CHAPTER-1

INTRODUCTION

CHAPTER 1

INTRODUCTION

A propellant is a low explosive material, which undergoes rapid and

predictable combustion without detonation, resulting in generation of a large

volume of hot gases. These gases are used to propel a projectile or warhead

to the target. In order to produce gas quickly, propellant must carry its own

oxygen together with suitable quantities of fuel elements like carbon,

hydrogen etc. In homogeneous propellants, fuel and oxidizer are contained

in the same molecule e.g. nitrocellulose (NC) and nitroglycerine (NG).

However, in case of heterogeneous propellants, fuel and oxidizer are

contained in separate compounds. Gun propellants are traditionally known

as homogeneous propellants. Rocket propulsion uses both heterogeneous

and homogeneous propellants.

1.1 Gun Propellants and their characteristics

Gun propellants are designed to provide large quantities of gases,

which are used to propel projectiles at high kinetic energy. The velocity of

the projectile is dependent on the rate at which the gases are produced. This

in turn is dependent on the amount of chemical energy released. The

efficiency of gun (E) is given by the equation,

E = ^ ^ (1)

Where, E = Efficiency of gun

V = Muzzle velocity of the projectile

Y = Specific heat ratio of gases

W = Weight of projectile

C = Charge mass of propellant

F = Force constant of propellant

The amount offeree exerted on the projectile by the combustion gases

is dependent on the quantity and temperature of the gases and is expressed

as,

F = nRTo = ^ ^ - -(2)

M

where, F = Force constant of propellant

n = No. of moles of gas produced from propellant (mol./g)

To= Adiabatic flame temperature (Kelvin) of combustion gases

M = Mean molar mass of combustion gases

The value of 'F' is a useful parameter for comparing the performance

of gun propellants and can be determined experimentally by burning a known

quantity of propellant inside a closed vessel bomb (CV). The main

constituents of the propellant combustion gases are H2, CO, CO2, H2O and N2.

The search is always in the direction of low molecular weight gas

products, because the sonic velocity in the combustion gases should be as

high as possible to minimize the pressure gradient between the chamber

pressure and the pressure acting on the base of an accelerating projectile.

There is always an undesirable pressure drop at the projectile base, which is

inversely proportional to the local velocity of sound in the gases. The rate of

rise of pressure in the gun is controlled by the rate of gas generation from the

burning propellant and the rate of volume increase by the moving projectile.

The rate of gas generation is described by the equation ',

m = p s r - (3)

where, m = rate of mass generation of propellant gas

p = density of solid propellant

s = surface area of the propellant

r = linear burning rate of the propellant

Thus, surface area of the propellant is one of the important parameter

to control bum rates and depends upon propellant geometry. The different

geometries used for gun propellants are ribbon, tube, multi-tube (seven,

nineteen and thirty seven perforations). These are shown in Fig. 1.1. The

relationship between the fraction of propellant burnt and the fraction of

ballistic size remaining at any moment is known as the form function. The

rate of gas generation must be controlled through control of the surface area,

because the linear burning rate and solid propellant density are physical

constants for a given type of propellant. Most important aspect of any grain

design is the surface progressivity i.e. the increase in surface area as a

function of the distance burned into the grain. A thin disc maintains almost

the same surface area throughout the burning process and its form function

of burning rate is close to zero, whereas tubular granules are less regressive

than cylindrical cords. Multitubular propellants possess negative form

function coefficient and are called to be progressive. The multi-tube granules

produce more progressive burning.

Conventional solid gun propellants continue to be investigated for

improvements. However, primary focus of the various studies has been to

reduce cost, increase performance reliability and survivability, enhanced

muzzle velocity and muzzle energy as well as system effectiveness and

efficiency.

In all guns, the performance is also constrained by the attainable

maximum pressure and acceleration that can be tolerated by the mechanical

parts and the projectile. Higher performance requires more energy and

proper programming of total energy release.

Fig 1.1 Different shapes of Gun Propellant

®1 ®1 I® iS^

Tubular Propellaut

Propellant with 7 hole geometry

Propellant with 19 hole geometry

Propellant with 37 hole geometry

Gun propellants are broadly divided into four different classes

namely, single base propellant (SBP), double base propellant (DBP), triple

base propellant (TBP) and nitramine based propellants or advanced

propellants. Single base propellants (SBP) are used in all kinds of guns from

pistol to artillery. The main ingredient of SBP is 90% of nitrocellulose (NC)

having nitrogen content from 12.2 to 13.2 %. Nitrocellulose is gelled with

solvent such as ether /alcohol mixture in 60 : 40 ratio and a plasticizer like

diethyl phthalate (DEP), dibutyl phthalate (DBP) and a stabilizer like

carbamite (symmetrical diethyl diphenyl urea) are added and propellant is

extruded into long strands and cut into required shape and size. The

operations after cutting and vacuum drying include surface moderation of

propellant in a sweetie pan, water steeping for enhancing the burning rate of

propellant and final drying, followed by glazing by graphite. Final

operations are blending and packing.

The force constant of single base propellants varies between 900 and

1000 J/g. Single base propellants have low flame temperature and hence are

less erosive. The burning rate of single base propellants can be controlled

by moderation level. They are less sensitive to impact and friction.

However, due to hygroscopicity in the propellant, irregularity in burning is a

major drawback of single base propellants. The scope of improving the force

constant of single base propellants is limited due to acute oxygen deficiency

inNC.

In order to raise the force constant of single base propellant, NC is

plasticised with nitroglycerine (NG) to form double base propellants (DBP).

