chapter 1 introduction -...
TRANSCRIPT
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
12
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
13
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
16
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
REFERENCES
1. Ramanujachari V., “Introduction to Gun Interior Ballistics”, Modelling
and Performance Prediction in Rockets and Guns, Course Notes,
Edited by S.R. Chakravarthy and S. Krishnan, Allied Publishers Ltd, 6 -
7 (Dec 1998), p 67-90.
2. Akhavan J., “Introduction to Propellants and Pyrotechnics”, The
Chemistry of Explosives, The Royal Society of Chemistry, UK (2004), p
149-164.
3. Boyars C, Kiager K., “Propellants Manufacture Hazards and Testing”,
Advances in Chemistry Series, American Chemical Society, Vol.88,
Washington DC, (1969).
4. Frankel M.B., Witucki E.F., “Processing for Preparing 1,5 -diazido-3-
nitra-zapentane”, US Patent No. 4761250, (1988).
5. Karanjule N.S., Talwar M.B. , Sivabalan R., Gore G.M., Dhavale D.D.
and Asthana S.N., “Synthesis and Characterization of 1- azido, 2,3-
dihydroxy 2-azidomethyl propane (ADMP): A New Organic Azide”,
Indian Journal of Chemical Technology, Vol.14, (Jan 2007), p 34-38.
6. Hoffman D.M., Hawkins T. M., Lindsay G.A., Wardle R.B.and Manser
G. E., “Clean Agile Plasticizers for Propellant Propellant and
Explosive Formulations”, LLNL Document, UCRL-JC- 119253, (1994).
7. Kawaguchi S. , Kusumoto K., Hisada T., Noda H., Ota T., Kato K.,
“Synthesis of New Azido Plasticizer”, Journal of Japan Explosives
Society, Vol.61, No.5, (2000), p 242-247.
47
8. Dress D., Loffel D., Messmer A., Schmid K., “Synthesis and
Characterization of Azido Plasticizer”, Propellant, Explosives,
Pyrotechnics, Vol. 24, Issue 3, (1999), p 159-162.
9. Bailey A. and Murray S.G., “Explosives, Propellants and
Pyrotechnics”, Brasey’s World Military Technology, (1989), p 79-80.
10. Krishnan S., Chakravarthy S.R., Athithan S.K., “Propellant and
Explosive technology”, (1998), p48-51.
11. Krier H. and Summerfield Martin, “Interior Ballistics of Guns”, AIAA
Series, Vol.66, (1979), p 13-15.
12 Holt R.B. and Phillips C.M., “Energetic Plasticized Propellant”, US
Patent No. 5500060, (1996).
13. Pillai A.G.S., Dayanandan C.R., Joshi M.M., Patgaonkar S.S. and
Karir J.S., “Studies on the effects of RDX particle size on the burning
rate of Gun Propellants”, Defence Science Journal, Vol 46, N0.2,
(1996), p 83-86.
14. Kirk-Othmer, “Encyclopedia of Chemical technology”, Wiley- Inter-
Science Publication Vol.10, 4th Edition, (1993), p 1-8.
15. Ritchie P.D., “Physics of Plastics”, (1965), p 134 -135.
16. Manfred O and Obrist J., Kolloidzeitschrift, 41,348 (1997).
17. Doolittle A.K., “The Technology of Solvents and Plasticisers”, John
Viley, New York, Ch.14 & 15 (1954).
18. Strecker R.A. and Verderame F.D.“Isotactic And Syndiotactic Polyvinyl
Nitrates And Processes For their Formations”, US Patent No. 3965081
(1976).
48
19. Flanagan J.E.,Wilson E.R.and Frankel M.B.,”1,5-Diazido-3-
nitrazapentane and Method of Preparation thereof”US Patent No.
5013856 (1991).
20. Leilich N., Kolloidzeitschrift, 99(1), 107 (1943).
21. Kirkpatrick A, Jr. of Applied Physics, 11, p225, (1940).
22. Clark F.W. Chem. Ind., 60,p225, (1941).
23. Houwink R., Proc. XI Int. Cong. Pure Appl. Chem., London, p575-583,
(1947).
24. Handbook of Plasticizers, Editor George Wypych, William Andrew
Publishing, Toronto, New York, p 20-23 (2004).
