Burning Rate Characterization of GAP/HMX Energetic Composite

Materials

Naminosuke Kubota*

Mitsubishi Electric Corporation, Kamimachiya 325, Kamakura 247-0065 (Japan)

Ichiro Aoki

Hosoya Kako Co., Ltd., Sugao 1847, Akiruno, Tokyo 197-0801 (Japan)

Charakterisierung der Abbrandrate von energetischen Komposit-

Materialien mit GAP=HMX

Die Abbrandrate von energetischen Materialien aus Glycidylazid-

polymer (GAP) und HMX-Partikeln wurde charakterisiert, um den

WaÈrmefreisetzungsprozeû waÈhrend des Abbrandes zu klaÈren. Das

energetische Polymer GAP brennt selbststaÈndig. Die Zugabe von

HMX erhoÈht die Flammentemperatur und aÈndert das Abbrandverhal-

ten. Experimentelle Beobachtungen deuten auf eine zweistu®ge Gas-

phasenreaktion hin: Die erste Reaktionsstufe kontrolliert die

Abbrandrate. Eine sichtbare Flamme bildet sich erst in der zweiten

Stufe aus. Der WaÈrme¯uû von der ersten Reaktionszone zur

Abbrandober¯aÈche erhoÈht sich mit steigendem Druck, dagegen ist die

WaÈrmefreisetzungsrate an der Abbrandober¯aÈche vom Druck unab-

haÈngig.

CaracteÂrisation des vitesses de combustion de mateÂriaux compo-

sites eÂnergeÂtiques aÁ base de GAP/octogeÁne

La vitesse de combustion de mateÂriaux eÂnergeÂtiques aÁ base de

polyglycidylazide (GAP) et de particules d'octogeÁne a eÂte caracteÂriseÂe

en vue d'expliquer le processus de deÂgagement de chaleur pendant la

combustion. Le polymeÁre eÂnergeÂtique GAP bruÃle de manieÁre auton-

ome. L'addition d'octogeÁne augmente la tempeÂrature de ¯amme et

modi®e le mode de combustion. Des observations expeÂrimentales

laissent supposer qu'une reÂaction de phase gazeuse en deux eÂtapes a

lieu: la premieÁre eÂtape de combustion controÃle la vitesse de combus-

tion. Ce n'est qu'au cours de la deuxieÁme phase qu'une ¯amme visible

se forme. Le ¯ux de chaleur de la premieÁre zone de reÂaction vers la

surface de combustion augmente avec la pression ; en revanche, le

deÂgagement de chaleur de la surface de combustion ne deÂpend pas de

la pression.

Summary

The burning rate of the energetic materials composed of glycidyl

azide polymer (GAP) and HMX particles was characterized in order to

elucidate the heat release process during burning. Since GAP is an

energetic polymer and burns by itself, the addition of HMX increases

the ¯ame temperature and alters the burning rate characteristics.

Experimental observations indicate that the gas phase structure con-

sists of a two-staged gas phase reaction: the burning rate is controlled

bythe ®rst-stage reaction zone and the ®nal ¯ame is formed at the

second-stage reaction zone. The heat ¯ux transferred back from the

®rst-stage reaction zone to the burning surface increases as pressure

increases and the heat released at the burning surface remains

unchanged when pressure is increased.

1. Introduction

Glycidyl azide polymer (GAP) is a unique energetic

material that burns with a relativelyhigh burning rate even

though the energycontent within a unit mass of GAP is not so

high. Cyclotetramethylene tetranitramine (HMX) is a crys-

talline energetic material composed of a stoichiometrically

balanced molecule. When HMX particles are mixed with

GAP, an energetic material, the so-called ``GAP=HMX

energetic composite material (ECM)'' is obtained.

It has been reported that the burning rate of GAP=HMX-

ECM depends largelyon the mass fraction of HMX mixed

within GAP, x (HMX). In order to characterize the burning

rate of this class of ECM the heat release process was

determined bythe measurement of the ¯ame structure of

GAP=HMX-ECM.

