Indian Journal of Engineering & Materials Sciences
Vol. 18, October 2011, pp. 393-398
Thermal decomposition mechanism of particulate core-shell KClO3-HMX
composite energetic material
Lin-Quan Liaoa,b, Qi-Long Yanc*, Ya Zhenga, Zhen-Wei Songb, Jun-Qiang Lib & Peng Liub
a
College of Astronautics, Northwestern Polytechnical University, Xi'an, China, 710072
b
Xi’an Modern Chemistry Research Institute, Xi'an, China, 710065
c
Institute of Energetic Materials, University of Pardubice, CZ-532 10 Pardubice, Czech Republic
Received 25 April 20011; accepted 11 November 2011
The thermal decomposition mechanism of a newly designed composite material KClO3-HMX (KC-HMX) is investigated
by combined TG-DSC-FTIR technique and T/Jump in-situ thermolysis cell/FTIR (T/Jump FTIR) technique．It is shown that
KC-HMX began to decompose at about 266°C without melting, and the fast stage of mass loss at the temperature range of
268.4~290.1°C is considered to be the result of the thermolysis and complex reactions of KClO3 and HMX with energy release
of 1859 J.g-1, which exceeded that of pure HMX about 40%. It is also shown that CO, CO2, NO2 and H2O were the main
gaseous products. The T/Jump FTIR analysis showed that the competing reactions of N-N and C-N bonds cleavage occurred in
initial stage of HMX decomposition are greatly affected by KClO3. In contrast of pure HMX, there is no CH2O and HCN
detected in its thermolysis products. In presence of electronegative oxygen radical produced by thermolysis of KClO3 oxidized
CH2O and HCN through gas-phase reaction “(NO2+4O2) + (2N2O+5CH2O) → 5NO+3CO+2CO2+5H2O”, which is probably
the dominating reaction, being immediately followed by the decomposition reaction of HMX.
Keywords： TG-DSC-FTIR, HMX, Potassium chlorate, Decomposition mechanism
HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane)
is one of the highly energetic material which can be
used as a major ingredient of solid propellants in such
applications as guns and rocket motors. Therefore,
over the past several decades, many studies have been
devoted to its ignition, decomposition and combustion
behavior1,2. As its known to all, most of the high
explosives including HMX are oxygen-lean
compounds and it always burn without any oxygen
support from the air due to the design of the rocket
motor. The heat releases of these energetic materials
are usually restricted due to incomplete oxidation
reactions. In order to make the oxidation reactions
more complete, the oxygen must be incorporated with
such oxygen-lean energetic materials. One feasible
way is combining an oxygen rich compound with
them such as HMX to form new composite energetic
materials. For instance, ammonium perchlorate (AP)
and potassium chlorate (KClO3) are oxygen rich
compounds. In fact, AP has been widely used as an
ingredient of propellants and high explosives.
Compared with AP, it is more appropriate for KClO3
to be used in pyrotechnic mixtures due to its lower
energy content, and the combustion behavior of the
_________________
*Correcpoding author (E-mail: [email protected])
mixtures could be greatly improved by using this
compound. Undoubtedly, by compatibility and
processability, the potassium chlorate could also be
considered as the oxygen rich compound which could
be used in modified double base propellants
containing HMX.
In order to make the oxygen element more
effective in the propellant, KClO3 should be combined
with HMX at molecular level. In this study, a new
core-shell KC-HMX composite energetic material
was prepared by recrystalization method and a
combined use of different thermoanalytical methods
was taken to characterize its extensive thermal
properties. In fact, a thermogravimetric analyzer
coupled with Fourier transform infrared analysis of
evolving products (TG-FTIR) was also used to
analyze the volatile products relevant to KC-HMX
pyrolysis. Significant efforts have been devoted to
identifying an initial stage of the thermal
decomposition pathways in the condensed phase and
its thermolysis mechanism.