DBP are manufactured by using NC-NG paste which is made by slurry

method. NG is absorbed in NC fibers in water medium and NC gets

gelatinized. This is filtered and dried. NC-NG paste is further mixed with

solvents like mixture of ether and alcohol. After addition of other ingredients

to this NC-NG paste, dough is formed and extruded into required shape.

Force constant of double base propellants varies between 950 and 1200 J/g

with comparatively higher flame temperature. These propellants are used in

pistols, mortars and tank guns. Double base propellants are highly energetic

by virtue of high flame temperature.

Due to inclusion of NG, the major disadvantage of double base gun

propellants (DBP) is the excessive erosion of the gun barrel, caused by the

higher flame temperature and the muzzle flash, which reveals gun location.

The muzzle flash is the result of fuel-air explosion of the combustion

products (carbon monoxide and hydrogen gases). In order to reduce the

muzzle flash and flame temperature of double base propellants,

nitroguanidine (NQ) to the tune of 30 - 55% is added to NC and NG matrix

to form triple base propellants (TBP). Introduction of nitroguanidine results

in lowering of flame temperature by about 300 K, without any penalty on

energy. Consequently, gun barrel erosion and muzzle flash are reduced.

TBP offers numerous advantages such as cool flashless burning, lower flame

temperature, non-hygroscopic nature, better shelf life, higher density, ease of

manufacture, safety and regularity in ballistic performance even at subzero

level temperatures. Triple base propellants are used in tank guns, large

caliber field guns and naval guns.

High energy propellants containing nitramines (RDX/HMX) are used

in tank guns, where higher energy is needed mainly for fin stabilized armour

piercing discarding sabot (FSAPDS) type projectiles.^ The main source of

energy in high energy propellants is RDX along with NC. The high energy

propellants exhibit higher energy (1100 to 1200 J/g), thereby imparting very

high muzzle velocity of the order of 1600-1800 m/s to FSAPDS type

projectile, but suffer from the drawback of causing high barrel erosion due to

high flame temperature^. However, type of propellants discussed above

suffers from the possibility of accidental initiation from fire, impact, electric

spark etc. Hence, a new class of propellants is under focus of development

for insensitive munitions (IM) with particular emphasis on low vulnerability

ammunition (LOVA). LOVA propellants contain RDX or HMX as filler, an

inert or energetic polymeric material as binder with suitable plasticizer. The

1̂ ' generation LOVA propellants were made with polyurethane binders,

which provided good mechanical strength to the propellant. Since

polyurethane binders are cross linked, they cannot be recycled. The 2"

generation of LOVA propellants utilize cellulosic binders like Cellulose

Acetate (CA), Cellulose Acetate Butyrate (CAB) etc. whereas 3'̂ '' generation

LOVA propellants use the energetic thermoplastic elastomeric binders.

These composite propellants are less vulnerable to initiation than

nitrocellulose based propellants. The important properties of different

ingredients used in propellants are given in Table -1.1 and that of different

classes of propellants are given in Table-1.2 .

Table-1.1 : Characteristics of Gun Propellant Ingredients

Ingredients

Nitrocellulose (NC)

Nitroglycerine (NG)

Nitroguanidine (NQ)

Cyclotrimethylene trinitramine (RDX)

Heat of formation

(k Cal/mol) -149.77

-83.97

-19.24

-16

Density (g/cc)

1.66

1.59

1.77

1.80

Heat of combustion (k Cal/mol)

650

364

208

501

Oxygen balance

(OB) -24.2

+3.5

-30.75

-21.6

TabIe-1.2 : Properties of Conventional Propellants

Propellant

Single base

(SBP)

Double base

(DBP)

Triple base

(TBP)

High energy

Propellant

Density (g/cc)

1.58

1.61

1.62

1.65

Flame temperature (K)

2500-3000

2600-3600

2400-3300

3200-3300

Force Constant (J/g)

940-1020

940-1180

950-1140

1150-1180

Gun propellants can also be classified based on the method of

processing Solvent, Solvent-less and Semi-solvent.

Single base propellants are processed with the mixture of ether and

alcohol, whereas double base propellants are processed with the mixture of

acetone-water or ether-alcohol. For triple base propellants, solvents used

include ether - alcohol or acetone - alcohol mixtures, depending upon the

type of NC used. The solvent processed propellants are required to be dried

upto 0.5% of volatile matter (VM) in the propellant. Otherwise, there is

possibility of deformation and change in ballistics after curing of the

propellant. The main advantage of solvent process is, the safety of

processing. In semi-solvent process, mixture of ingredients is converted into

dough form and further extruded by hot extrusion technique. Gelatinisation

is achieved by hot rolling (60-70°C). For solvent-less propellant, NC - NG

paste is rolled at high temperatures in a rolling mill and sheets of uniform

thickness are obtained. These sheets are further used for pressing into a

propellant by hot extrusion technique. Semi-solvent and solvent-less

propellants offer an advantage of regular ballistics by virtue of non-

shrinkage during processing and drying of propellants. Propellants with

higher web sizes are produced by solvent less technique.

Existing ammunitions containing conventional gun propellants are

highly vulnerable to external stimuli. This is due to the presence of nitric

esters (NC and NG), which make them sensitive to initiation by high

velocity impact of hot spall fragments and shaped charge jet. This results in

an accidental and unplanned initiation of stored ammunition in the armoured

vehicle, resulting in high casualty and loss of costly equipment'*. Hence, the

concept of Low Vulnerability Ammunition (LOVA) has been evolved.

