 However, friction sensitivity data

and ballistic parameters like force constant, burning rate coefficient, pressure

index have not been studied.

Manning et al30 studied high energy gun propellants comprising of

Hexanitro hexaazaisowurtzitane (CL-20), TNAZ, RDX, HMX and mixtures

thereof. The plasticizers selected for this class of propellant include TMETN,

TEGDN, BTTN, TNAZ, BDNPA/F and Alkyl NENAs in the range of 5-30 % by

weight of the propellant. The high energy gun propellant having an impetus of

at least 1350 J/g has been reported by the inventor. The propellant

25

composition comprised of NC 51%, ethyl centralite 1%, TNAZ 15% and NG

and DEGDN 33%. The burn rate exponent is reported as 0.97.

Highsmith et al31 also reported BTTN (61.85%) as plasticizer for NC

(12.6% N, 32.45%) propellant formulations. However, the ballistic parameters

like force constant, burning rate coefficient have not been reported.

Debenhem32 formulated a gun propellant composition having force

constant 1100 J/g to 1200 J/g with reduced vulnerability to shaped charge

attack. The energetic plasticizer studied by Debenhem includes one nitrate

plasticizer such as BTTN or more preferably at least one nitro plasticizer such

as bis (2, 2-dinitro propyl)- formal/ acetal or mixture of both.

BTTN is less sensitive, less volatile than NG and has residual

hydrogen to contribute to lower gas molecular weight 26.1 Vs 29.0 for NG.

TMETN has a higher hydrogen content and lower CO2 content than NG or

BTTN contributing to further lower gas molecular weight (23.1). This

compound produces a favourable energy release in the mixtures while

keeping the flame temperature lower than NG or BTTN. Compositions based

on (PGN:10%, BTTN:30%, HMX:10%, TAGNAT:50%) where BTTN is used as

plasticizer generate substantially more nitrogen (34.2) and have increased

amount of hydrogen (22.8%)than that of NC-NG (nitrocellulose:80% and

nitroglycerine:11%) based compositions (N2:10.6% and H2:15.6%). TMETN

formulations are similar with lower gas molecular weight (average 23.1). The

order of decreasing chemical energy release for the selected plasticizer is

NG>BTTN>TMETN>GAP>Azide.33 The calculated energy, flame temperature

and molecular weight of binder as monopropellant is given in Table 1.6.

26

Table- 1.6 : Monopropellant calculations

Compound Impetus (J/g)

Temperature (K)

Gas average molecular weight

BAMO/NIMMO 861.7 2334 17.09

NC 1083.7 3320 25.47

PGN 971.3 2344 20.06

BTTN 1287.8 4047 26.13

GAP 892.7 2339 17.55

NG 1149.8 3001 29.00

TMETN 1255.1 3492 23.13

HMX 1366.5 4060 24.35

ANT 1157.6 2932 21.06

TAGNAT 864.4 2015 19.38

BAMO/NIMMO : Copolymer of 3,3-bis(azidomethyl) oxetane and 3-nitratomethyl –3-methyl oxetane

NC: Nitrocellulose PGN : Polyglycidyl nitrate BTTN : 1,2,4-Butane triol trinitrate

GAP : Glycidyl azide polymer NG : Nitroglycerine TMETN : Trimethylol ethane trinitrate

HMX : Cyclotetramethylenetetranitramine ANT : Ammonium 5-nitraminotetrazole

TAGNAT : Triaminoguanidium nitraminotetrazole

Nitrocellulose produces a hot gas mixture (3320 K) with CO2 and H2 as

major components with high average gas molecular weight (20.1). 3,3-bis-

(azidomethyl) oxetane -3-nitratomethyl-3-methyl oxetane (BAMO - NIMMO)

combustion is also cooler with substantial N2 and H2 to decrease the

molecular weight (17.1)