25. Meyer R., Kohler J., Homburg A. “Explosives”, 6th Edition, Wiley
Publication (2007).
26. Fordham S., “High Explosives and Propellants” Vol1, Pergamon Press,
Oxford (1996).
27. Adolph H.D. and Kim K.E., “Energetic Polynitro Formal Plasticizers”,
United States Statutory Inventory Regulations No.H 350 (1987).
28. Flanagan J.E. “Ultra high Energy Azide Containing Gun Propellants”,
US Patent No. 5053087 (1991).
29. Rockwell International Corporation, “Triamino Guanidine Nitrate
containing Gun Propellants” UK Patent GB No. 1362506 (1974).
30. Manning T., Strauss B., Prezelski J. P., Moy S., ”High Energy TNAZ
Nitrocellulose Gun Propellant”, US Patent No. 5798481, (1998).
49
31. Highsmith T. K., Doll D. W., Cannizzo L. F., “Energetic Plasticizer and
Explosive Propellant Composition Containing Same”, US Patent No.
6425966 (2002).
32. Debenham D. F., “Extrudable Gun Propellant Composition” US Patent
No. 6228190 (2001).
33. Zentner B. A. and Reed R., “Calculated Performance of Gun Propellant
Composition Containing High Nitrogen Ingredients”, 5th International
Gun Propellant and Propulsion Symposium, (1991).
34. Cliff M., “PolyGLYN Binder Studies and PBX Formulations: Technical
Achievements from a LTA to DERA Fort Halstead”, DSTO-TR-0884,
DSTO, Salisbury, SA.
35. Adolph H.G., “Bis (dinitropropyl) formal/dinitro butyl dinitro propyl
formal plasticizers”, US Patent No. 4997499, (1991).
36. (USA) , Aerojet, Material Data Sheet.
37. Witucki E. F., Flanagan J. E., “Azido Esters” US Patent No. 4432814,
(1984).
38. Flanagan J. E.and Gray J. C., “Gun Propellants Containing Polyglycidyl
Azide Polymer”, US Patent No. 4288262, (1981).
39. Ampleman G., “Synthesis of a Diazido Terminated Energetic
Plasticizer”, US Patent 5124413, (1992).
40. Proatus A., “Energetic Polymers and Plasticizers for Explosive
Formulations – A Review of Recent Advances”, DSTO, TR-0966,
(2000).
50
41. Urenovitch J. V., “Low Vulnerability Propellant Plasticizers”, US Patent
No. 5482581, (1996).
42. Wright G.F. and Chute W. J. “Nitramines and their Preparation”, US
Patent No. 2461582, (1949).
43. Blomquist A.T. and Fiedorek F.T.,”Nitramines”, US Patent No.
2485855, (1949).
44. Blomquist A.T. and Fiedorek F.T.,”Process of Preparing Nitroxy Alkyl
Nitramines” US Patent No.2678946 (1944).
45. Lutz R.G, “Low Vulnerability Propellants”, US Patent No. 5520757,
(1996).
46. Jold S.K. Eriend, J. , Gjersoe R., Halvorsen T. and Berg Alf, “Process
of Preparing a High – Energy softening agent”, European Patent
Specification No. EP 1 196 374 B1, (2003).
47. Simmons R.L., “NENAs-New Energetic Plasticisers”, NIMIC –S- 275-
94, NATO, Brussels, Belgium, (1994).
48. Simmons R.L., “Thermochemistry of NENA Plasticisers”, 25th Int.
Ann.Conf. ICT, Karlruhe, Germany, Fraunhofer Institute for Chemische
Technologie, 1994, 10-1 -10-10.
49. TTCP, W-4, Energetic Materials and Propulsion Technology, 22nd
Meeting, TTCP, p112.
51
50. Flower P. and Garaty B., “Characterisation of Poly NIMMO and Poly
GLYN Energetic Binders”, 25th Int. Ann. Conf, ICT, Karlsruhe, (1994).
51. Strauss B., Moy S. M., “Insensitive Gun Propellant”, US Patent No.
5325782, (1994).
52. Gill R.C. and Nauflett G. W., “New Plasticizers for Nitro Polymers”, US
Patent No. 4457791, (1984).
53. Brown L.C. “High Energy Gun Propellants”, US Patent No. 6241833
B1, (2001).