LOVA propellants are specially designed to mitigate, projectile and

fragment impact threats, under condition of ammunition storage. To achieve

these objectives, conventional propellant ingredients such as NC, NG and

NQ are reduced or replaced by other materials such as RDX or HMX. The

most significant current trend in munition and propellant technologies is the

development of insensitive munition (IM) LOVA propellants contains fine

particle size of RDX or HMX in an inert or energetic binder along with other

additives. Polymeric binder systems have a profound effect on almost all

properties of LOVA propellants. Judicious combination of high energy

oxidizers, binders and plasticizers are employed to obtain required

insensitive high energy LOVA propellants. Replacement of existing

conventional propellants by LOVA propellants is highly advantageous in

terms of life, safety, ballistics and ageing characteristics. Thus, life cycle

cost of LOVA propellants is another major attraction^.

IMs are expected to bring about a true revolution in handling and

stocking procedures of propellants and ammunitions.

1.2 LOVA Propellant Ingredients

1.2.1 Oxidiser

RDX and HMX are widely reported to be used as an energetic

component of LOVA gun propellants to achieve low vulnerability due to

their thermal stability (decomposition > 200°C) and positive heat of

formation (+14 & +17 k Cal/mol, respectively). XM-39 and M-43

propellants developed by USA are well reported in literature^. XM-39

consists of 76% RDX, 12% cellulose acetate butyrate (CAB), 4% NC and

7.6 % plasticizer namely, acetyl triethyl citrate (ATEC) and 0.4 %

stabilizer, carbamite. For XM-43, the inert plasticizer ATEC is replaced by

an energetic plasticizer, 1:1 mixture, of bisdinitro propyl acetal (BDNPA)

and bisdinitro propyl formal (BDNPF). During 1999, a new LOVA

propellant was designed with 2,4,6,8,10,12- hexanitro-2,4,6,8,10,12-

hexaaza tetracyclo 5,5 dodecane (CL-20) as main energetic ingredient by

Mueller . The force constant achieved was 1253 J/g with flame temperature

(Tf) of 3700°C. This formulation consists of CL-20, RDX, NC and

BDNPF/A .

Schedlbauer^ studied LOVA gun propellants containing RDX, HMX

nitroguanidine (NQ), triamino guanidine nitrate (TAGN) and guanidine

nitrate (GN) with thermoplastic materials like cellulose acetate butyrate

(CAB) and energetic binders like glycidyl azide polymer (GAP). He found

that the impetus for 8% TAGN based propellant was 1164 J/g. However, the

pressure index was more than one. New energetic ingredients emerging from

research will form the basis for the next generation of solid LOVA gun

propellants, where impetus values could be more than 1400 J/g with low

flame temperature (Tf).

A number of methods have been described to improve the

performance output of gun propellants.^ Composite propellants with

energetic nitramine litce triamino guanidine nitrate (TAGN), exhibit higher

performance, when they are combined with energetic binder-plasticizer

systems. The calculations of performance parameters showed that GAP

binder exhibits excellent specific energy as compared to polybutadiene (PB)

based propellants. The main reason being better oxygen balance of GAP

(-41.15%) than PB (-53.2%) and high energy release due to azide group.

Compositions based on RDX/TAGN/GAP (43:43:14) showed specific

energy of the order of 3949 J/g with force constant 1244 J/g and flame

temperature 2952 K.

Solid oxidizers examined by Simmons^ include CL-20, RDX, 1,33-

trinitroazetidine (TNAZ), ammonium dinitramide (ADN), triamino

guanidine azide (TAGAZ), triamino guanidine nitrate (TAGN),

nitroguanidine (NQ) and diazido nitramines namely 1,7-diazido- 2,4,6-

trinitrazaheptane (DATH), l,6-diazido-2,5-dinitrazahexane (DADNH) and

1,5 -diazido 2,4-dinitrazapentane (DADZP) and a liquid oxidizer like 1,3-

diazido-2-nitrazapropane (DANP). Among these ingredients, DANP is a

liquid, while DATH, DADNH and DADZP are solids. The impetus values

reported for DANP, DADZP, DADNH, and DATH are 1761,1650, 1504J/g

and 1481 J/g, respectively. Due to higher density of CL-20, propellant

containing CL-20 will have the highest volumetric energy.

Physico-kinetical mechanisms of combustion of energetic oxidizer

CL-20 was investigated with 3,3-bis (azidomethyl)oxetane / tetrahydrofuran

(BAMO-THF), 3,3-bis (azidomethyl) oxetane / 3 - azidomethyl - 3 -

methyloxetane (BAMO-AMMO),cyclic nitramines like HMX and

polycyclic caged nitramines. HNIW (CL-20) as an oxidizer, mixed with

U

modem active binders offer many advantages like higher power, superior

performance, higher safety, environmental friendliness and possibility of

recycling. Zenin et al'* investigated combustion of these propellants using

micro thermocouple techniques. Physical parameters of burning waves of

CL-20 based mixture showed that surface burning temperature was 100° C

lower than HMX based mixtures. Heat feedback was also lesser at elevated

pressures, whereas heat release in solid phase was about twice lower than

HMX based compositions and rate of release of heat was thrice than HMX

based mixtures. Different gasification laws were established for CL-20 and

HMX based compositions. Burning rate control regions in the combustion

waves were the regions of the heat release in solid and thin low temperature

gas layer near the burning surface and wide reaction zone of chemical

reactions with distributed heat release rate in the gas phase.

An improved gun propellant with TNAZ and NC has been patented

by Manning et.al " , wherein the impetus of propellant was claimed to be 1 7

1350 J/g. Schaffers and Stein studied different LOVA compositions in 35

mm LSP (Less Sensitive Propellant ) test generally applied for the

investigation of the propellant for vulnerability against shaped charge jet .

