168 Propellants, Explosives, Pyrotechnics 25 168Ä…171 (2000)
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)
 Â
Charakterisierung der Abbrandrate von energetischen Komposit- Caracterisation des vitesses de combustion de materiaux compo-
  Á Á
Materialien mit GAP=HMX sites energetiques a base de GAP/octogene
   Á
Die Abbrandrate von energetischen Materialien aus Glycidylazid- La vitesse de combustion de materiaux energetiques a base de
Á Â Â Â Â
polymer (GAP) und HMX-Partikeln wurde charakterisiert, um den polyglycidylazide (GAP) et de particules d'octogene a ete caracterisee
È È È Â
Warmefreisetzungsprozeû wahrend des Abbrandes zu klaren. Das en vue d'expliquer le processus de degagement de chaleur pendant la
È Á Â Â Ã Á
energetische Polymer GAP brennt selbststandig. Die Zugabe von combustion. Le polymere energetique GAP brule de maniere auton-
È È Á Â
HMX erhoht die Flammentemperatur und andert das Abbrandverhal- ome. L'addition d'octogene augmente la temperature de Żamme et
Â
ten. Experimentelle Beobachtungen deuten auf eine zweistu®ge Gas- modi®e le mode de combustion. Des observations experimentales
 Â
phasenreaktion hin: Die erste Reaktionsstufe kontrolliert die laissent supposer qu'une reaction de phase gazeuse en deux etapes a
Á Â Ã
Abbrandrate. Eine sichtbare Flamme bildet sich erst in der zweiten lieu: la premiere etape de combustion controle la vitesse de combus-
È Á
Stufe aus. Der WarmeÅ»uû von der ersten Reaktionszone zur tion. Ce n'est qu'au cours de la deuxieme phase qu'une Å»amme visible
È È Á Â
AbbrandoberŻache erhoht sich mit steigendem Druck, dagegen ist die se forme. Le Żux de chaleur de la premiere zone de reaction vers la
È È
Warmefreisetzungsrate an der AbbrandoberŻache vom Druck unab- surface de combustion augmente avec la pression ; en revanche, le
È Â Â
hangig. degagement de chaleur de la surface de combustion ne depend pas de
la pression.
Summary It has been reported that the burning rate of GAP=HMX-
ECM depends largely on the mass fraction of HMX mixed
The burning rate of the energetic materials composed of glycidyl
within GAP, x (HMX). In order to characterize the burning
azide polymer (GAP) and HMX particles was characterized in order to
rate of this class of ECM the heat release process was
elucidate the heat release process during burning. Since GAP is an
determined by the measurement of the Żame structure of
energetic polymer and burns by itself, the addition of HMX increases
the Żame temperature and alters the burning rate characteristics. GAP=HMX-ECM.
Experimental observations indicate that the gas phase structure con-
sists of a two-staged gas phase reaction: the burning rate is controlled
by the ®rst-stage reaction zone and the ®nal Å»ame is formed at the
second-stage reaction zone. The heat Żux 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 2. Physicochemical Properties of GAP and HMX
unchanged when pressure is increased.
Glycidyl azide polymer (GAP) is a unique polymeric
material characterized with a 2N3 chemical bond as shown
2
in Table 1. The decomposition of 2N3 bond generates a
2
signi®cant heat without oxidation reaction. The bond break-
1. Introduction
age of2N3 is the initial step of the reaction including melting
2
and gasi®cation processes. The formation of gaseous frag-
Glycidyl azide polymer (GAP) is a unique energetic
ments occurs when the GAP surface is heated, and numerous
material that burns with a relatively high burning rate even
chemical species are formed from the reacting surface. The
though the energycontent within a unit mass of GAP is not so
heat transfer process from the high-temperature zone to the
high. Cyclotetramethylene tetranitramine (HMX) is a crys-
reacting surface determines the burning rate of GAP. In this
talline energetic material composed of a stoichiometrically
studythe combustion mechanisms of the ECM made of GAP
balanced molecule. When HMX particles are mixed with
GAP, an energetic material, the so-called ``GAP=HMX
Table 1. Chemical Properties of GAP
energetic composite material (ECM)'' is obtained.
Chemical formula C3H5ON3
Molecular mass 1.98 kg=mol
Heat of formation 957 kJ=kg
* Corresponding author;
Flame temperature at 5 MPa 1465 K
e-mail: [e-mail]
# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0721-3115/00/0409 Ä… 168 $17.50‡:50=0
Propellants, Explosives, Pyrotechnics 25, 168Ä…171 (2000) Burning Rate Characterization of GAP/HMX 169
and crystalline particles are examined in order to gain a wide reaction by NO2. When HMX is mixed with GAP, HMX
range of burning rate characteristics. acts as an energy addition on the combustion products
GAP monomer is synthesized by replacing C2Cl bonds of because no excess oxidizer components are available.