Linear 3- nitratomethyl oxetane (NIMMO) oligomer (polymer consisting

of 1-10 monomer units), another class of nitrate esters was used as plasticizer

27

in oligomeric binder system. Oligomeric NIMMO has lower Tg than the cyclic

tetramer. The derivative of glycidyl nitrate described as GLYN dimer was used

for plasticization of polyether binder system such as poly GLYN and Poly

NIMMO. The linear GLYN dimer is normally a mixture of oligomers and has

low Tg (-64.9 0C) & better impact sensitivity compared to nitrate esters such as

BTTN and TMETN.34

GLYN dimer exhibits superior explosive performance compared to

K-10, Bu-NENA, and BDNPA/F. It is reported that the GLYN dimer is less

susceptible to migration than most of the conventional plasticizers.35

Highsmith et al30 have reported the synthesis of 2,2-dinitro-1,3-

propanediol-diformate (ADDF). Its heat of formation is (-) 145.5 kcal/mol. It

can be used in combination with conventional or novel propellant for

formulating high performance insensitive propellant. It is also used for

minimum smoke propellants in 0–30 % wt. The poor thermal stability and

shock sensitivity characteristics associated with conventional nitrate ester

plasticizers such as NG, DEGDN and TEGDN can be overcome with ADDF

without sacrificing energetic properties. It can be used in single and double

base propellants.

In the same patent, Highsmith et al have reported that BDNPA/F has

the ability to lower viscosity and improve workability of polymeric

compositions. Their synthesis can be conducted in an environment friendly

manner. BDNPA/F is oxygen deficient, relatively low in energy capacity. It has

good chemical compatibility with only selected binders.

28

1.11.2 Nitro plasticizers

Plasticizers composed of bis-(2, 2-dinitropropyl)- acetal (BDNPA) and

bis (2, 2-dinitropropyl)- formal (BDNPF) have found widespread application in

energetic formulations. The molecular structures of BDNPA/F are given in

Fig1.3.

NO2

(CH3CCH2O)2CHC H3

NO2

BDNPA

NO2

(CH3CCH2O)2CH2

NO2

BDNPF

Fig. 1.3: Structures of some Nitro Plasticizers

BDNPA/F plasticizers are typically 50:50 mixtures. The formal is a

solid, ( ∆Hf : -142 kJ/ mol) slightly more energetic than the liquid acetal (∆Hf :

-154kJ/ mol)and is used to form a eutectic mixture to lower the melting point

(making plasticizer usable at lower temperatures). These plasticizers always

contain 8-10% diformal that may slightly decrease the energy of plasticizer.

Recent report illustrates that diformal content may be reduced by controlling

the reaction conditions. BDNPA/F exhibits poor plasticizing properties in terms

of lowering Tg and viscosity of uncured PBX formulations. Furthermore,

BDNPA/F may become unstable under severe conditions such as a

combination of elevated temperatures (>74 0C) and high shock loading (8

blasting cap) with 33 g composition C-4 booster.36

Recent applications of BDNPA/F include low vulnerability gun

propellants, HMX based insensitive explosives PAX-2A, M 900 tank program

29

and it is in service in warheads for torpedoes, missiles and projectiles. Some

properties of 1:1 mixture of BDNPA and BDNPF are given in Table 1. 7.

Table 1. 7 : Properties of Mixture of BDNPA and BDNPF(1:1)

Freezing point, 0C -18

Density at 250C ,g/cm3 1.39

Viscosity at 250C, cPs 260

Impact sensitivity, h 50 cm 170

Friction sensitivity, kgf Insensitive upto 36 kgf

Decomposition temperature, 0C 240

1.11.3 Azido Plasticizers

Low molecular weight azido plasticizers are used to enhance the

processability of gun and rocket propellants by lowering viscosity during

mixing and increasing the mobility of polymer chains for improved elasticity.

Azide group has a high positive heat of formation and can produce more

gases in the combustion products thereby increasing the work capacity of

propellant. Azido compounds offer exclusively smokeless combustion

products with high amounts of nitrogen which are advantageous for specific

applications. Karanjule et al5 have reported the synthesis and characterization

of 1-azido-2,3-dihydroxy-2-azidomethyl propane (ADMP) which acts both as

30

plasticizer and monomer for further polymerization to high molecular weight

polymeric molecules.