Experimental Procedure
Materials and apparatus
KClO3-HMX composite material (KC-HMX; Mass
ratio of HMX/KC was about 2/1) was prepared and
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INDIAN J. ENG. MATER. SCI., OCTOBER 2011
studied with regard to the thermal behavior and
thermolysis mechanism by using thermogravimetry
(TG), infrared spectroscopy (IR) and differential
scanning calorimetry (DSC). The IR series spectra of
gaseous thermolysis products of KC-HMX were also
recorded and the bands assigned.
Experimental conditions
The particulate KC-HMX（about 35.5 mg）was
analyzed in the PDSC (TA910s, made in America)
instrument was introduced in the static nitrogen
atmosphere, and the pressure was 0.1 MPa. The
sample quantum was about 1.5 mg, and the heating
rate is about 5°C min-1. It was also analyzed in
TG 209 coupled to a TENSOR 27 FTIR spectrometer.
The sample was heated at a rate of 5°C min-1 in a
nitrogen flow of 45 mL·min-1 up to 400°C.
The in-situ T/Jump FTIR instrument (Nexus 870,
Nicolet America) was introduced to detect its
condensed thermolysis products with the data
collection rate of 1.89 cm s-1, the differentiating rate
of 4 cm-1 and the heating rate of about 10°C min-1.
Results and Discussion
Thermal properties of pure compounds
DSC and TG analysis of KClO3 particle
As for the pure potassium chlorate, no thermal
event is observed prior to its melting point near
356°C, at which potassium chlorate undergoes a sharp
endothermic phenomenon including melting. In fact,
there is a long interval of temperature change from
when potassium chlorate undergoes fusion at 356°C
and first rapidly decomposes exothermically at 472°C.
Thus, 472°C corresponds to an ignition temperature.
Such behavior suggests that pure potassium chlorate
is kinetically stable at its melting point. After
complete decomposition of the sample, oxygen and
potassium chloride are produced through
2KClO3 → 2KCl + 3O2
… (R1)
This process is globally recognized and the
reaction mechanism is easy to obtain. As a oxygen
rich compound, KClO3 could provide a large amount
of free oxygen radicals in its thermolysis process
which could be used to oxidize the energetic
intermediates produced by thermal decomposition of
explosives such as HMX.
structures. For its thermolysis process, the bond
breakage of N-N produces NO2, which acts as an
oxidizer. The remaining hydrocarbon fragments act as
fuel components. Previous theoretical studies have
included electronic structure calculations of various
decomposition channels of the gas-phase HMX
molecule. Lewis et al.3 calculated four possible
decomposition pathways of the HMX polymorph:
N-NO2 bond dissociation, HONO elimination, C-N
bond scission, and the concerted ring fission. Based
on the energetics, it was determined that N-NO2
dissociation was the initial mechanism of
decomposition in the gas phase, while they proposed
HONO elimination and C-N bond scission to be
favorable in the condensed phase. The FTIR/DSC
combination technology was introduced by Kimura
and Kubota4 to judge the thermolysis mechanism of
HMX, and it was indicated that the initial step was the
cleavage of N-NO2 and N2O, CH2O, CO, CO2 and
H2O was its main gaseous products.
Recent experiments using thermogravimetric
modulated beam mass spectrometry and isotope
scrambling identified gaseous pyrolysis products such
as H2O, HCN, CO, CH2O, NO, and N2O between
210 and 235°C5-7. Brill suggested two competing
global mechanisms for thermal decomposition8, the
first leading to 4HONO and 4HCN while the second
leading to the formation of 4CH2O and 4N2O. The
above noted experimental work on thermal
decomposition of condensed phase HMX is largely
restricted to relatively low temperature (277°C) and
pressure (0.1 MPa) regimes.
In the condensed phase, however, Farber9 made the
observation
that
alternative
decomposition
mechanisms can occur for thermolysis of pure HMX.
The deposed NO2 fragment can recombine as a
nitride, which is then decompose by breaking the O-N
bond to form NO, or attract weakly hydrogen atoms
and form HONO. The HONO molecules can then
rapidly equilibrate to form water via reaction (R2) as
follows.