2. Physicochemical Properties of GAP and HMX

Glycidyl azide polymer (GAP) is a unique polymeric

material characterized with a 2

2N

3

chemical bond as shown

in Table 1. The decomposition of 2

2N

3

bond generates a

signi®cant heat without oxidation reaction. The bond break-

age of2

2N

3

is the initial step of the reaction including melting

and gasi®cation processes. The formation of gaseous frag-

ments occurs when the GAP surface is heated, and numerous

chemical species are formed from the reacting surface. The

heat transfer process from the high-temperature zone to the

reacting surface determines the burning rate of GAP. In this

studythe combustion mechanisms of the ECM made of GAP

* Corresponding author;

e-mail: [e-mail]

Table 1. Chemical Properties of GAP

Chemical formula

C

3

H

5

ON

3

Molecular mass

1.98 kg=mol

Heat of formation

957 kJ=kg

Flame temperature at 5 MPa

1465 K

# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000

0721-3115/00/0409 ± 168 $17.50‡:50=0

168

Propellants, Explosives, Pyrotechnics 25 168±171 (2000)

and crystalline particles are examined in order to gain a wide

range of burning rate characteristics.

GAP monomer is synthesized by replacing C2

2Cl bonds of

polyepichlorohydrin with C2

2N

3

bonds. The three nitrogen

atoms in the form of N

3

were attached linearlywith ionic and

covalent bonds. The bond energyof 2

2N

3

is reported to be

378 kJ per azide group

(1 ± 3)

. GAP monomer was polymerized

with the terminal 2

2OH groups with hexamethylene diiso-

cyanate (HMDI) and crosslinked with trimethylolpropane

(TMP). The GAP polymer has one 2

2N

3

bond in every

monomer unit and the bond energyis responsible for the

positive heat of formation. Thus, the GAP polymer was

chosen as a fuel component that acts also as an adhesive

binder of crystalline particles to formulate ECM. The

physicochemical properties of GAP and GAP polymer are

shown in Table 1 and Table 2, respectively.

The adiabatic ¯ame temperature of the GAP polymer is

1365 K at 5 MPa and large amounts of C

(s)

, H

2

, and N

2

are

formed as combustion products. It should be noted that the

combustion products are mostlyfuel components and very

small amounts of CO

2

and H

2

O are formed.

Table 3 shows the physicochemical properties of HMX.

HMX is a cyclic nitramine represented by N2

2N

3

groups. The

combustion products of HMX are stoichiometrically

balanced and the adiabatic ¯ame temperature is 3255 K at

5 MPa.

The overall initial decomposition reaction is
3…CH

2

NNO

2

†

4

! 4NO

2

‡ 4N

2

O ‡ 6N

2

‡ 12CH

2

O

and produces oxidizer and fuel fragments. Since nitrogen

dioxide reacts quite rapidlywith formaldehyde, the gas

phase reaction

7NO

2

‡ 5CH

2

O ! 7NO ‡ 3CO ‡ 2CO

2

‡ 5H

2

O

is probablythe dominating reaction immediatelyfollowed

bythe decomposition reaction. The reaction product of NO

oxidizes the remaining fuel fragments such as H

2

and CO.

However, the oxidation reaction byNO is reported to be

slow to produce the ®nal combustion products. The dom-

inating gas phase reaction on the burning rate of HMX is the

reaction byNO

2

. When HMX is mixed with GAP, HMX

acts as an energyaddition on the combustion products

because no excess oxidizer components are available.

3. Experimental

3.1 GAP/HMX-ECM Samples Tested in this Study

The mass fraction of HMX mixed within GAP,x (HMX),

was 0.8 used for the examination of the combustion wave

structure. The physicochemical properties of GAP=HMX-

ECM are shown in Table 4.

3.2 Burning Rate and Combustion Wave Structure

In order to understand the combustion mechanisms of

GAP=HMX-ECM formulated in this studythe burning rate

and combustion wave structure were measured. Strand-

shaped samples (7 mm67 mm in cross-section and 70 mm

in length) were made. The ignition of the samples was

conducted using an electricallyheated chromium ®ne-wire

set on the top end of each sample.

