The effect of glycidyl azide polymer on the stability and explosive properties of different interesting nitramines

Article abstract of DOI:10.1039/c8ra02994f

Preparation of glycidyl azide polymer (GAP) and its influence on the stability and explosive properties of polymer bonded explosives (PBXs) based on several cyclic nitramines, namely β-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (β-HMX), 1,3,5-trinitro-1,3,5-triazinane (RDX), ?-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (?-CL-20) and cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole (BCHMX) are discussed. Impact and friction sensitivity were determined. Combustion heat and detonation velocity of the studied samples were measured. The detonation parameters were obtained by the EXPLO 5 thermodynamic code. The compatibility between the energetic polymeric matrix and the studied nitramines was discussed following a vacuum stability test. The relationship between performance and sensitivity was studied in comparison with literature HTPB compositions. The results showed that the GAP matrix increased both the detonation velocities of its PBXs by more than 500 m s^-1 and the heat of explosion by nearly 1.13-1.16 times in comparison to PBXs based on HTPB for each individual explosive. The compatibility of BCHMX to the GAP matrix seems to be better than that of CL-20/GAP.

Full text of DOI:10.1039/c8ra02994f

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PAPER
The effect of glycidyl azide polymer on the stability
and explosive properties of different interesting
nitramines
Cite this: RSC Adv., 2018, 8, 17272
ab
b
a
*
and Svatopluk Zeman
Ahmed K. Hussein,
Ahmed Elbeih
Preparation of glycidyl azide polymer (GAP) and its influence on the stability and explosive properties of
polymer bonded explosives (PBXs) based on several cyclic nitramines, namely b-1,3,5,7-tetranitro-1,3,5,7-
tetrazocane (b-HMX), 1,3,5-trinitro-1,3,5-triazinane (RDX), 3-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-
hexaazaisowurtzitane (3-CL-20) and cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole (BCHMX)
are discussed. Impact and friction sensitivity were determined. Combustion heat and detonation velocity
of the studied samples were measured. The detonation parameters were obtained by the EXPLO 5
thermodynamic code. The compatibility between the energetic polymeric matrix and the studied
nitramines was discussed following a vacuum stability test. The relationship between performance and
sensitivity was studied in comparison with literature HTPB compositions. The results showed that the
GAP matrix increased both the detonation velocities of its PBXs by more than 500 m sꢀ1 and the heat of
explosion by nearly 1.13–1.16 times in comparison to PBXs based on HTPB for each individual explosive.
The compatibility of BCHMX to the GAP matrix seems to be better than that of CL-20/GAP.
Received 7th April 2018
Accepted 27th April 2018
DOI: 10.1039/c8ra02994f
rsc.li/rsc-advances
velocity is 9800 m sꢀ1.8 The high price of 3-CL-20 is the main
problem which limits its application. These advanced explo-
1. Introduction
sives have been studied in comparison with traditional nitr-
amines by several researchers.1–4,9 The detonation parameters of
these explosives bonded by several polymeric matrices were
determined.3,4,10 Elbeih et al.10–16 have studied the explosive
properties of different plastic bonded explosives (PBXs) based
on several nitramines including BCHMX. In addition, a thermo-
analytical study of these PBXs was conducted by Yan et al.17–19
The penetration performance of the advanced nitramines has
been studied,20–22 while different relationships have been pre-
sented and discussed by Zeman et al.23–26 based on the reactivity
and sensitivity of the studied PBXs. BCHMX as part of a low
sensitivity composition was studied by Hussein et al.27–29 From
the previously mentioned research, it was concluded that the
polymeric matrices have a signicant inuence on the different
explosive properties of the studied PBXs. Glycidyl azide polymer
(GAP) is a high energy polymer which has different applications,
especially in composite solid rocket propellants (CSRP).30 It can
be used as a binder or plasticizer for different types of rocket
propellant and PBXs.31–33 The synthesis, structure and thermal
behavior of the GAP binder system have been discussed in
several publications.29,34–36 From ref. 29, 37 and 38, it was
concluded that the GAP polymeric matrix is an interesting
system which could have different applications with energetic
materials. However, studies on the explosive properties of
advanced explosives bonded by the GAP polymeric system and
their compatibility remain limited in the literature. In this
work, the effect of the GAP binder system on the explosive
The discovery of new energetic compounds and mixtures is
a motivation for many scientists and researchers. However,
Licht had proved1 that increasing of the explosive performance
is accompanied by increasing of its sensitivity.1–4 He has also
stated that this characteristic cannot be proved by any theory. It
is also important to note that many factors, including the
density of the crystals, affect the performance of the explosives.