Azido esters are used as a means of reducing or minimizing the

amount of flame in the exhaust gases generated during the operational phase

of gun, missile and rocket propellants. Witucki and Flanagan37 studied a novel

family of azido esters which are energetic liquids and find particular utility as

energetic plasticizers in advanced solid propellants. 6-Azidohexyl-6-

azidohexanoate (AHAH) is an example of this family which has been found to

be unexpectedly effective in overcoming the problem of flame in the exhaust

gases produced during the operational phase of solid propellant compositions.

The propellant composition consisted of HMX-75%, polyester resin-10% and

AHAH-15%.

Flanagan and Gray38 formulated a gun propellant consisting of 50% by

weight of NC and 40% by weight of GAP. This propellant yielded an isochoric

flame temperature of 2321 K. A gun propellant consisting of 80% by weight of

NC and 20% by weight of GAP yielded an isochoric flame temperature of

2647 K. Another gun propellant consisting of 18% by weight of NC, 20% by

weight of GAP, 20% by weight of TAGN and 22% by weight of HMX yielded

an isochoric flame temperature of 2483 K.

Flanagan et al39 reported gun propellant formulations based on

nitrocellulose binder matrix containing a variety of azide components to

provide reduced isochoric flame temperature and ultra high mass impetus.

The propellant formulations based on binder (NC/DANPE/NIBTN) in

combination with other azido compounds DADNH, DATH and DATN gave

31

impetus of the order of 1451-1497 J/g (flame temperature 3667-3872 K) as

against impetus of 1395 J/g (flame temperature 4306 K) of baseline

composition containing 75% RDX and 25% binder.

The study revealed that new azide propellant formulations based on

DANPE, NIBTN, DADNH, DATH, DATN are less erosive compared to RDX

based propellant. However, the other ballistic parameters like burning rate

coefficient, etc are not reported. Further more, Ampleman40 synthesized GAP

at a low cost without terminal hydroxyl groups, which gave low glass transition

temperature in propellant compositions.

A new class of energetic plasticizers based on azido- acetate esters

has been recently reported. These energetic plasticizers give binders with low

glass transition temperature (Tg), good thermal stability and compatibility. Four

new compounds, viz., ethylene glycol bis (azidoacetate) – EGBAA,

diethyleneglycol bis (azidoacetate)-DEGBAA, trimethylol nitromethane tris

(azidoacetate)-TMNTA and pentaerythritol tetrakis (azidoacetate) - PETKAA

are presented in Table 1.8. 8, 41

32

Table-1.8 : Properties of Azido Plasticizers

Property EGBAA DEGBAA TMNTA PETKAA

Density at 200C, g/cm3 1.34 1.00 1.45 1.39

Oxygen balance, % -84.15 -99.92 -71.95 88.82

∆ H, kJ/mol -167.36 -328.86 -230.54 -215.2

Viscosity, at 200C, mPa-s 23.4 29.2 1288 2880

Tg , (0C) -70.8 -63.3 -34.1 -35.4

Deflagration temperature, 0C 232 235 214 234

Weight loss,

(900C@ ca. 80 days, %)

0.9 0.48 0.25 -

Impact sensitivity, Nm 5.5 > 10 16 60

Friction sensitivity, N 165 160 192 360

1.11.4 Nitramine Plasticizers

Nitratoethyl nitramines are effective plasticizers in energetic

formulations particularly in NC based systems. NENA are hybrid molecules

which contain a nitro ester group (as in nitroglycerine) and nitramine group (as

in HMX / RDX). The general molecular structure of NENA is as follows:

R- N (NO2) CH2CH2ONO2.