These compositions of LOVA propellants contained FOX-7 / NC / RDX.

FOX-7 (dinitro diamine ethylene) based composition was found to be less

sensitive than RDX based composition with lower performance.

New insensitive Modular Charge System, UNIFLEX 2, is based on

the mixture of GUDN, a stable salt of dinitramide C2N4OH7 (N[N02])2 also

referred as FOX-12, with RDX and the binder system of nitrocellulose

plasticized with butyl-NENA'^. A typical composition with 60% GUDN

generates a burning rate of approximately 40 mm/s at 100 MPa with two

12

different burning rate exponents over the pressure regime. Delay time for

ignition of GUDN is also half that of RDX.

Susan peters et al''* have reported that an advanced LOVA propellant

comprising of 76% RDX with 24% cellulosic binder gave impetus of 1157

J/g and Tf of 3042 K, whereas 24% BAMO-AMMO produced impetus of

1182 J/g with Tf of 2827 K. Replacement of RDX by CL-20 in the same

formulation realized impetus of 1291 J/g and Tf of 3376 K. LOVA

propellant with 56% CL-20 and 24% BAMO-AMMO gave force constant of

1247 J/g with Tf of 3217 K .These gun propellants were processed by

solvent method.

1.2.2 Binders

A binder plays an important role in resisting conductive ignition by

hot metal particles. Propellants whose binders decompose endothermically

are considered excellent. On the other hand, exothermically decomposing

binder ignite as easily as standard nitrate ester propellants.

The V^ generation LOVA propellants were made with polyurethane

binders, which provided good mechanical strength to the propellant'^ The

2"'' generation of LOVA propellant utilize cellulosic binders like cellulose

acetate (CA), cellulose acetate butyrate (CAB) etc. The 3'̂ '' generation

LOVA use energetic thermoplastic elastomeric binders. The solvent less

process of producing a solid extruded propellant grain having a smooth

surface and predictable burn characteristics is patented by Kristofferson et

al ' . Inclusion of HMX is reported to enhance pre-cure extrudability of the

propellant grain. Ulrike has developed LOVA gun propellants with inert

binders CA, CAB and HTPB with 60-80 % nitramines, whose force

constants were comparable to conventional JA-2 propellant (1140 J/g).

These propellant formulations were evaluated for ballistics in closed vessel

13

(CV) and results were matching with theoretical predictions. These

propellants were examined for the mechanical stability by the static

compression of the propellant grains at 300 bar at +21 and -40°C and then

evaluated in closed vessel for dynamic vivacity in comparison to the original

propellant at 0.2 g/cc loading density at 2 r C . The increase in surface area of

compressed propellant was calculated by linear regression method. The

propellants were dynamically tested in 40 mm stimulator. It was

recommended that these propellants can be adopted for middle and large

caliber ammunition. Similar studies using HTPB and CAB were carried out

by Mackowick'^. The propellants with polybutadiene binder cured at 50°C

using non toxic components for curing were evaluated. The propellant had

impetus in the range of 884 to 1050 J/g, where RDX percentage varied from

71 to 83, and binder (HTPB) varied between 15 and 21%. Flame

temperature (Tf) was in the range of 2200 to 2500 K. However, the pressure

exponent (a) was more than one. The dynamic vivacity for HTPB based

propellant was greater than CAB, which in turn was more than GAP.

Compressive strength was very high for polybutadiene propellants .

Crosslinked LOVA gun propellants with HTPB as a binder, toluene-

diisocyanate (TDI) as curing agent, pentaerythritol (PET) or pyrogallol (PG)

as crosslinker have been evaluated ". PET formulations gave better results

than Pyrogallol in all respects namely force constant (1055/980 J/g), flame

temperature (2132 / 2156 K), height for 50% explosion (84/90 cm) and

friction insensitiveness (up to 36/32.4 kg).

Recent trend in development of LOVA propellants is the use of

energetic binders to obtain improved impetus level. Energetic polymers

containing azide (-N3) / nitrato (O-NO2) groups namely glycidyl azide

polymer (GAP), 3,3-bis (azidomethyl) oxetane (BAMO), 3-azidomethyl-3-

14

methyl oxetane (AMMO) and 3-nitratomethyl methyl oxetane (NIMMO)

polymers are emerging as potential binders . They are expected to

facilitate the combustion due to positive heat of formation and superior

oxygen balance than HTPB. Among these, GAP and poly BAMO based

propellants compositions have been widely evaluated. Major source of

energy of azido polymer is exothermic cleavage of azide bond (-N3)

(N=N=N), resulting in overall high energy of the propellant system. Azido

polymer extend the pressure limit of combustion to lower pressure zones and

enhance the bum rates of propellant systems.

Energetic polymers, poly bis ethoxy methyl oxetane (BEMO) and

poly bisazido methyl oxetane (BAMO) have been synthesized by varying

molecular structure to correlate the physical properties to structural changes

made^'. The effect of structure and molecular weight on melt viscosity and

morphology has been examined. BAMO-AMMO copolymer was

synthesized and used as an energetic binder in gun propellant formulations

by Wardle and co-workers . The propellant was extruded and evaluated for

compressive strength and bum rates. The oxetane block co-polymer exhibit

attractive properties in gun propellant formulations. BAMO produces the

best mechanical properties also. The energy of the oxetane block co­

polymer based propellant is evident due to inherently high bum rates of

derived propellant. A comparison has been made with M-43 propellant

(RDX-76%, CAB-24%) to the propellant with 24% BAMO-AMMO and

76% RDX / CL-20 or a mixture thereof The impetus for these propellants

was found to be 1182, 1291 and 1247 J/g, respectively. The propellants were

made by solvent process. Similar studies using BAMO-AMMO are reported

by the same author^^. In these studies, the oxidizer used was CL-20. The

propellant was extruded into multiple grain sizes and configurations.