Increasing the density of the crystals can be done gradually
from cyclic to polycyclic and cage structures. b-1,3,5,7-
tetranitro-1,3,5,7-tetrazocane (b-HMX) and 1,3,5-trinitro-1,3,5-
triazinane (RDX) are well known nitramines which have been
used in several applications for more than 70 years. During the
last few decades, several advanced nitramines have been re-
ported and attracted interest from researchers, such as 3-
2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (3-
CL-20) and cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imid-
azole (BCHMX). Our research group successfully prepared
BCHMX by a simple method, as described in ref. 5 and 6. The
performance of BCHMX is high in comparison to the perfor-
mance of traditional explosives, and its sensitivity is at same
level as pentaerythritol tetranitrate (PETN).7 3-CL-20 is a prom-
ising explosive; at a density of 2.04 g cmꢀ3 its detonation
aInstitute of Energetic Materials, Faculty of Chemical Technology, University of
Pardubice, 53210 Pardubice, Czech Republic
bMilitary Technical College, Kobry Elkobbah, Cairo, Egypt. E-mail: elbeih.czech@
gmail.com
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properties of several interesting explosives such as RDX, HMX,
BCHMX and 3-CL-20 is presented. In addition, the compatibility
of the GAP binder system with the selected explosives is
discussed.
2. Experimental work
2.1. Materials
Scheme 1 GAP preparation with diethylene glycol as the starter.
BCHMX is a bright white crystal. Similar shape and optimal
particle size were obtained by recrystallization using the solvent/
antisolvent (acetone/heptane) technique. HMX (b-modication,
with particle size close to Class 3) was imported from Russia,
RDX (with particle size close to Class 5) was a product of Dyno
Nobel, and 3-CL-20 (3-modication) was prepared in an Explosia
pilot plant in the Czech Republic. The chemical structures of the
studied explosives are shown in Fig. 1.
condenser. 190 g of sodium azide was added into this solution,
and then the mixture was mixed and heated to 125 ꢁC by an oil
bath for 11 hours. Then 1250 ml of water was added and mixed
ꢁ
for 1 hour at 50 C, followed by cooling to room temperature.
500 ml of dichloromethane was added and the phases separated
using a separating funnel. The organic phase containing GAP
was neutralized by washing with water. The obtained washed
phase was desiccated by the addition of Na2SO4, then ltrated,
and evaporated under vacuum to obtain 228 g of GAP. The
theoretical yield was 243 g, and thus the yield was 93%.
The number average molecular weight (Mn) was 1757 kg
molꢀ1 and the weight average molecular weight (Mw) was 2459
kg molꢀ1. The polydispersity index (PDI) is 1.4. These results
were obtained aer analysis by GPC against polystyrene stan-
dards. The viscosity of GAP was 5000 cP at 25 ꢁC, the measured
value of mg equivalents of OH per g of GAP was 0.825 mg eq.
per g, and the measured nitrogen content was 40.3%.
2.2. Preparation of GAP polymer and its PBXs
GAP was produced in our laboratories in two consecutive steps.
Scheme 1 shows the GAP preparation method. The rst step was
the preparation of poly(epichlorohydrin) (PECH). The second
step was the preparation of GAP from the PECH polymer.
