Effect of Inert Plasticizers on Mechanical, Thermal, and Sensitivity

Properties of Polyurethane-Based Plastic Bonded Explosives

€ rkan Atınç Yılmaz,1,2 Deg
Gu  er Şen,1 Zekeriya Taner Kaya,1 Teoman Tinçer2
1
Division of Material Technologies, Defense Industries Research and Development Institute, Tubitak Sage PK: 16 06261 Ankara,
Turkey
2
Department of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey
Correspondence to: G. A. Yılmaz (E - mail: gurkan.yilmaz@tubitak.gov.tr) and T. Tinçer (E - mail: teotin@metu.edu.tr)

ABSTRACT: Mechanical, thermal, and sensitivity properties of plastic bonded explosives (PBX) depend on the type of ingredients in
their formulation. Aim of the work is to develop aluminized cast PBX formulations and process conditions by using alternative inert
plasticizers to have similar or better properties than PBXN-109 without compromising sensitivity properties. Although very small
portion of total production of plasticizers is used for solid rocket propellant and explosive formulations, they play very significant
role in that area. Both inert and energetic plasticizers have involved propellant and explosive formulations to improve process param-
eters, mechanical properties, and even insensitivity properties of them. Isodecyl pelargonate and dioctyl adipate are the most preferred
inert plasticizers in polyurethane based thermoset propellant and explosive formulations. In addition to them, diisononyl adipate and
diisononyl phthalate were used and screened as inert plasticizer candidates for aluminized cast PBX formulations. Mechanical, ther-
mal, and sensitivity properties of PBX formulations were studied and compared in detail. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci.

2014, 131, 40907.

KEYWORDS: composites; plasticizer; polyurethanes; properties and characterization

Received 23 January 2014; accepted 19 April 2014

DOI: 10.1002/app.40907

INTRODUCTION Migration of plasticizer is one of the main problems in plastic

bonded explosives (PBX) and solid rocket propellants.2,3
Plasticizer is a low molecular weight additive which makes pro-
Depending on the rate of migration, polymer loses flexibility
duction easier for materials, hard to process by themselves
and cannot fulfill mechanical requirements. Volatility is the
because of their nature. Some of the important benefits gained
main reason for migration together with the washing of by a
by inclusion of plasticizer are given as: improved flow and flexi-
solvent like water, alcohols, etc. Volatility is directly related with
bility, decrease in glass transition temperature, improved
the molecular weight of the plasticizer.3
mechanical properties, and low temperature usage.
One of the advantages of plasticizer is to decrease glass transition
Phthalates and adipates are the major plasticizers used in daily
temperature (Tg) of a polymer by creating free volume. Hereby,
life. Dioctyl phthalate, diisononyl phthalate (DINP), dioctyl adi-
workability and low temperature usage of polymer is improved.
pate (DOA), and diisononyl adipate (DINA) are some of the
most common plasticizers among them. Both inert and energetic plasticizers have involved propellant and
explosive formulations to improve process parameters, mechani-
Chemical compatibility, miscibility, volatility, and glass transi-
cal properties and insensitivity properties of them.4–7 1,2,4-Buta-
tion temperature are the main parameters for the selection of
netriol trinitrate, trimethylolethane trinitrate, nitroglycerin,
plasticizer. First of all, a plasticizer should not react with the
n-butyl-2-nitratoethyl-nitramine, triethyleneglycol dinitrate, and
main polymer during production and in use.
bis(2,2-dinitropropyl)acetal or formal are energetic plasticizers
The second crucial criterion is miscibility which is based on used in especially smokeless or reduce smoke propellant formula-
“like dissolves like” principle. Polarities and solubility of plasti- tions to compensate energy loss due to reduction or removal of
cizer and the polymer should be similar for better miscibility.1 metallic fuel which is responsible for primary smoke generation.

Additional Supporting Information may be found in the online version of this article.
C 2014 Wiley Periodicals, Inc.
V

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Table I. Properties of Plasticizers12–18