15

Mechanical properties by compressive loading at 50 cm/min gave modulus

41 MPa and maximum stress of 12.2 MPa. Safety properties of coated CL-

20 with BAMO-AMMO binder were well within the acceptable limits. The

force constant of RDX-oxetane and CL-20-oxetane based propellants was

1182 and 1297 J/g, respectively with flame temperatures 2827 and 3412°K.

The density of propellants was 1.64 and 1.77 g/cc, respectively.

Nitratopolyethers are other class of energetic polymeric binders used

for LOVA propellants^"*. The gun propellant formulations based on the

combination of non energetic binder / energetic plasticizer and energetic

binder 3-nitratomethyl 3-methyloxetane (poly NIMMO) and glycidyl nirate

(Poly Glyn) and BDNPF/A have been extruded into slotted cord shape. The

satisfactory dough rheology was achieved by adjusting the filler particle size

and shape distribution and by partially replacing HMX by nitroguanidine.

Nitroguanidine addition binds the dough together strongly. The same

compositions were used for plastic bonded explosives also. The

vulnerability of the propellants was assessed by measuring the response of

the propellants to shaped charge attack. The propellants showed LOVA

properties, when subjected to shaped charge jet attack and performance was

superior to double base and conventional LOVA propellants.

Advances in Poly NIMMO composite LOVA gun propellants are

reported by Leach and Debenham ". A programme on the alternative

technology of composite LOVA propellants was undertaken by U.K.

research agency. A range of composite LOVA propellants based on the

energetic binder poly NIMMO in combination with various fillers and

plasticizers have been examined. According to this study, factors

responsible for affecting response to shaped charge attack are propellant

composition, impetus value, shatter properties, burn rates, ignition

16

temperature and critical diameter of propellant, geometry, shape, web size

and mass confinement. Loading density also affects LOVA properties.

Ballistic data of Poly NIMMO formulations showed that the pressure index

was more than 1.2, linear burning rate coefficient (61) was in the range of

0.150 - 0.215 cm/s/MPa. Impetus obtained was between 1200 and 1300 J/g

with flame temperature from 3037 to 3340 K. Gumming ^̂ has also reported

composite gun propellants based on poly NIMMO with force constant of

1230 J/g with low vulnerability to shaped charge attack. The modified

GUDOL (NG/DEGN/NQ/RDX) formulation "̂̂ used in 155 mm Modular

Gharge System (MTLS) showed equivalence to the polymer bonded LOVA

propellants. Different tests used to assess the vulnerability of propellants

were slow cook off, bullet impact, detonation shock, cook-off in weapon,

fragment impact and shaped charge impact test. Mainly two types of

propellants, nitramine filled polymer bonded and nitrocellulose based, in

LOVA category were studied. RDX / HTPB based propellants were least

sensitive in bullet and fragment impact, no reaction was observed.

RDX/GAB/NG based propellants were, however, much more sensitive.

DREV, Ganada, has been involved in the synthesis of high molecular

weight GAP, energetic thermoplastic elastomer (ETPE) obtained by macro-

polymerization of GAP having mean molecular weight (Mn) 500, 1000 &

2000 . Ballistic evaluation of gun propellant formulations, containing

linear GAP based ETPEs has revealed that LOVA propellants with 75-88%

RDX and remaining GAP binder had ignifion temperature (Ti) of 205°G and

pressure exponent greater than one. Mechanical properties observed are

between soft and rigid structure, often stiff and brittle behaviour at low

temperatures. Stiffening was observed during ageing due to post curing

reactions.

17

1.2.3 Plasticizers

The role of plasticizer in propellant formulation is multi-functional.

In order to increase the energy of gun propellants, energetic plasticizers can

be used. Well reported LOVA propellant XM-39 containing acetyl triethyl

citrate (ATEC) as a plasticizer. Compounds such as ATEC, triacetin (TA)

and dibutyl phthalate (DBP) have been used as plasticizers for cellulosic

binders.

Evaluation of different plasticizers for LOVA propellants was carried

out by Sanghavi and co-workers^^. Plasticizers evaluated were dioctyl

phthalate (DOP), triacetin (TA), tributyl phthalate (TBP), acetyl triethyl

citrate (ATEC) and GAP. Experimental data on comparative study indicate

that amongst inert plasticizers, triacetin is superior in terms of vulnerability

aspects and mechanical properties, whereas the energetic plasticizer GAP

(low molecular weight) plays a significant role in LOVA formulation by

imparting higher impetus to propellant formulation without compromising

on vulnerability aspects. GAP behaviour was evaluated by partial

replacement of triacetin by Sanghavi et al °̂ in LOVA propellant with 75%

RDX / CA and NC. The improvement in impetus was achieved by 65 J/g

without affecting the mechanical properties but it further reduced pressure

exponent of the propellant. Propellant formulations with 78% RDX, 4% CA,

12%) NC and TA partially replaced by GAP was also studied by Pillai et

al . Most of their findings are in agreement with the earlier researchers .

They have also shown that low molecular weight GAP is a potential

plasticizer for LOVA propellants.