First step. 9 g of diethylene glycol (0.087 mol) was added to
360 ml of dichloromethane in a ask connected with a ther-
mometer and CO2 inlet. 5 ml of cationic initiator BF3$OEt2
(boron triuoride diethyletherate) was then injected. Aer 30
minutes of mixing at room temperature the solution was cooled
by a mixture of water, ice, and NaCl crystals to 0 ꢁC, aer which
the slow dropwise addition (0.1 ml minꢀ1) of epichlorohydrin
(ECH) (2.46 mol) was commenced. These operations and our
own polymerization were conducted under a CO2 atmosphere;
the gas was sourced from unprompted evaporation of “dry ice”
(solid CO2). This dropping was continued for more than 10
The GAP polymeric matrix was fabricated with a BRABENDER
plastograph. Water ow was circulated through the mixer jacket
ꢁ
at 40 ꢃ 2 C. Mixing of the liquid ingredients, GAP, and dioctyl
adipate (DOA) [obtained from ACROS Organics] was done for 20
minutes under vacuum to remove trapped air. The solid ingre-
ꢁ
dient (pure explosive) was added at 40 C over 30 minutes, fol-
lowed by the addition of 1,1,1-tris(hydroxymethyl)ethane (THME)
as a cross linking agent (obtained from Merck & Co.), and the
curative hexamethylene diisocyanate (HMDI) (obtained from
ACROS Organics). The addition was performed at 55 ꢁC and
mixed for another 30 minutes under vacuum. Aer completion of
the mixing process, the mixture was cast under vacuum and
cured at 60 ꢃ 2 ꢁC for seven days. The ratio of NCO/OH was 0.85.
The masses of the plasticizer and the cross linking agent were 7.5
and 5.0 wt% of the total mass of the polymeric matrix respec-
tively. The weight percentage composition of the GAP PBXs was
16 wt% GAP binder, and 84 wt% explosive. The PBXs are desig-
nated as RDX/GAP, HMX/GAP, BCHMX/GAP, and CL-20/GAP.
ꢁ
hours at 0 to +3 C in order to ensure a constant temperature
and to minimise the amount of ECH in the reaction mixture.
Then the mixture was le to reach room temperature. The
mixing process was continued for another 6 hours. Then 700 ml
of distilled water was added with ꢂ5 g of Na2CO3 to the mixture,
and was mixed for 1 hour. The organic phase containing PECH
was extracted into methylene chloride using a separating fun-
nel, and was washed several times by distilled water until the
pH was neutral. The washed organic phase was dried by adding
sodium sulphate (Na2SO4), then ltered, and evaporated under
vacuum to obtain 230 g of poly-epichlorohydrin.
Second step. 230 g of poly-epichlorohydrin (PECH) was dis-
solved in 800 ml of dimethylformamide (DMF) in a three-
necked ask tted with a thermometer, CO2 inlet, and water
2.3. Elemental analysis
C, H, and N elements of the prepared PBXs were measured using
Fisons EA-1108 CHNS–O elemental analyzer. Recalculation of
the matched nitrogen content was used to obtain the formula for
the individual explosives. The formulas were used as input data
to calculate detonation parameters using the EXPLO 5 code.
2.4. Heat of combustion determination
A high pressure bomb calorimeter, model BCA 500, obtained
from OZM Research, Czech Republic, was used to determine the
Fig. 1 Structural formulas of the studied cyclic nitramines.
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heat of combustion of BCHMX/GAP, as well as the other PBXs. charge, with the rst sensor being placed at a distance of 50 mm
The sample was inserted into the bomb and lled with an excess from the surface containing the detonator. Detonator no. 8 was
of oxygen,39 and then ignited. The enthalpy of formation was used as an initiator. Each sample was measured three times and
obtained aer calculation of the output data obtained from the the mean values (max. ꢃ76 m sꢀ1) are reported in Table 2.
instrument. The enthalpy of formation was used for the calcu-
lation of the detonation characteristics of the studied samples.
2.7. Calculation of the detonation properties
The theoretical detonation properties (detonation velocity, D,
heat of detonation, Q, and detonation pressure, P) of the
studied compositions, in addition to the individual nitramines,
were calculated by the EXPLO5 code version 5.04.42 The
following BKWN set of parameters was used: a ¼ 0.5, b ¼ 0.176,
k ¼ 14.71, Q ¼ 6620.42 The heat of explosion is the heat released
at a constant volume of explosion and is determined by sub-
tracting the heats of formation of the explosive (reactants) from
the sum of the heats of formation of the detonation. The
theoretical calculation and the error between the experimental
results are listed in Table 2.