Properties DOA IDP DINA DINP

Mwt 370.6 298.5 398 418
Density (g/cm3) 0.927 0.867 0.950 0.975
  
Viscosity (cP) 22 @25 C 4.2 @40 C 18–22 @20 C 68–82 @20 C

Pour point ( C) 275 272 268 245
Flash point ( C) >190 172 >204 >210

Effect of ingredients on mechanical and physical properties of 60–90 g samples were measured at room temperature in Helium
polyurethane based energetic composites (propellants and explo- atmosphere and average of five measurements were reported.
sives) was studied in many aspects before. It has been shown that
Viscosity measurements of uncured explosive were done by
type, amount, ratio, and particle size of ingredients have substan-
using HBDV-II Brookfield Digital Viscometer at 50 C and with
tial effects on mechanical and physical properties of them.8–10
T-B type T-bar spindle. Uncured explosive has paste type behav-
Aim of this study is to develop aluminized polyurethane based
ior that cause cavities during viscosity measurement. Helipath
PBX formulations and process conditions by using alternative
stand was used to solve that problem. That stand raise and
inert plasticizers to have similar or better properties than PBXN-
lower the spindle to create a helical movement and prevent cav-
109 without compromising insensitivity properties. Isodecyl pel-
ity formation (channeling effect). Pot life and end of mix vis-
argonate (IDP) and DOA are the most preferred inert plasticizers
cosity (EOMV) measurements were done by one rpm rotational
in polyurethane based thermoset propellant and explosive formu-
speed. Additional to EOMV measurements, viscosities of explo-
lations. In addition to them, DINA and DINP were chosen and
sives with different rotational speeds (2, 3, 5, 7, 10 rpm) were
screened as inert plasticizer candidates. Those plasticizers were
measured and reported as well. All viscosity measurements were
proposed as alternative plasticizer for PBX formulation very
done by using 500 mL uncured explosive samples.
recently by United States Patent number 8,172,96511.
Uniaxial tensile tests were done at room temperature by using
EXPERIMENTAL Chatillon LTCM-100 instrument and Chatillon 100 LBF (445
Chemicals N) AMETEK load cell. All tensile measurements for explosives
The binder used for PBX preparation was military grade were done with 50 mm/min testing speed as described in STA-
hydroxyl-terminated polybutadiene (HTPB R45M), with 1 wt % NAG 4506.20
2,20 -methylenebis (4-methyl-6-tertbutylphenol) (AO-2246), was Autoignition temperatures of explosives were measured accord-
supplied from Cray Valley Company, USA. DOA, DINA, and ing to STANAG 4491 “Explosives, Thermal Sensitiveness and
DINP were the plasticizers used in PBX formulation were com- Explosiveness Tests, Annex B-1: Temperature of Ignition” by a
mercial product of Plastifay Kimya, Istanbul. The other plasti- test set up designed and produced in Turkey.21
cizer IDP was production of Cognis Corporation. Some of the
properties of inert plasticizers either provided by the supplier or LINSEIS L75 Vertical Bench Top Platinum Series dilatometer
from the referred literature are shown in Table I. Chemical with 100 mN force gauge was used to measure glass transition
structures can be given as Supporting Information. temperatures (Tg) of explosives with 6 mm diameter and
5.69 6 0.07 mm length samples. First, samples were cooled
Isophorone diisocyanate was used as a curing agent and supplied down to 2110 C by 2 C/min cooling rate, then waited for sta-
from Bayer Material Science. Reduced sensitivity cyclotrimethy- bilized temperature and heated to 220 C by 2 C/min heating
lene trinitramine (RS-RDX) was used as an energetic filler and rate.
provided by Chemring Nobel with two different particle sizes,
Class 1 and Class 5 as defined by MIL-DTL-398D “Detail Specifi- Decomposition temperature determination of explosive samples
cation RDX”.19 di–(2–hydroxyethyl)–5, 5-dimethylhydantoin was performed by Mettler-Toledo TGA/DSC 1 thermogravimet-
(DHE or Dantocol DHE) was supplied from Lonza. Ferric acety- ric analyzer. A total of 50 mL/min nitrogen gas was flowed over
lacetonate (FeAA) with > 99.9 purity was supplied from Aldrich. 3–5 mg samples in a 70 mL platinum pan. First, samples were
2,20 -Methylenebis (4-methyl-6-tertbutylphenol) AO-2246 also flashed for 10 min under nitrogen gas at 30 C. Then, 5 C/min
known as Vulkanox BKF was provided by Rhein Chemie Ger- heating rate was applied to attain 550 C.
many. Type IV spherical aluminum powder was supplied from C-Therm Tci Thermal Conductivity Analyzer was used to mea-
Toyal America, according to MIL-PRF-23950B (AS) “Performance sure thermal conductivity of explosive samples (25 3 25 3
Specification Aluminum Powder, Spherical.” All chemicals were 12) 6 2 mm in dimensions.
used as received in formulation studies.
Impact sensitivity of explosives was tested by BAM impact
Instruments and Analysis machine according to STANAG 4489 “Explosives, Impact Sensi-
Quantachrome Manuel Gas Stereopycnometer (Model: SPY-3) tivity Tests, Annex C: BAM Impact Machine.”22 Solid samples
with 135 cm3 (5 cm diameter 3 7.5 cm length) stainless steel with 3 3 4 3 4 mm dimensions and 10 mm3 powder samples
sample cell was used for density measurement. Approximately with particle size between 1.0 and 0.5 mm mesh screen were