2
polyepichlorohydrin with C2N3 bonds. The three nitrogen
2
atoms in the form of N3 were attached linearlywith ionic and
covalent bonds. The bond energy of 2N3 is reported to be 3. Experimental
2
378 kJ per azide group(1 Ä… 3). GAP monomer was polymerized
with the terminal 2OH groups with hexamethylene diiso- 3.1 GAP/HMX-ECM Samples Tested in this Study
2
cyanate (HMDI) and crosslinked with trimethylolpropane
(TMP). The GAP polymer has one 2N3 bond in every The mass fraction of HMX mixed within GAP,x (HMX),
2
monomer unit and the bond energy is responsible for the was 0.8 used for the examination of the combustion wave
positive heat of formation. Thus, the GAP polymer was structure. The physicochemical properties of GAP=HMX-
chosen as a fuel component that acts also as an adhesive ECM are shown in Table 4.
binder of crystalline particles to formulate ECM. The
physicochemical properties of GAP and GAP polymer are
shown in Table 1 and Table 2, respectively. 3.2 Burning Rate and Combustion Wave Structure
The adiabatic Żame temperature of the GAP polymer is
1365 K at 5 MPa and large amounts of C(s), H2, and N2 are In order to understand the combustion mechanisms of
formed as combustion products. It should be noted that the GAP=HMX-ECM formulated in this study the burning rate
combustion products are mostly fuel components and very and combustion wave structure were measured. Strand-
small amounts of CO2 and H2O are formed. shaped samples (7 mm67 mm in cross-section and 70 mm
Table 3 shows the physicochemical properties of HMX. in length) were made. The ignition of the samples was
HMX is a cyclic nitramine represented by N2N3 groups. The conducted using an electrically heated chromium ®ne-wire
2
combustion products of HMX are stoichiometrically set on the top end of each sample.
balanced and the adiabatic Żame temperature is 3255 K at The burning rate of these materials was measured using a
5 MPa. chimney-type strand burner that was pressurized by nitrogen.
The overall initial decomposition reaction is The Żame structure was observed through a transparent
window attached on the side of the burner. The regressing
3CH2NNO2Ä…4 ! 4NO2 ‡ 4N2O ‡ 6N2 ‡ 12CH2O
burning surface of the samples was recorded bya high-speed
and produces oxidizer and fuel fragments. Since nitrogen
video camera through the window. The measurements of the
dioxide reacts quite rapidly with formaldehyde, the gas
temperature pro®les in the combustion wave were conducted
phase reaction
byusing ®ne thermocouples which were threaded through the
samples in order to determine the heat release process of this
7NO2 ‡ 5CH2O ! 7NO ‡ 3CO ‡ 2CO2 ‡ 5H2O
class of ECM.
is probably the dominating reaction immediately followed
by the decomposition reaction. The reaction product of NO
oxidizes the remaining fuel fragments such as H2 and CO.
4. Results and Discussion
However, the oxidation reaction by NO is reported to be
slow to produce the ®nal combustion products. The dom-
4.1 Burning Rate Characteristics and Combustion Wave
inating gas phase reaction on the burning rate of HMX is the
Structure
Figure 1 shows the burning rates of GAP binder and
HMX(1,2). The burning rate of GAP binder is higher than
Table 2. Physicochemical Properties of GAP polymer
that of HMX even though the Żame temperature of GAP
Chemical formula C3.3H5.6O1.12N2.63
binder is 1890 K lower than that of HMX (see Tables 2 and
Molecular mass 1.27 kg=mol
3). The bunring rate of GAP=HMX-ECM is shown in Fig. 2
Flame temperature at 5 MPa 1365 K
as a function of xHMXÄ… at different pressures. The burning
Combustion products (mole fractions) at 5 MPa
rate decreases as xHMXÄ… increases in the range of
N2 C(s) CO CO2 CH4 H2
19.02 29.83 13.93 3.68 1.59 31.52
xHMXÄ…50:6 and increases as xHMXÄ… increases in the
range of 0:65xHMXÄ…. The measurement results of the
Table 3. Physicochemical Properties of HMX Table 4. Physicochemical Properties of GAP=HMX-ECM Tested in
this Study
Chemical formula C4H8N8O8
Density r (6103 kg=m3) 1.90 x (HMX) 0.4 0.6 0.8
Flame temperature Tf (K) 3255 Flame temperature at 5 MPa Tf (K) 1628 1836 2574
Molecular mass Mf (kg=kmol) 19.2 18.9 21.1
Molecular mass Mf (kg=kmol) 24.24
Heat of formation (kJ=kg) ‡252.8 Density r (6103 kg=m3) 1.46 1.58 1.77
170 Naminosuke Kubota and Ichiro Aoki Propellants, Explosives, Pyrotechnics 25, 168 Ä… 171 (2000)
burning rate of GAP=HMX-ECM x0:8Ä… are shown in Fig. 3
as a function of pressure pÄ…. The burning rate rÄ… increases
linearly in a ln p versus ln r plot.