A US patent claimed that the LOVA propellants with RDX/HMX and

the mixture of at least two different dinitro diaza compounds as a mixed

plasticizer for the propellant produces low temperature sensitive coefficient

18

and pressure remains almost constant in tlie temperature range of -50 to

+70°C ^̂ . Other energetic plasticizers reported for LOVA propellants are bis

dinitro propyl acetal / formal (BDNPA / F), alkyl nitratoethyl nitramines

(NENA's) and l,5-diazido-3,3-nitroazapentane (DANPEf'*. Nitrate

plasticizers, trimethylo-ethane-trinitrate (TMETN), triethylene glycol

dinitrate (TEGDN), diethylene glycol dinitrate (DEGDN), butane triol

trinitrate (BTTN) have been studied in LOVA propellants with RDX.

Replacement of the inert plasticizer in LOVA formulations by energetic

plasticizer increases impetus level by 8%. Ballistic studies showed that

formulations containing TEGDN have higher pressure (1.13) exponent than

others (1.00)

1.3 Processing of LOVA propellants

LOVA propellants are processed by different techniques depending

upon the binder used in the propellant compositions. Polybutadiene based

propellants are processed through a solvent less process. The manufacture

of cured system involve addition of an oxidizer to pre-polymer (HTPB /

CTPB / CTBN), which are liquid of low viscosity. After efficient mixing in

a sigma blade incorporator, curative isocyanates (TDI, HMDI) for HTPB

and epoxy or aziridines for CTPB are added. The propellant is extruded and

final curing is carried out at elevated temperatures (75-80°C). This method

eliminates the drying step after extrusion, ensuring thereby uniform

ballistics. Details of solvent less preparation of LOVA gun propellants are

reported by Kristofferson et al ^̂ . Blending of oxidizer (HMX or fine RDX)

with polyurethane binder and cure catalyst is carried out below 50°C . A safe

process technology has been established for LOVA gun propellant with CA

/TA / RDX by Pillai et aP^. Manufacture of propellant formulation using CA

and RDX was tried by conventional solvent process using two different

19

methods. In the first method, fine RDX was first desensitized with

plasticizer coating, which in turn was incorporated with the inert binder. In

the second method, a two-stage process technology was adopted. In the first

stage, the basic composition was prepared by wet mixing process and in the

second stage the dry basic mix is incorporated along with solvent and

additives and extruded into the required size and shape.LOVA propellants

based on CAB/NC were processed by twin screw extruder by Dilleby .

The process uses a lacquer of all the ingredients except nitramines. The

lacquer is pumped as a liquid into a twin screw compounder and RDX is

added as a dry powder. The ingredients are compounded, granulated and

dried to a free flowing powder. The dry powder is fed to a twin screw

extruder with the optimum amount of solvent for extrusion. This process is

more effective and is of low cost. All the process steps including

granulation are continuous.

A LOVA propellant composed of RDX, NC and CAB was

manufactured using standard ball powder propellant technology and tailored

to match the existing for M-792, 25 mm round . Ball powder technology

involves the preparation of a lacquer which is extruded through a die head

and cut into cylinder. The cylinders are then rounded through the action of

osmotic pressure and shear forces. Once the proper shape has been

obtained, solvent is removed by distillation. In this process, critical steps are

shaping and retardation of the RDX leaching. Similar studies have been

carried out by Canterberry and Mrazek ^̂ and ballistic parameters have been

determined. The ball powder in 76/19/5 (RDX/CAB/NC)composition was

found to be extremely impact insensitive (353 kg/cm ) as against 29 kg/cm

for RDX with 4 kg drop weight tester. In bullet impact test no LOVA round

(25 mm) detonated or resulted in fire.

20

LOVA propellants with thermoplastic elastomers, which are

physically crosslinked, can be processed by solvent less process. Elastomers

are required to be heated to elevated temperatures to achieve desired fluidity.

Oxidizer and additives are then mixed with the melt. The mix is allowed to

cool down and extruded after attainment of desired consistency. These

systems offer excellent mechanical properties. High processing temperature

is the main drawback for explosives and propellants. However plasticizers

are added to bring down the process temperatures.

1.4 Ballistic properties of LOVA propellants

Ballistic behaviour of a variety of LOVA propellants has been

examined using closed vessel technique '*". The slope break behaviour

frequently observed in nitramine based propellant burning could be

eliminated by using multi modal mixtures of 2|im and 10 |im size. Bum

rates versus pressure curve exhibits linear behaviour on the conventional

log-log plot. But bum rate curve does not show any slope break. Pillai and

co-workers '** have reported the use of bimodal RDX for elimination of slope

break in P-t curve of LOVA propellants. The thermo-chemical constants of

propellants were computed by them. Results of studies on the role of fine

RDX in determining the buming rate and ballistics of LOVA gun propellants

are also discussed by these authors. Fine RDX of 4.5, 6, 13 and 32)am size

were used. This study reveals that 4.5 to 6 fim size is most suitable to get

desired buming rate behaviour. Thermodynamic properties for LOVA

propellant were calculated and compared with M-30, JA-2 and XM-39

propellants ^^. The results indicate flame temperature of 3000 K against

2671 K for XM-39. Similarly, force constant was found to be 1160 J/g for

XM-39 propellant. Thus, only limited studies have been carried out on the

ballistic behaviour of a few selected LOVA propellants.

21

1.5 Vulnerability Test

Studies on high energy LOVA compositions with respect to energetics

and sensitivity characteristics have been carried out by Choudhari etal '*̂ .

Results obtained reveal that height for 50% explosion for CAB/RDX and

CAB/GAP based compositions more than 60 cm and decomposition

temperature > 216°C, higher from the conventional propellants (160°C).

Cook and Harbersat '*'* used the laser ignition technique on LOVA

propellants for ranking of their vulnerability. LOVA propellants studied

contained CA/RDX, EC/NC/RDX, HTPB /HMX etc. Comparisons were

made with conventional triple base propellants M-30 and M-26. The test

used was the ignition delay. All LOVA propellants exhibited significantly

longer delay time compared to standard propellants and therefore need more

powerful ignition system.