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Table II. Explosive Codes Containing Different Plasticizers Table III. Plastic Bonded Explosive Ingredients

Plasticizer type DOA IDP DINA DINP Ingredients Percentage (wt %)

Explosive code P-1 P-2 P-3 P-4 RS-RDX II Class 1 58.18
RS-RDX II Class 5 5.82
Aluminum powder, spherical, Type IV 20.00

tested. Bruceton up and down procedure was used to obtain HTPB R45M (with 1.0% AO 2246) 7.47
50% probability of reaction. Isophorone diisocyanate 0.93
Ferric acetyl acetonate 0.0008
Friction sensitivity tests were done by BAM friction method
Plasticizer 7.33
according to STANAG 4487 “Explosives, Friction Sensitivity
Tests, Annex A: BAM Friction Machine.”23 Approximately 10 di–(2–hydroxyethyl)–5, 5-dimethylhydantoin 0.27
mm3 of the material, maximum 1 mm thick by 5 mm in diam-
eter, were used as solid samples. Powdered substances were curing process at 50 C to ensure complete curing of the explo-
sieved through a 0.5 mm mesh screen. A measuring spoon fab- sive. FeAA was used as catalyzer to accelerate curing. Curing
ricated out of conductive plastic was used to measure 10 mm3 process was followed by mechanical, thermal, and sensitivity
of powder. characterization of explosives.
Electrostatic discharge tests were done by using ESD 2008 “A
small-scale electrostatic spark sensitivity tester” which is devel- RESULTS AND DISCUSSION
oped by OZM research s.r.o in cooperation with “Institute of Compatibility of the ingredients was the first issue that should
Energetic Materials,” University of Pardubice, according to AOP be carried out before formulation studies. Reactions arising
7 “Manual of Data Requirements and tests for The Qualification from incompatibility have significant effect on shelf life of
of Explosive Materials for Military Use” and the manual written explosives. Incompatibility reactions between plasticizer and
by that company.24,25 Samples less than 10 mg were tested at other main ingredients (binder, energetic filler, and metal pow-
relative humidity <50% and at room temperature. der) of explosive can further accelerate the rate of ageing. Com-
Hardness measurements were performed by Bareiss Shore-A patibility of an explosive is the primary issue and has to be
Durometer according to MIL-E-82886(OS) “Military Specifica- entirely investigated for every step.27
tion, Explosive, Plastic Bonded, Cast PBXN-109.”26 After 10 Therefore, compatibility is one of the important criteria for
days of curing process, explosive blocks were sliced into 6–7 convenient plasticizer in PBX formulation. Vacuum stability test
layers, upper and lower levels were discarded and hardness data (VST) is the most widely used and accepted method for chemi-
was taken from the rest of the layers after 24 h conditioning at cal compatibility because it is reliable and relatively short test.
room temperature. In each test, after 30 s values were recorded The amount of sample used in VST test is much more represen-
for appropriate measurement. tative than the amount used in TGA or DSC method.28 Higher
STABIL Vacuum Stability Tester made by OZM Research Com- sample size increases the possibility of physical contact in
pany was used to survey compatibility of ingredients, as between the materials tested, and ensures the reliability. On the
described in MIL-E-82886(OS).26 All samples were tested in the other hand, the foremost advantages of using DSC and TGA
calibrated test tubes at 100 C for 48 h. methods are to make experiments quicker and safer.29 However,
TGA and DSC methods are suitable for the testing of primary
FEI Quanta 400F model Scanning Electron Microscope was explosives and pyrotechnics, their applicability for propellants,
used for surface analysis of explosives. Explosive samples were and PBX can be questioned.
inserted in liquid nitrogen, and then broken at their midpoint
and plated 7 nm with Au-Pd by sputter method before analysis. VST compatibility test results of HTPB, plasticizers, RDX, and
SEM analysis was done under low vacuum (0.8 mbar) and 10 aluminum are given in Table IV. All values are found to be well
kV was applied from 10.8 to 13.4 mm distance. Field emission below the 5 mL and even less than 0.150 mL. This proves that
gun was used during analysis. Photos of those tests can be given all plasticizers in question are compatible with the HTPB, RDX,
as Supporting Information. and aluminum. VST test results less than 1 mL can also be
commentated as insignificant incompatibility according to crite-
Explosive Preparation ria put forth by a study published in 2006.30 The results with
After optimization of formulation and process parameters, four
batches of explosive production were done by using one US gal- Table IV. Compatibility of Plasticizers with Main Explosive Ingredients
loon vertical planetary mixer with four different types of plasti- According to VST (mL gas)
cizer. The explosive formulations were given codes to
distinguish them as shown in Table II. Explosive mixtures were Components DOA IDP DINA DINP
produced at 50 C according to typical explosive preparation HTPB 20.249 0.116 20.021 20.148
procedure with the ingredients given in Table III. Right after
RDX 20.030 21.989 20.100 20.234
the mixing process, EOMV and pot life measurements were
Al powder 20.339 21.836 20.309 20.281
done. Other measurements were performed after 10 days of