The burning rate of these materials was measured using a

chimney-type strand burner that was pressurized by nitrogen.

The ¯ame structure was observed through a transparent

window attached on the side of the burner. The regressing

burning surface of the samples was recorded bya high-speed

video camera through the window. The measurements of the

temperature pro®les in the combustion wave were conducted

byusing ®ne thermocouples which were threaded through the

samples in order to determine the heat release process of this

class of ECM.

4. Results and Discussion

4.1 Burning Rate Characteristics and Combustion Wave

Structure

Figure 1 shows the burning rates of GAP binder and

HMX

(1,2)

. The burning rate of GAP binder is higher than

that of HMX even though the ¯ame temperature of GAP

binder is 1890 K lower than that of HMX (see Tables 2 and

3). The bunring rate of GAP=HMX-ECM is shown in Fig. 2

as a function of x…HMX† at different pressures. The burning

rate decreases as x…HMX† increases in the range of

x…HMX†50:6 and increases as x…HMX† increases in the

range of 0:65x…HMX†. The measurement results of the

Table 2. Physicochemical Properties of GAP polymer

Chemical formula

C

3.3

H

5.6

O

1.12

N

2.63

Molecular mass

1.27 kg=mol

Flame temperature at 5 MPa

1365 K

Combustion products (mole fractions) at 5 MPa

N

2

C

(s)

CO

CO

2

CH

4

H

2

19.02

29.83

13.93

3.68

1.59

31.52

Table 3. Physicochemical Properties of HMX

Chemical formula

C

4

H

8

N

8

O

8

Density r (610

3

kg=m

3

)

1.90

Flame temperature T

f

(K)

3255

Molecular mass M

f

(kg=kmol)

24.24

Heat of formation (kJ=kg)

‡252.8

Table 4. Physicochemical Properties of GAP=HMX-ECM Tested in

this Study

x (HMX)

0.4

0.6

0.8

Flame temperature at 5 MPa T

f

(K)

1628

1836

2574

Molecular mass M

f

(kg=kmol)

19.2

18.9

21.1

Density r (610

3

kg=m

3

)

1.46

1.58

1.77

Propellants, Explosives, Pyrotechnics 25, 168±171 (2000)

Burning Rate Characterization of GAP/HMX 169

burning rate of GAP=HMX-ECM x…0:8† are shown in Fig. 3

as a function of pressure …p†. The burning rate …r† increases

linearlyin a ln p versus ln r plot.

A typical example of the temperature pro®le in the

combustion wave of GAP=HMX-ECM x…0:8† is shown in

Fig. 4. The gas phase reaction occurs with two-stage zones: at

the ®rst-stage reaction zone the temperature increases rapidly

on and just above the burning surface. At the second-stage

reaction zone the temperature increases also rapidlyat some

distance from the burning surface. In the preparation zone

between the ®rst-stage and the second-stage the temperature

increases veryslowly. At the second-stage reaction zone a

luminous ¯ame is produced. The ¯ame stand-off distance,

L

g

, of x…0:8† decreases linearlyas pressure increases in a log

L

g

versus log p plot as shown in Fig. 3.

The overall reaction rate in the second-stage reaction zone

(preparation zone), o

g

, is determined bythe use of mass

conservation equation as

o

g

L

g

ˆ rr

…1†

where r is the densityof ECM. The reaction rate was cal-

culated

using

the

experimental

values

and

r ˆ 1:77 10

3

kg=m

3

as shown in Fig. 5. It is evident that

the reaction rate increases linearlyas pressure increases in a

log o

g

versus log p plot.

4.2 Analysis of Heat Release Process in the Combustion

Wave

The burning rate is represented by

(1)

r ˆ a

s

fyc

…2†

a

s

ˆ l

g

yc

p

r

…3†

f ˆ …dT ydx†

s

‡

…4†

c ˆ T

s

T

0

Q

s

yc

p

…5†

Figure 2. Burning rate versus mass fraction of HMX, x…HMX†, mixed

within GAP=HMX-ECM at different pressures showing that the

minimum burning rate is observed at about x…0:6†.