Relative sensitivity of a number of LOVA propellants is reported by

Barnes etal '^^ LOVA propellants proved to be superior to M-30 propellant

with respect to impact sensitivity (32 cm/16 cm), exotherm in explosion

temperature, auto ignition temperature (192 / 167°C), explosion temperature

(336 / 254°C) hot fragment conductive ignition test (775° / 363°C) . DDT

test showed that LOVA propellants exhibit low vulnerability.

Hot fragment conductive ignition test of nitramine propellants XM-39

and M-43 in partially and highly confined status were carried out by Huang

et al ^ '̂̂ ' GO, NO-GO boundaries were predicted. The effects of initial

temperature of spall particle, and size of chamber exhaust port were

examined. Good agreement was found between calculated and experimental

ignition boundaries at 1 atm pressure. XM-39 propellant was found to be

more susceptible to ignition by the spall fragment than M-43 propellant at

low pressures ,because binder decomposition of the M-43 propellant is more

endothermic than of the XM-39 propellant.

22

1.6 Ignition/ combustion studies

Since LOVA propellants are generally formulated using inert binder

and nitramines, they possess high ignition threshold and require more heat

input for their smooth ignition. Ignition studies *^ for HTPB and CAB based

propellants reveal that the relative delay was 5.3 ms as against 1.0 ms for

NC based propellants. Delay time increases by a factor of around five. XM-

39 propellant is more susceptible to conductive ignition by spall fragment

than M-43 propellant due to difference in the binder composition. Thus, a

small change in binder composition has a pronounced effect on ignition

boundary .Vamey "*' has reported that for LOVA propellants, black powder

and boron-potassium nitrate based igniter materials were most effective than

magnesium-Teflon-Viton (MTV) based igniters. For simultaneous and

smooth ignition of LOVA propellants (CA/RDX), combustion products of

igniter play an important role. Various igniter materials namely B-KNO3-

ehylcellulose, magnesium-potassium nitrate-ethyl cellulose, magnesium-

teflon-viton (MTV) etc. were evaluated for the ignition of LOVA propellants

containing 78% RDX, by closed vessel technique.^*' The rate of rise of

pressure with respect to time (dP/dt), was used to assess the comparative

performance of various igniter materials. Results of ignition study reveal

superior performance of gun powder over other igniter compositions since

both had given nearly same ignition delay of 2-3 ms.

Regarding usage of LOVA propellants in artillery applications,

Minor '̂ has opined that a detailed investigation need to be carried out to

study LOVA propellant performance during critical ignition and the flame

spread portion of the interior ballistics cycle in order to assess their

suitability as artillery propellant. He has recommended a simulator for 155

mm Howitzer for these studies involving monitoring of flame spread

through high speed photography.

23

The ignition properties of LOVA propellant M-43 in the simulator

for 120 mm tank gun charge were investigated by Lee et al . These studies

were conducted on the ignition process for a 120 mm APFSDS projectile.

The diagnostics provide insights into the ignition process occurring during

the early phase of the interior ballistics cycle. Correlation between the

pressure data in the simulator and that of initial phase has also been

investigated. It was observed that flame front advanced to the forward part

of the chamber nearly normal to the chamber axis. Breech pressure rose

slowly indicating the difficulty in ignition of propellant and time lapsed

between temperature detectors was more as compared to single, double and

triple base propellants.

The influence of ageing on the burning rate parameters for LOVA

propellants comprising cellulosic binders and thermoplastic elastomers was

carried out at 65.5°C by Strauss et al . The combustion characteristics

were observed at different time intervals. Reduction in relative vivacity and

relative force from zero to 12 months was about 5.5% as against 1.3% of

M-30 conventional propellant. Pressure exponent for LOVA propellant

decreases from 1.16 to 0.88 as against slight upward change for M-30

propellant. Majority of LOVA propellants are inherently more chemically

stable than M-30 composition.

The combustion behaviour and thermo-chemical properties of XM-39

and M-43 revealed that activation energy for XM-39 was 4 k Cal /mol,

whereas it was 8.1 k Cal /mol for M-43 propellant '̂*. Scanning Electron

Microscope (SEM) pictures of burning surfaces of recovered propellant

samples demonstrated significant alteration of surface structure of M-43

propellant due to the use of energetic plasticizer. Increase in heat release on

the burning surface of M-43 propellant was also observed.

24

Different sensitivity characterization techniques for LOVA

propellants have been described by Kirshenbaum et al̂ .̂ The laboratory

sensitivity and thermal stability tests included impact sensitivity, DTA,

TGA, vacuum stability, hot fragment conductive ignition and deflagration to

detonation (DDT) test which can be used to select the propellant for scale

up.

LOVA composition useful for a tank gun ammunition comprising

RDX, a new cyclic nitramine, 2-nitroimino-5-nitrohexahydro-l,3,5-triazine

(NHNT), NC, TMETN, BDNPA/F has been patented by Strauss et al ^^

Low pressure exponent (a) and high linear bum rate coefficient (Pi) values

are important parameters for its multizone unicharge application. This

propellant is claimed to be less sensitive to shaped charge jet impact.