33

These compounds are less sensitive to impact and friction than NG

and some have a lower freezing point than NG.42 However, colloidal mixtures

of NENA with NC give lower impetus than those of NG with NC. Various

NENA compounds and process for producing them are discussed under US

patents.43-45

Alkyl NENAs were first discovered in the early part of World War II by

Wright and Chute46 at the University of Toronto. Scientists, then looking

forward for a new flashless propellant found that alkyl NENAs appear to be

promising solution. Recently, Bu-NENA has been found promising plasticizer

as an alternative to NG. It contributes towards enhancing safety and fulfils the

requirements of advanced ammunitions. Bu-NENA has been reported to

improve not only the thermo-chemical properties but also is effective as good

plasticizer. In the present study, NENA compounds were synthesized from

commercially available alkyl amino ethanol using concentrated nitric acid in

the presence of acetic acid / anhydride.43,46

In continuation to this work further, Erlend et al47 have studied a

continuous process for preparing alkyl NENAs and found 99.5% purity as

against 97% purity in batch process. Typical NENA derivatives in use include

(R=) methyl, ethyl, propyl, iso propyl, butyl and pentyl. It was not until the late

1970s, where researchers at Eglin AFB used NENAs in gun propellants that

required low flame temperatures and low molecular weight combustion

products.30, 35 The use of NENAs as plasticizer in gun and rocket propellants

offer excellent properties such as high burn ratio, reduction in flame

temperature and produces lower gas molecular weight and higher specific

34

impulse based on moderate loading of 60-70% RDX in NC/NENA binder.48

NENAs possess good thermal stability, readily plasticize nitrocellulose and

other polymers, generate low molecular weight combustion gases and give

good impact sensitivity.49 The important properties of NENAs are given in

Table 1.9.

Table-1.9 : Properties of NENA Plasticizers

Property Methyl NENA

Ethyl NENA

Propyl NENA

Butyl NENA

Pentyl- NENA

Molecular weight 165.1 179.1 193.2 207.2 221.1

Density, g/cm3 1.53 1.32 1.264 1.211 1.178

Melting point, 0C 38-40 1.5 -2 -27 to -28 -8 to -5

Oxygen balance, % -43.6 -67.0 -87.0 -104.0 -119.1

DSC exotherm ,0C 218 210 210 210 -

∆ H, kJ/mol 1113 784 503 259 47

The work carried out by Lutz46 reveals that LOVA propellant formulation

based on mixture of NENA and bis (2-nitroxyethyl)- nitramine - DINA exhibit

higher energy than NG. The preferred composition revealed 10-45 % DINA

and 45-90% methyl NENA. This study further revealed that maximum energy

output obtained to the order of 1300 J/g. However, the higher freezing point of

35

methyl NENA and DINA offers less choice from processing and conditioning

point of view, though they exhibit higher energy.

One of the main disadvantages of NENAs as plasticizers is the

migration from compositions on standing or on long – term ageing. Though

the NENAs offer excellent initial plasticizing effect, but due to low molecular

weight, they are volatile and tend to migrate from the polymer matrix.47 Also,

the propellant formulations based on NENAs are reported to have difficulty in

achieving 10 years service life. Moreover, researchers recently found that

Bu-NENA significantly decreases Tg without plasticizer migration when poly

NIMMO and poly GLYN binders are used.50 Bernard et al51 studied novel gun

propellant formulations using NC as the major binder along with 2-nitro amino

5,5-dinitro hexahydro-1,3,5- triazine (NNHT) and a liquid energetic nitramine

plasticizer including Me-NENA and Et-NENA. The formulation reported by

Bernard et al was found to exhibit reduced sensitivity and impetus of the order

of 1108-1172 J/g compared to 1085 J/g for triple base formulation (N30A1).

The flame temperature was in the range of 3042-3106 K. However, this study

did not reveal the independent and obvious effect of NENA plasticizer.

Gill52 reported 2,4-Dinitro-2,4- diaza pentane (DMMD) as the first

nitramine plasticizer which does not contain a nitroxy group. It is a novel

plasticizer for nitrocellulose. Its structure is as follows

2,4 -Dinitro-2,4-diaza pentane (DMMD)

NO2 NO2 CH3- N- CH2 – N-CH3

36

The CH2 group present in between two nitrogen is activated by 2

electron attracting substituents and forms hydrogen bond with OH group or

ONO2 group on NC. The adjacent nitramine groups do not make hydrogens

in CH2 group acidic but only polarize them.