2HONO → H2O + NO2 + NO
… (R2)
The results also showed that the formation of
CH2O and N2O could occur preferably from
secondary decomposition of methylenenitramine. The
final thermolysis reaction equation could be
established as (R3):
Thermal decomposition mechanism of HMX
HMX as a nitroamine is characterized by -N-NO2
chemical bonds that are attached to hydrocarbon
HMX → CH2O + N2O + CO + CO2 + N2 + H2O+ NO
… (R3)
Liao et al.: KClO3-HMX COMPOSITE ENERGETIC MATERIAL
(f(α) =1/2α; one dimension diffusing chemical
reaction; Ea = 165.1 kJ.mol-1 [10]
The reaction models above (conditions: multiple
heating rate non-isothermal kinetics; Coats-Redfern
method; 20% < α (conversion) < 80%) could
accommodate the thermal decomposition of HMX, as
well as subsequent reactions in the foam layer.
Thermal behavior of KC-HMX composite material
TG and DSC analysis
TG and DSC curves for HMX and KC-HMX
(Figs 1 and 2) show a residue-free decomposition. By
applying the TG and DSC method, only one
decomposition steps can be clearly observed and
quantitatively described for both HMX and
KC-HMX.
As shown in Fig. 1 and Table 1, the TG curve of
HMX in dynamic nitrogen atmosphere at a heating
rate of 5°C min-1 showed one stage of very fast mass
loss of 96.6% which is attributed to the thermolysis of
HMX. The mass change occurred only in the
temperature range of 251.3 - 301.3°C with energy
release of 1239 J.g-1. The thermal decomposition of
KC-HMX composite material is similar to HMX, and
there is one stage of more fast mass loss of 72.1% in
395
its thermolysis process. The fast stage of mass loss is
considered to be the result of the thermolysis and
complex reactions of KClO3 and HMX at the
temperature range of 268.4~290.1°C with energy
release of 1858 J.g-1. As shown in Fig. 2 and Table 1,
the thermolysis process of KC-NC/NG could be
divided into as two stages, the thermolysis of “NG +
KClO3” with a heat release of 1755 J.g-1 and
thermolysis of NC with 499.7 J.g-1. The heat release
from the thermolysis of NG made the thermolysis of
KClO3 take place at a lower temperature range. It was
also indicated that the residue of pure HMX
decomposition is less than 3.4%, while KC-HMX
shows a residue of 27.9% which was probably due to
the formation of KCl. However, when the initial mass
ratio of HMX/KC is 2/1, the calculated residue
(KCl and remain of HMX thermolysis) should be
22.7%. Hence the actual percentage of KC in the
composite might be less than 1/2 due to the slight
mass loss of HMX during the preparation process.
Besides, as shown in Fig. 2, KClO3 has a
significant influence on the thermolysis behavior of
HMX. In addition to lowering of the thermolysispoint of HMX, a more intensive exothermic peak
could be observed at lower temperature for KC-HMX
Table 1TG and DSC results of HMX, KC-HMX and KC-NC/NG under pressure of 0.1 MPa
samples
Tfw,°C
HMX
KC-HMX
KC-NC/NG
P1
P2
251.3-290.1
268.4-301.3
-
Parameters of TG
La, (%)
TVmax °C
96.6
72.1
-
285.4
272.9
-
Mass remain
T0, °C
3.4 %
27.9 %
-
251.3
268.4
188.4
352.1
Parameters of DSC
Tp, (%)
△Hd J·g-1
286.6
274.7
204.9
333.8
1239
1859
1755
499.7
*Note: Tfw– temperature range; La– percentage of mass loss; TVmax– temperature of maximum mass loss rate; heating rate: 5°C.min-1
Fig. 1 Characteristic TG curve of KClO3-HMX and pure HMX
(heating rate 5°C/min; sample mass about 2 mg; nitrogen flow of
45 mL·min-1)
Fig. 2 Characteristic DSC curve of KClO3-HMX, HMX and
KC-NC/NG (heating rate 5°C /min; sample mass 2.15 mg)
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INDIAN J. ENG. MATER. SCI., OCTOBER 2011
composite material than that of pure HMX. On one
hand, the presence of KClO3 lowered the peak
temperature of HMX, and the oxygen produced by
KClO3 accelerated the thermolysis velocity of HMX
meanwhile made the exothermic reaction more
complete, the heat release of HMX was thereby
enhanced. On the other hand, there are phase changes
for pure HMX at 219.2°C and 248.3°C, while HMX
decomposed rapidly before change of phase in
presence of KClO3.