LOVA propellants with energetic plasticizers have been reported by

Urenovitch . The plasticizers used were alkyl NENA and l,3-dmitroxy-3-

nitrazapentane (DINA) and the mixtures there of The development of

LOVA gun propellants at Eureco Bofors with l,l-diamino-2,2-

dinitroethylene (FOX-7) and FOX-12 as energetic oxidizers with NC and CO

NENA plasticizers has been reported . A typical composition with 60 %

FOX-7 generates a burning rate of approximately 55 mm/s at 100 MPa with

pressure exponent of about one. FOX-7 reduces the delay time for ignition

significantly in comparison to the RDX based reference composition. The

gun performance equals that of a double base propellant but the flame

temperature as well as temperature dependence are considerably lower. The

ballistic details of some of the LOVA propellants developed by advanced

countries are given in Table 1.3.

Thus, LOVA propellants containing energetic ingredients (high

explosives) as oxidizers, energetic binders and energetic plasticizers with

suitable additives have very bright future for usage in field gun, tank guns

25

and for naval applications. These propellants provide superior ballistics with

highest order of insensitivity.

Table-1.3 : Different LOVA Propellants Developed by Advanced Countries

Country

USA

USA

USA

USA

USA

USA

USA

USA

USA

Canada

Germany

Germany

Year

1983

1983

1978

1978

1978

1976

1978

1982

1995

1990

1992

1992

Ingredients (%)

RDX-74/EC-12/NC-8/DBP-6

RDX-75/C A-16/TA-8/K2SO4-1

HMX-80/HTPB-20

RDX-80/CTBN-20

HMX-80/CTBN-15/Additives-5

HMX-75/Polymer-12/IPDI-

lO/Additives-3

HMX-80/Polymer-13/Additives-7

XM-39(RDX/76/CAB-12/NC-

4/ATEC-7.6/Carb.0.4

M-43(RDX-76/CAB-12/NC-

4/BDNPF/A-7.6)

XM-39 - ATEC replaced by GAP BuNENA (N-n-butyl-N-(2-nitroxyethyl nitramine) TEGDN DANPE (1,5-diazido-3-nitraza pentane) RDX-78/HTPB-11/Picrite-

8/Additives-3

RDX-78/HTPB-11/TAGN-

8/Additives-3

Force Constant (J/g) 1055

1000

1008

1009

1005

1000

1038

1069

1157

1159 1173

1179 1210

1164

1180

Flame Temp. K 2536

2548

2363

2387

2379

2350

2434

2671

3042

2927 2975

3054 3107

2869

2880

26

1.7 Objective of Present Study

In view of the review presented above, bringing out limitations of

earlier studies, the present study was undertaken to generate comprehensive

and exhaustive information on vulnerability and energetics of low

vulnerability gun propellants based on cellulose acetate, BAMO-THF

copolymer and GAP (high molecular weight) as binder and GAP (low

molecular weight) and BuNENA as energetic plasticizers. Plasticisers play

a dual role, as processing aid and as a desensitiser and improve mechanical

as well as ballistic properties.

1.8 Plan of Present Study :

The present thesis is divided into five chapters.

Chapter 1 : General Introduction

This gives a brief account of conventional gun propellants and concepts used

for energetic gun propellants.

LOVA propellants : An overview of R&D efforts attempted so far towards

improvements in propellant performance and reduction in vulnerability. This

also includes processing details and description of oxidizers, binders and

plasticizers used. Objective of the present study is given at the end of the

chapter.

Chapter-2 : Experimental

This chapter describes broad specification of the materials used and

methods and methodologies adopted during the present study for generation

27

of information on gun propellants witii polymeric binders like HTPB,

BAMO-THF and GAP as binders. Method of processing of propellants as

well as instrumental methods used for determination of ballistics and

vulnerability (impact, friction, ignition temperature) of the formulations are

also elaborated in this chapter. Salient aspects of closed vessel test and

determination of mechanical properties are brought out in this chapter.

Chapter-3 : Results and Discussion

This chapter gives details of ballistic results along with results of

sensitivity and mechanical properties. Results of propellant formulations

evaluated during this research programme are also included. The results

obtained for various LOVA propellants based on different energetic binders

and plasticizers are presented in this chapter including results of ballistic

performance in closed vessel at 0.1 g/cc loading density, sensitivity

characterization of propellants, their thermal characteristics and mechanical

properties.

Section-3.1 :

It contains results of the studies carried out on the effect of non-

energetic binders, cellulose acetate (CA) and hydroxyl terminated

polybutadiene (HTPB) and their mixture on LOVA propellant compositions

containing on RDX (70-80%), NC(5-15%), Carbamite (0.5%) and low

molecular weight GAP (5.5%)) as an energetic plasticizer .

28

Section-3.2 :

This section contains results of studies carried out on the effect of

BuNENA as an energetic plasticizer on LOVA propellants containing CA

and HTPB and their mixture as binders.

Section-3.3 : This section contains results of studies carried out on the

effect of energetic binder and plasticizer GAP on LOVA propellant

compositions.

Section-3.4 : This contains results of the studies carried out on the effect of

energetic binder glycidyl azide polymer(GAP-2000) on LOVA propellant

compositions containing BuNENA as an energetic plasticizer.

Section-3.5:

This section contains results of the studies carried out on the

propellant compositions based on BAMO-THF co-polymer as energetic

binder and low molecular wt. GAP as plasticizer,

Section-3.6 :

This contains results of studies carried out on the propellant

compositions containing BAMO-THF co-polymer as energetic binder and

BuNENA as an energetic plasticizer.

Chapter-4 : General Discussion

The results obtained in the present study are discussed in the light of

information reported in the literature so far.

Chapter-5 : Summary

This chapter summarizes various findings of the present study.

29

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57. Urenovitch J.V., "Low Vulnerability Propellants Plasticizers", US

Patent 5,482,581 dt. Jan 9, 1996.

58. Per Sjoberg, Eurenco Bofors's news letter .October 2004.

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