Brown53 formulated a composition based on RDX in combination with

NENAs particularly Et-NENA and Me-NENA to replace a fraction of

nitrocellulose and nitroglycerine and entire amount of diethylene glycol

dinitrate (DEGDN) in JA-2 composition. The amount of RDX was 20-40% and

amount of NENAs was 15-22%. The formulations demonstrated feasibility of

combining RDX and NENAs to increase impetus of propellant 100 units &

1.7 % increase in muzzle velocity.

Some of the advantages and disadvantages of energetic plasticizers

are summarized in Table 1.10.

37

Table-1.10: Advantages and disadvantages of Energetic Plasticizers

Nitrate Ester Plasticizers

Advantages Disadvantages General category

Used in conventional propellant formulations. H.D. 1.1, possess low critical diameters, high volatility and high sensitivity making them difficult to handle.

Nitroglycerine It is the most commonly employed energetic plasticizer in gun propellants.

a) It is highly sensitive to shock, impact and friction. b) Unstable upon storage at temperature 70-80 0C. c) Exhibits physiological effects, causes dilation of arteries and severe headaches. d) Prone to accidental detonation.

TMETN, TEGDN, EGDN

a) They are chemically stable. b) They possess properties of NG without severe hazards of NG. c) EGDN is more efficient plasticizer of nitrocellulose than NG. d) EGDN is less sensitive to impact than NG. e) EGDN has more energy than NG. EGDN possesses lower density and greater volatility in comparison to NG.

a) TMETN has low volatility. b) TEGDN has poor thermal stability and high

shock sensitivity. Hence compositions are dangerous to handle and extremely prone to accidental detonation.

BTTN a) It can replace NG. b) It has improved stability than NG. c) It is less sensitive and less volatile than NG. d) Compositions using BTTN generate substantially more N2 and have increased amount of H2.

Its density is lower than NG. Migrating, volatile and unstable at high temperature

38

Nitrate Ester Plasticizers Advantages Disadvantages TMETN a) Has higher H2 content and lower CO2 content than

NG or BTTN Therefore contributes lower gas molecular weight (23.1) b) Produces favourable energy release in the mixtures keeping flame temperature lower than NG or BTTN.

It gives low energy. Migrating, volatile and unstable at high temperature.

PGN a) Better impact insensitivity compared to BTTN & TMETN. b) It has superior explosive performance compared to Bu-NENA & BDNPA/F. c) Binders with PGN have good mechanical properties.

PGN loses more weight compared to BTTN (at 0.2mm Hg pressure)

Nitro Plasticizers BDNPA/ F Plasticizers are made usable at lower temperatures. They exhibit poor plasticizing properties in terms of

lowering Tg & viscosity of uncured PBX formulations .These may become unstable under severe conditions such as elevated temperature >740C and high shock loading.

Azido Plasticizers GAP a) GAP + NC compositions have isochoric flame

temperatures 2321, and 2483 and 2647 K with high impetus. b) Preferred if higher solid energetic nitramine content is desirable. c) Compositions have low Tg.

Compositions with GAP give poor mechanical properties.

EGBAA, DEGBAA, TMNTA, PETKAA

Good thermal stability and compatibility. Chemically incompatible with double bonds of polybutadiene and therefore cannot be employed in such compositions.

39

Azido Plasticizers AHAH Reduction of flame in exhaust gases during

combustion

DANPE, NBTN,DATH DADNH & DATN

Propellant formulations are less erosive compared to RDX based propellants

Exhibit higher flame temperature (>3300K) compared to conventional non energetic plasticizer (2900K).

Nitramine Plasticizers Me-NENA, Et-NENA, Pr-NENA, Bu-NENA & Pentyl NENA

Good thermal stability, generate low molecular weight combustion gases and give reasonable impact insensitivity.

Me-NENA is a crystalline solid at room temperature and melts at 38-400C. Et-NENA is a liquid at room temperature and freezes at 2-40C. Bu-NENA is also a liquid at room temperature and freezes at -90C.