2.5. Sensitivity to impact and friction
A BAM impact sensitivity instrument with an exchangeable drop
weight was used to perform standard impact tests;40 the volume
of each tested sample was 50 mm3, and different weights were
used as the dropping hammer. The probability of initiation was
determined by Probit analysis. Only 50% probability of initia-
tion (H50) was determined, and the results are listed in Table 1.
Sensitivity to friction using standard test conditions40 was
determined by a BAM friction test apparatus. Sensitivity to
friction for the studied PBXs was detected by spreading 0.01 g of
the sample on the rough surface of the porcelain plate. Change
to the normal force between the porcelain pistil and the plate
was applied by the use of different loads. Any sort of decom-
position (such as smoke, sound or smell) was considered as
a positive sample initiation. Using Probit analysis,41 50% of
initiations are specied and are listed in Table 1 as the friction
sensitivity.
2.8. Vacuum stability test
The compatibility of the PBXs was tested by a vacuum stability
test (VST) with a modernized STABIL 16-Ex apparatus43 (man-
ufactured by OZM Research; the original apparatus is described
in ref. 44). The measurement procedures have previously been
reported.23,25,45,46 The amount of sample used for a measure-
ment was 2 g. The duration of the test was 40 hours. The
2.6. Detonation velocity measurements
The detonation velocity of the prepared compositions was temperature for the isothermal measurements was chosen to be
determined by the ionization copper probe method.40 The setup 120 C. Samples in evacuated glass test tubes were placed into
ꢁ
required for the measurement consists of an oscilloscope the heating block and heated to the desired temperature.
(Escort EUC-3200) with an electronic counter connected to
The increase in pressure was continuously measured by
a capacitor. The capacitor was connected to the ionization pressure transducers inside the glass tubes. This test deter-
probes, which were placed inside the explosive charge. Twisted mines the gas pressure evolution per 1 g of sample in depen-
copper wire with 0.25 mm radius was used. The PBXs were cast dence with time. A linear relationship was observed based on
in the form of cylinders with 200 mm length and 21 mm each curve being tested isothermally for 60–2400 minutes. The
diameter. The measurements were determined by measuring slope of these lines, k, corresponds to the reaction velocity of
the exact distance between the two probes, and then recording gaseous product evolution in a zero-order reaction,23,25,45,46 and
the time at which the detonation wave moved between the two in this case, the slope k expresses the specic rate constant (in
probes. The twisted ionization copper was located in each kPa gꢀ1 minꢀ1).
Table 1 Results of the experimental measurements of the studied samples, and literature data of HTPB compositions of 18% weight binder
Heat of combustion
Enthalpy of formation
Impact sensitivity
[J]
Friction sensitivity
[N]
No.
Code designation
Summary formula
[J gꢀ1
]
[kJ molꢀ1
]
1
2
3
4
5
6
7
8
BCHMX
b-HMX
RDX
3-CL-20
BCHMX/HTPB
HMX/HTPB
RDX/HTPB
CL-20/HTPB
BCHMX/GAP
HMX/GAP
RDX/GAP
CL-20/GAP
C4H6N8O8
C4H8N8O8
C3H6N6O6
C6H6N12O12
—
—
—
—
9124
9485
9522
8311
13 798
14 118
14 162
13 255
11 789
11 893
12 011
10 922
236.5
77.3
66.2
397.8
—
—
—
—
414.8
256.4
198.3
597.4
3.2a
6.4a
5.6a
4.1b
9.6c
88a
95a
120a
69b
322c
>360c
>360c
214c
294
15.2c
14.6c
10.8c
7.7
9
C4.94H7.89N8O7.27
C4.84H8.48N8O7.25
10
11
12
11.2
11.5
8.4
338
>360
247
C3.67H7.12N6O5.39
C7.47H9.33N12O11.17
a
b
Value sourced from ref. 10. Value sourced from ref. 12. c Value sourced from ref. 25.
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Table 2 Detonation parameters of the studied samples, and literature data of HTPB compositions of 18% weight binder
Studied sample
Experimental
Detonation parameters calculated by Explo5
Detonation
velocity [m sꢀ1
]
Density r
Detonation pressure
P [GPa]
Heat of Explosion
No.