In order to clarify how the heat release of HMX
was enhanced by KClO3 and which are the main
chemical reactions for the extra heat release, the
detailed chemical reaction pathways should be
brought forward. Therefore, the evolved gases and
condensed products of KC-HMX decomposition were
further analyzed.
The change of concentration with the temperature
for the main gaseous products of KC-HMX and pure
HMX mentioned in (R3) and (R4) are also shown in
Figs 4 and 5. As shown in Fig. 5, the maximum
concentrations for all the gaseous products appeared
at about 287-288℃ for the pure HMX, which are a
little lag compared with the peak temperature
(285.6°C) of the HMX thermolysis.
But for decomposition of KC-HMX, the maximum
concentration for all the gaseous products appeared at
about 290°C which is higher than that of pure HMX,
even though the peak temperature of its
decomposition is lower than that of pure HMX. This
might be caused by the difference of the sense organ
for different test facility. However, there is no CH2O
and N2O for decomposition of KC-HMX, and the
Evolved gas analysis (EGA)
With FTIR spectroscopic EGA, the gaseous
decomposition products of KC-HMX can be
identified. Figure 3 shows the IR gas-phase spectra
which were obtained online during linear heating of
KC-HMX.
As shown in Fig. 3, the main gaseous products for the
decomposition of KC-HMX should be CO2, CO, NO2,
and H2O, and about 72.1% mass loss occurred during
the thermolysis process. The possible decomposition
pathway of KC-HMX pyrolysis could be described as
reactions (R4)-(R6). However, mass spectrum (Fig. 5)
show that N2O, CH2O, CO2 and H2O were detected as
the main thermal decomposition gases of HMX.
HMX + KClO3 → KCl + CH2O +
N2O + NO + HCN + O2
CH2O + N2O + HCN + O2 → NO +
CO + CO2 + H2O
NO + CO + O2 → NO2 + CO2
… (R4)
Fig. 4 EGA profiles of the KClO3-HMX decomposition
gases H2O (2000-1300 cm-1), CO (2239-2173 cm-1), CO2
(2390-2280 cm-1) and NO2 (1210-1510 cm-1)
… (R5)
… (R6)
Fig. 3 FTIR of the main gaseous products of KClO3-HMX at
temperature of 280°C
Fig. 5 Fragments concentration curves of gaseous products for
pure HMX thermolysis
Liao et al.: KClO3-HMX COMPOSITE ENERGETIC MATERIAL
curve of CO2 concentration rebounded after its first
tiptop at the points of 381.3 and 573.9°C. This should
be caused by continued oxidation reactions in
presence of oxygen, such as:
HCN + O2 → CO + NO (or NO2) + H2O
CH2O + O2 → NO (or NO2) + CO + H2O
CO + O2 → CO2
… (R7)
… (R8)
… (R9)
The above mentioned chemical reactions are
exothermic which could enhance the whole heat
release of KC-HMX decomposition which is
confirmed in its DSC traces (Fig. 2). The EGA/MS
profiles of gaseous products confirm its degradation
due to consecutive gas-phase reactions for HMX and
a large amount of data with regard to its thermal
decomposition mechanism are available.
Thermal decomposition mechanism of KC-HMX composite
material
The thermal decomposition mechanism of
KC-HMX could not be clarified just based on its
gaseous thermolysis products. The autocatalytic
acceleration of the thermal decomposition process of
KC-HMX is further confirmed by in-situ T/Jump
FTIR measurements (Figs 6 and 7). It was shown that
there is a quantitative change for the main peak
intensity at about 266°C, following which some new
peaks (905-1006 cm-1) appeared, which are
characteristic peaks of N=O. In thermolysis of
potassium chlorate, the Cl- is adequate to exhaust K+
producing KCl, while the strong electronegative O2ion was produced to react with other fuel free
radicals11.