Readily plasticize NC & other polymers. Migrate from composition during ageing. High burning rates and reduction in flame

temperature. Colloidal mixtures of NENA compounds with NC exhibit lower impetus than those of NG and NC.

NENAs are better than BDNPA/F at reducing viscosity and Tg of poly NIMMO cured rubbers.

Volatile, low molecular weight materials.

The vacuum stability of Me-NENA is superior to either DINA or NG.

Incompatible with ammonium perchlorate.

Less sensitive to impact & friction than NG Difficulty in achieving service life of 10 years.

Some NENAs have lower freezing point than NG . eg Et-NENA & Bu-NENA

DINA DINA can replace NG. Capable of plasticizing NC. It has excellent impetus.

a) DINA is a crystalline material at room temperature and melts at 50-520C .It tends to crystallize out of NC matrix of propellant composition. b) It has high melting point which is disadvantageous for many propellant applications

40

1.12 Objective and scope of the present study

Propellants are low explosives which, due to regularity of burning

produce large volume of gases at high temperature and pressure in the

chamber of the gun in order to accelerate projectile with high velocity.

Conventional gun propellants, viz., single base propellant containing

nitrocellulose (NC) as major ingredient, double base propellant containing

nitrocellulose and nitroglycerine as major energetic ingredients along with

other additives and triple base gun propellants consisting of nitrocellulose,

nitroglycerine and nitroguanidine (picrite) as major ingredients have been

extensively used for gun applications. They are homogeneous mixtures of

these ingredients along with various additives which are formulated and

carefully processed to burn smoothly without detonation and requirement of

oxygen from external source. Depending on the constitution, nitrocellulose,

nitroglycerine and nitroguanidine form the major ingredients of gun propellant.

The other ingredients include stabilizers, plasticizers, flash reducing agents,

ballistic modifiers, surface moderants, lubricants, etc. These are required in

less quantity and they impart specific properties to the propellant.

Plasticizer is one of the key ingredients in propellant composition. They

are generally high boiling organic liquids or low melting solids, which when

added in propellant composition influence the mechanical properties,

processability, brittleness and sensitivity of the propellant. Plasticizers are

generally classified as inert or non-energetic and energetic plasticizers. The

non-energetic plasticizers modify tensile strength, elongation, toughness and

softening point and reduce energy of the systems. Generally, phthalates and

41

sebacates are used as non-energetic plasticizers in gun propellant

formulations. However they do not impart energy to the systems. The

energetic plasticizers enhance flexibility and elasticity in addition to increase

in the overall energy of the system in which they are used. In explosives and

propellants, they are preferred over non-energetic plasticizers because of

their contribution to energy. The energetic plasticizers contain functional

moieties such as nitro, nitrato, fluoronitro, fluoroamino, azido etc. in addition to

long carbon-carbon chains

Conventional gun propellants consist of non-energetic plasticizers, viz.,

diethyl phthalate, dibutyl phthalate, dioctyl phthalate etc. These are

moderately high molecular weight esters compatible with nitrocellulose (NC)

and nitroglycerine (NG). They are used to reduce the sensitivity of NG,

improve the mechanical properties of the propellant and to adjust the energy

level & burn rate of the propellant. Propellant compositions based on above

mentioned ingredients have reached to saturation level in terms of energy out-

put. The design of future weapon systems requires the use of propellant

formulations having enhanced performance and reduced vulnerability during

storage and transportation. As per the current trend, there is a need to

enhance the energy out-put of the gun propellants. This can be achieved by

replacing non-energetic plasticizer with energetic plasticizer which is used in

conventional gun propellants.

It is, therefore, imperative to develop energetic plasticizers, which are

reported to contribute 10 -15% higher energy to the propellant formulations.

As the propellant formulations are primarily for defence applications, the

42

technical details are secured in the form of classified reports or patents.

Therefore, a systematic study was undertaken on azido and nitramine based

energetic plasticizers for their synthesis and finally evaluation in gun

propellant formulations to achieve higher performance, i.e., improved force

constant in addition to better shelf life which forms the major objectives of the

present work.