Code of samples
[g cmꢀ3
]
Dexp
Dcal.
Error%
Q [kJ kgꢀ1
]
1
2
3
4
5
6
7
8
BCHMX
b-HMX
RDX
3-CL-20
BCHMX/HTPB
HMX/HTPB
RDX/HTPB
CL-20/HTPB
BCHMX/GAP
HMX/GAP
RDX/GAP
CL-20/GAP
1.79a
1.90b
1.76b
1.98b
1.56c
1.57c
1.52c
1.63c
1.62
8650
9100
8750
9473
7746
7812
7526
8167
8292
8384
8074
8676
8840
9225
8718
9407
7593
7627
7449
7919
8261
8313
8099
8482
+2.19
+1.37
ꢀ0.40
ꢀ0.60
ꢀ1.97
ꢀ2.36
ꢀ1.02
ꢀ3.03
ꢀ0.37
ꢀ0.85
+0.31
ꢀ2.23
33.9
38.0
32.1
41.7
21.2
21.3
20.1
23.7
28.6
28.4
26.2
32.1
6447
6075
6085
6455
5744
5453
5453
5786
6658
6297
6152
6559
9
10
11
12
1.64
1.59
1.73
a
b
Value sourced from ref. 7. Value sourced from ref. 12. c Value sourced from ref. 16.
detonation (the product of loading density and the volume
explosion heat rQ/MJ mꢀ3) and the logarithm of impact sensi-
tivity was studied in ref. 48, while the same relation based on
the logarithm of friction sensitivity is presented in Fig. 3. Three
independent lines are presented in Fig. 3. One of them connects
the pure nitramines, and the performance of both BCHMX and
HMX is approximately the same despite the higher density of
HMX. The other two lines connect the PBXs based on HTPB on
the le, and the PBXs based on the GAP binder on the right. It is
clear that all of the studied PBXs based on the GAP binder have
a higher volume heat of explosion compared to the PBXs based
on the HTPB binder. In addition, PBXs based on BCHMX have
a higher heat of explosion than PBXs based on RDX and HMX
for the same individual binder. These results are clear due to
the high heat of formation of BCHMX compared to HMX (see
Table 2). Also, due to the presence of the energetic GAP matrix,
the heat of explosion of BCHMX/GAP is 1.15 times higher than
that of BCHMX/HTPB.
3. Results and discussion
The performance of an explosive material has a signicant
effect on its impact and friction sensitivity, which is attested to
by recently published literature,47 that is to say, high perfor-
mance of an explosive is accompanied by an increase in
explosive sensitivity. However, modern techniques can be used
to combine a high performance value with a reduction in
sensitivity. In order to study the sensitivity of the studied
samples, a comparison between the impact sensitivity (initia-
tion due to uniaxial compression) and the friction sensitivity
(initiation due to shear slide of a xed volume) of all the studied
samples is shown in Fig. 2 It is obvious that the sensitivity of the
nitramines in the GAP matrix have been changed signicantly.
Nevertheless, BCHMX/GAP and CL-20/GAP still have relatively
high sensitivity compared to HMX/GAP and RDX/GAP. It is also
important to mention that the sensitivity to friction of RDX is
improved by its incorporation into GAP matrix. In the sensitivity
evaluation, comparison with performance should be repre-
sented. Thus, the relation between the volume heat of
3-CL-20 based PBXs have the highest friction sensitivity of all
the studied samples. The depicted relationship attests to a clear
Fig. 2 Semi-logarithmic relationship between friction and impact Fig. 3 Semi-logarithmic relationship between friction sensitivity and
sensitivities.
volume heat of explosion.
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link between sensitivity and performance, which has been
pointed out by Licht,1 who has reported that increasing the
performance of an explosive is accompanied by an increase in
its sensitivity.1–4
Fig. 4 represents the well-known relationship between
experimental loading density and velocity of detonation. The
velocities of detonation of the studied GAP bonded explosives
are higher than those of HTPB bonded explosives of each cor-
responding explosive. CL-20/GAP is in the same range as pure
RDX and BCHMX. Also, BCHMX/GAP has a higher detonation
velocity than CL-20/HTPB, which means that BCHMX/GAP
might be able to replace the expensive composition CL-20/
HTPB, given its performance. BCHMX/GAP has a higher deto-
nation velocity than BCHMX/HTPB by nearly 500 m sꢀ1. It
means that GAP binder has a greater positive inuence on
performance compared to HTPB binder. Also, the velocity of
detonation of BCHMX/GAP is considered to be between the
detonation velocity values of RDX/GAP and HMX/GAP.