In order to make sure what had happened in this
thermolysis process, the initial IR spectra and
intermediate condensed products were collected and
Fig. 6 In-situ T/Jump FTIR spectra of KC-HMX pyrolysis
(heating rate 5°C/min)
397
identified which is shown in Fig. 7. It was observed
that there is a broad peak at about 3630-2950 cm-1
which is the characteristic peak of crystal H2O, and it
disappeared at about 110°C at which the water would
sublimate.
Generally, there are two competing pathways for
HMX decomposition. One is aroused by a cleavage of
C-N bond while the other is started with a uniform
cleavage of N-N bonds. However, in presence of
potassium chlorate, the mechanism of HMX
decomposition is influenced by high concentration of
electronegative O2- ion. Based on the investigation
made by Palopoli12, for pure HMX, when the pressure
was elevated, more CH2O was produced by cleavage
of C-N, and the decomposition mechanism is
dominated by cleavage of C-N. In this way, more fuel
was produced and the heat release is thereby
enhanced at higher pressure. It would be in favor of
C-N cleavage when potassium chlorate was used as
oxidizer. If the cleavage of C-N were considered as
the initial way of its decomposition, an intermediate
product
hydroxide
methyl
methacylamine
(HMFA, HOCH2NHCHO) would be formed
according to Palopoli et al.13. The dominant reaction
in this oxidation stage should be described as (R10).
HMX → HOCH2NHCHO → 4CH2O + 4N2O … (R10)
Without potassium chlorate, NO2 and N2O would
act as oxidizers and CH2O as fuel component. When
electronegative O2- ion was produced by potassium
chlorate, both N2O and CH2O would act as fuel,
minor NO2 and the entire O2- ion would act as
oxidizers. Some other gaseous products of HMX
Fig. 7 FTIR of the condensed intermediate products of
KClO3-HMX
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INDIAN J. ENG. MATER. SCI., OCTOBER 2011
decomposition would also be oxidized by oxygen
through the reactions of (R6)-(R9). Since nitrogen
dioxide and oxygen reacts quite rapidly with
formaldehyde, the overall gas phases reaction
(NO2 + 4O2) + (2N2O + 5CH2O) → 5NO
+ 3CO + 2CO2 + 5H2O
… (R11)
Is probably the dominating reaction, being
immediately followed by the decomposition reaction of
HMX. Therefore, the predicted thermolysis pathways of
KC-HMX could be established as shown in Scheme 1.
The reaction pathway was based on a cleavage of
N-N bond releasing NO2 as an initial decomposition
product which is conventionally observed for organic
nitroamines.
Conclusions
The thermal decomposition mechanism of a newly
designed composite material KC-HMX was
investigated by combined TG-DSC-FTIR techniques
and T/Jump FTIR analysis．The following
conclusions could be made:
(i) KC-HMX began to decompose at 274.7°C
without melting, and the fast stage of mass loss at the
temperature range of 268.4~290.1°C is considered to
be the result of the thermolysis and complex reactions
of KClO3 and HMX with energy release of 1859 J.g-1,
which exceeded that of pure HMX about 40%;
(ii) The CO, CO2, NO2 and H2O were detected as
the main gaseous products of KC-HMX
decomposition, and the competing reactions of N-N
and C-N bonds cleavage occurred in initial stage of
HMX decomposition are greatly affected by KClO3;
(iii) In contrast of pure HMX, there is no CH2O
and HCN was detected in its thermolysis products. In
presence of electronegative oxygen radical produced
by thermolysis of KClO3 oxidized CH2O and HCN
through
gas-phase reaction “(NO2+4O2) +
(2N2O+5CH2O) → 5NO + 3CO + 2CO2 + 5H2O”,
which was probably the dominating reaction, being
immediately followed by the decomposition reaction
of HMX.
Acknowledgement
The authors gratefully acknowledge helpful
discussion with Dr Liu Zi-Ru and Dr Li Ji-zhen of
Xi’an Modern Chemistry Research Institute. Special
thanks to Wang Xiao-Hong, who performed the DSC
and TG experiments.
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