Literature survey indicates that the current trend in gun propellant

research is to replace the non-energetic plasticizer with energetic plasticizer.

This leads to improved performance of gun propellants in terms of higher

force constant and improved shelf life. To enhance the performance of gun

propellants, numbers of energetic plasticizers are synthesized globally to

evaluate them in futuristic gun propellants.

A global scene of new energetic propellant ingredients like polymeric

binders, energetic plasticizers and oxidizers are being emerged out from

research in laboratories all over the world which will form the base for the next

generation of gun propellants.

The use of energetic additives mainly binder & plasticizers containing

groups such as nitro, nitrato, azido, etc., is considered to be one of the

practical ways to improve the energy level & other technical performance of

solid propellants. These compounds have positive heat of formation, good

thermal stability and low melting points. They are therefore potential

candidates for use as energetic plasticizers in gun propellants.

A review of the literature reveals that most of the studies carried out in

gun propellants using energetic plasticizers are in the form of classified

43

reports and patents due to its classified application in defence and very limited

information is available in open literature. Hence, the present study was

undertaken for generation of systematic data on energetic plasticizers

including DANPE, DAPOL, EGBAA, alkyl NENAs, etc and evaluated their

performance in gun propellant formulations. The performance of these

energetic plasticizers has been compared with conventional plasticizers. The

improved performance parameters (energy, sensitivity, thermal characteristics

and mechanical properties) are also highlighted in the thesis.

1.13 Scheme of the thesis

The thesis is divided into 5 chapters.

CHAPTER 1: INTRODUCTION

This chapter gives the brief introduction of solid gun propellants

encompassing single base, double base and triple base propellants. The

ingredients, requirements and characteristics of gun propellants are also

included in this chapter. This chapter covers a brief resume of role of

plasticizers, the mechanism of plasticization and literature study of new

energetic plasticizers suggested for futuristic gun applications. The overview

of experimental and theoretical work carried out by various researchers and

objective of present investigation is also included in this chapter.

CHAPTER 2: EXPERIMENTAL

This chapter mainly describes the specifications of materials used for

synthesis of energetic plasticizers and propellant formulations adopted in the

present study. A brief description of the processing of the propellant

44

formulations is included in this chapter. The methods and methodology for the

synthesis work adopted for determination of ballistic performance, sensitivity

(friction and impact), deflagration temperature, decomposition temperature,

thermal stability an mechanical properties of propellant formulations are also

discussed. It also includes the characterization of the energetic plasticizers

used in the present study.

CHAPTER 3: RESULTS AND DISCUSSION

This chapter exclusively deals with the technical data generated during

the course of study and the results are presented in tables, figures and

graphs. The results are analyzed and discussed in detail with respect to

ballistic performance, mechanical properties, stability and sensitivity aspects

in this chapter.

Results and discussion on five different propellant series are presented

in five sections.

3.1: It includes studies carried out on propellant formulations based on 1,5-

diazido -3- nitraza pentane (DANPE).

3.2: It includes studies carried out on propellant formulations based on

Ethylene glycol bis azido acetate (EGBAA).

3.3: It includes studies carried out on propellant formulations based on 1,3-

diazido -2-propanol (DAPOL).

3.4: It includes studies carried out on mixed plasticizer system based on

alkyl NENAs in triple base propellant.

3.5: It includes studies carried out on propellant formulations based on

Butyl NENA.

45

CHAPTER 4 : GENERAL DISCUSSION

This chapter broadly discusses the effect of incorporating energetic

plasticizers in the propellant formulations and compared with energetic

plasticizers reported in the literature so as to highlight the major benefits of

plasticizers under study. The chapter also highlights the special features of

the present research work. It also includes the applications of the energetic

plasticizers selected for the present study along with futuristic research which

may be carried out in the field of gun propellants. Other challenges in the

field of gun propellants are also discussed in this chapter.

CHAPTER 5 : SUMMARY

This chapter summarizes the findings of the present study.

References are given at the end of each chapter and the thesis ends

with publications of the research candidate.

46

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