Regarding the theory of detonation, the detonation pressure
of the energetic materials could be represented by the product
of the square of the detonation velocities and the loading
densities. Fig. 5 presents a comparison between the calculated
detonation pressures and the mentioned measured products.
The good t of this relationship demonstrates a good match
between the calculated results by EXPLO5 and the measured
data. The calculated detonation pressures of all the studied
samples based on GAP binder are higher than the samples
based on HTPB binder.
In order to check the compatibility between the polymeric
matrix and the nitramines practically, a vacuum stability test
was conducted to determine the amount of gas that evolved
from the compositions, which was compared to that for the
pure nitramines. From the obtained results, the amount of the
gas liberated from RDX was the highest compared to those of
the other nitramines in this study. In addition, 3-CL-20 is
considered to be more stable than pure BCHMX, while HMX is
the most stable explosive of the studied pure nitramines.
Nevertheless, these characteristics are changed in the presence
of the GAP matrix; it was observed that the amount of gas
liberated from CL-20/GAP reached 12 ml gꢀ1, while BCHMX/
Fig. 5 Relationship between calculated detonation pressure and the
square of the experimental detonation velocity multiplied by the
loading density.
GAP produced half the amount liberated from CL-20/GAP.
RDX/GAP produced less gas than both BCHMX/GAP and CL-
20/GAP, while HMX/GAP remained the most stable composi-
tion compared with the studied compositions. Also, it is clear
that the presence of the GAP matrix signicantly increased the
amount of gas that evolved from the individual explosives,
however, the chemical structure and the physical stability of the
nitramine plays a role in the presence of GAP.29
To check the possibility of linking the results of the vacuum
stability test with the energetic content of the studied explo-
sives, Fig. 6 presents the relationship between the heat of
formation and the logarithm of the specic rate constant (i.e.
the slope from Table 3). This gure claries the good evidence
for the link between increases in reactivity and the increase of
the energy content of the energetic material molecules. The line
connecting the GAP nitramines with pure RDX in Fig. 6 repre-
sents the order of stability of these studied explosives. It is
observed that the BCHMX/GAP explosive is more stable than
CL-20/GAP, while it is less stable than both pure RDX and the
composition RDX/GAP. Pure HMX is the most stable explosive
of all of the studied samples. These results are also conrmed
by the amount of gas evolved per gram of sample (see Table 3).
Fig.
6 Relationship between the logarithm of the specific rate
Fig. 4 The relationship between the density and the detonation constant and heat of formation of the studied PBXs and pure
velocity of all the studied explosives.
nitramines.
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Table 3 Summary of the vacuum stability test results of the nitramine explosives studied; with exposure to 120 ꢁC for 40 hours
Gas evolved per
Intersection
[kPa]
Abbreviation
BCHMX
gram [ml gꢀ1
]
Slope [kPa gꢀ1 minꢀ1
]
R2
0.117
0.131
0.041
0.036
0.367
0.34
0.0002
0.0002
0.00007
0.00007
0.0005
0.0005
0.0001
0.0002
0.0019
0.0017
0.0036
0.0031
0.001
0.5641
0.5831
0.2707
0.6728
1.7608
2.0799
0.4599
0.6334
0.0693
0.0275
0.6122
0.8656
0.0932
0.1391
2.3946
2.2903
0.9811
0.9941
0.9174
0.9013
0.9144
0.9193
0.9506
0.957
0.9990
0.9990
0.9927
0.9939
0.9961
0.9947
0.9552
0.954
HMX
RDX
3-CL-20
0.09
0.089
6.746
6.496
1.291
1.225
4.374
3.673
12.358
11.894
BCHMX/GAP
HMX/GAP
RDX/GAP
CL20/GAP
0.001
0.003
0.0028
4. Conclusions
References
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