Figure 3. Burning rate and ¯ame stand-off distance of GAP=HMX-

ECM x…0:8† as a function of pressure.

Figure 4. A typical example of temperature pro®le in the combustion

wave of GAP=HMX-ECM x…0:8†.

Figure 1. Burning rate characteristics of GAP binder and HMX.

170 Naminosuke Kubota and Ichiro Aoki

Propellants, Explosives, Pyrotechnics 25, 168 ± 171 (2000)

where T is temperature, x is distance, T

s

is burning surface

temperature, Q

s

is heat release at the burning surface, c

p

is

speci®c heat, l

g

is thermal conductivityin the gas phase, and

the subscript

s

‡

is the gas phase at the burning surface. As

shown in Eq. (2), the burning rate increases as f increases

and also c decreases.

In order to determine the values of the parameters in Eq.

(2), the temperature pro®le data measured with micro-

thermocouples were analyzed. The averaged burning

surface temperature, T

s

was determined to be approximately

695 K at 0.5 MPa. The temperature gradient at the burning

surface was also determined to 2.3610

6

K=m at 0.5 MPa.

The heat ¯ux transferred back from the gas phase to the

burning surface was 190 kW=m

2

. In the computations of

f ˆ …dT ydx†

s

‡

and Q

s

, the physical parameter values used

were: l

g

ˆ 8:4 10

5

kW=mK, r ˆ 1:77 10

3

kg=m

3

, and

c

p

ˆ 1:30 kJ=kgK. Substituting the measured values

T

0

ˆ 293 K, T

s

ˆ 695 K, and f ˆ …dT ydx†

s

‡

ˆ 2:3 106

K=m into Eq. (4), Q

s

is determined to be 369 kJ=kg.

It is evident from the measurement results that

f ˆ …dT ydx†

s

‡

and Q

s

playdominant roles on the determi-

nation of the burning rate of GAP=HMX-ECM. The simpli-

®ed analysis of burning rate and temperature sensitivity

described above is applied to determine the burning rate

characteristics of modern solid propellants.

5. Conclusions

The burning rate of GAP=HMX-ECM is dependent on the

mass fraction of HMX mixed within GAP. The gas phase

structure of GAP=HMX-ECM consists of a two-staged

reaction zone: the ®rst-stage zone is on and just above the

burning surface and the second-stage zone stands at some

distance from the burning surface. The luminous ¯ame is

produced bythe reaction at the second-stage zone. Though

the luminous ¯ame zone approaches the burning surface as

pressure increases, the heat ¯ux transferred back from the

®rst-stage zone to the burning surface plays a dominant role

for the determination of the burning rate. The burning rate

increases as the heat ¯ux increases due to the increased

reaction rate in the ®rst-stage reaction zone when pressure

is increased.

6. References

(1) N. Kubota, ``Surveyof Rocket Propellants and Their Combustion

Characteristics,'' in: K. K. Kuo and M. Summer®eld (eds),

``Fundamentals of Solid-Propellant Combustion'', Progress in

Astronautics and Aeronautics, Vol. 90, AIAA, Washington, DC,

1984, Chap. 1.

(2) N. Kubota and T. Sonobe ``Combustion Mechanism of Azide

Polymer,'' Propellants, Explosives, Pyrotechnics 13, 172 ± 177

(1988).

(3) N. Kubota, ``Combustion of Energetic Azide Polymers,'' J.

Propul. Power 11(4), 677 ± 682 (1995).

(Received October 10, 1999; Ms 71/99)

Figure 5. Reaction rate in the preparation zone of GAP=HMX-ECM

x…0:8† showing that the reaction rate increases as pressure increases.

Propellants, Explosives, Pyrotechnics 25, 168±171 (2000)

Burning Rate Characterization of GAP/HMX 171