A typical example of the temperature pro®le in the
combustion wave of GAP=HMX-ECM x0:8Ä… 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 very slowly. At the second-stage reaction zone a
luminous Żame is produced. The Żame stand-off distance,
L , of x0:8Ä… decreases linearlyas pressure increases in a log
g
L versus log p plot as shown in Fig. 3.
g
The overall reaction rate in the second-stage reaction zone
Figure 1. Burning rate characteristics of GAP binder and HMX.
(preparation zone), og, is determined by the use of mass
conservation equation as
ogL ˆ rr 1Ä…
g
where r is the density of ECM. The reaction rate was cal-
culated using the experimental values and
r ˆ 1:77 103 kg=m3 as shown in Fig. 5. It is evident that
the reaction rate increases linearly as pressure increases in a
log og versus log p plot.
4.2 Analysis of Heat Release Process in the Combustion
Wave
The burning rate is represented by(1)
r ˆ asfyc 2Ä…
as ˆ lgycpr 3Ä…
f ˆdT ydxÄ…s 4Ä…
‡
Figure 2. Burning rate versus mass fraction of HMX, xHMXÄ…, mixed
within GAP=HMX-ECM at different pressures showing that the
c ˆ Ts T0 Qsycp 5Ä…
minimum burning rate is observed at about x0:6Ä….
Figure 3. Burning rate and Å»ame stand-off distance of GAP=HMX- Figure 4. A typical example of temperature pro®le in the combustion
ECM x0:8Ä… as a function of pressure. wave of GAP=HMX-ECM x0:8Ä….
Propellants, Explosives, Pyrotechnics 25, 168Ä…171 (2000) Burning Rate Characterization of GAP/HMX 171
It is evident from the measurement results that
f ˆdT ydxÄ…s and Qs play dominant 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 Żame is
produced by the reaction at the second-stage zone. Though
Figure 5. Reaction rate in the preparation zone of GAP=HMX-ECM
the luminous Żame zone approaches the burning surface as
x0:8Ä… showing that the reaction rate increases as pressure increases.
pressure increases, the heat Żux 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
where T is temperature, x is distance, Ts is burning surface
increases as the heat Żux increases due to the increased
temperature, Qs is heat release at the burning surface, cp is
reaction rate in the ®rst-stage reaction zone when pressure
speci®c heat, lg is thermal conductivityin the gas phase, and
is increased.
the subscript is the gas phase at the burning surface. As
s‡
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.
6. References
(2), the temperature pro®le data measured with micro-
(1) N. Kubota, ``Survey of Rocket Propellants and Their Combustion
thermocouples were analyzed. The averaged burning
Characteristics,'' in: K. K. Kuo and M. Summer®eld (eds),
surface temperature, Ts was determined to be approximately
``Fundamentals of Solid-Propellant Combustion'', Progress in
695 K at 0.5 MPa. The temperature gradient at the burning
Astronautics and Aeronautics, Vol. 90, AIAA, Washington, DC,
surface was also determined to 2.36106 K=m at 0.5 MPa.
1984, Chap. 1.
(2) N. Kubota and T. Sonobe ``Combustion Mechanism of Azide
The heat Żux transferred back from the gas phase to the
Polymer,'' Propellants, Explosives, Pyrotechnics 13, 172 Ä… 177
burning surface was 190 kW=m2. In the computations of
(1988).
f ˆdT ydxÄ…s and Qs, the physical parameter values used
‡
(3) N. Kubota, ``Combustion of Energetic Azide Polymers,'' J.
were: lg ˆ 8:4 10 5 kW=mK, r ˆ 1:77 103 kg=m3, and
Propul. Power 11(4), 677 Ä… 682 (1995).
cp ˆ 1:30 kJ=kgK. Substituting the measured values
T0 ˆ 293 K, Ts ˆ 695 K, and f ˆdT ydxÄ…s ˆ 2:3 106
‡
K=m into Eq. (4), Qs is determined to be 369 kJ=kg. (Received October 10, 1999; Ms 71/99)