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Table V. Characterization Results of Explosives (Explosive Codes are

Shown in Table II)

Test results
Test name P-1 P-2 P-3 P-4
3
Density (g/cm ) 1.668 1.649 1.668 1.671
EOMV (Poise) 3845 5339 4126 2869
Pot life (min) 180 180 180 180
r @ max. load (MPa) 0.46 0.59 0.43 0.47
Modulus (MPa) 3.01 3.31 2.72 2.76 Figure 1. Variation of viscosity versus rpm of the explosive compositions
Elongation (%) 34.5 32.18 34.95 27.70 (symbols are shown in Table II).

Autoignition Temp. ( C) 224 222 224 228

While explosive containing IDP plasticizer (P-2) has highest
Tg ( C) 286.5 290.0 283.0 281.0
EOMV, explosive containing DINP (P-4) has the lowest. DINP
Decomposition Temp. ( C) 217.1 218.6 218.3 214.6
has aromatic ring in its structure and occupy more volume
K (W/m K) 0.81 0.71 0.80 0.84 than the other plasticizers. On the other hand, IDP has very
BAM impact (J) 19.98 17.27 17.89 16.34 long linear aliphatic chain in its structure. It is expected that
BAM friction (N) 360 >360 >360 >360 molecules with aromatic rings and branched chains create more
ESD (mJ) 117 114 119 116 free volume than linear chains and lower viscosity of the mix-
Hardness (Shore A) 44 49 41 42 ture more than the linear ones.34 EOMV results are lower than
VST (mL gas/g) 0.279 0.142 0.163 0.138 the viscosities measured in similar studies in the literature.
EOMV of PBXN-109 including insensitive RDX from Dyno
Nobel and DOA plasticizer has been measured as 6000 Poise
minus sign are also indication of compatibility and reported in which is 56% higher than the results obtained in this study.35
many studies.28,30,31 The viscosity (g) defined as the ratio of shear stress (s) to shear
After compatibility, formulation, and process optimization stud- rate (c). Uncured PBX show non-Newtonian (time independent,
ies, four different explosive formulations were prepared each shear dependent) flow behavior and generally power law model
containing different type of plasticizer. Explosive productions is used to define them as shown in eq. (1).
were done by using the same production parameters and recipe s K cn
except plasticizer. Characterization results of rheological, g 5 5Kcn21 (1)
c c
mechanical, thermal, and sensitivity properties of uncured and
cured explosives were summarized in Table V. where n 5 1, n < 1, and n > 1 represents Newtonian, pseudo-
plastic, and dilatant flows, respectively.
Average of five consecutive results showed that there is a direct
relationship between density of individual plasticizers and Viscosities of explosives were measured at 6–7 different viscom-
explosive formulations including those plasticizers. All of the eter speeds and results are shown in Figure 1. Results of viscos-
explosive formulations exceedingly meet the requirements (30 ity measurement proved that the flow behavior depends on
Shore-A) given in military standard (MIL-E-82886). shear rate which is a linear function of viscometer speed (rpm).

Pot life is described as the elapsed time until the viscosity of For a power law, fluid log-log plot of viscosity versus rpm is
explosive is still convenient for casting process.32 Accepted always linear as shown in Figure 2. Good linearity on slopes
upper viscosity limit for propellants and explosives is around 20 proves that data is well fitted with the power law.36,37 Slope is
kP for proper casting. Casting process can be done by several
methods. If viscosity of explosive is low enough (3000 poise),
only vacuum applied to the casting volume could be sufficient
for proper casting. If viscosity of explosive is high, pressure
casting technique accompanying vibration is one of the methods
studied for highly loaded propellants.33
Explosive viscosities less than 10 kP are accepted successful after
3 h lapsed from the mixing. Ranking of explosives containing
different type of plasticizer according to the EOMV measure-
ments were as follows: P-4 < P-1 < P-3 < P-2. Difference in
EOMV values can only be explained by the type of plasticizer.
It was interesting to note that explosives containing adipate
type plasticizers (P-1: DOA, P-3: DINA) have small difference in
EOMV, but DINA has totally two more ACH2A group in its Figure 2. Log-log viscosity versus rpm for explosives (symbols are shown
carbon chain. in Table II).

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Table VI. K and n Values of Explosives (Explosive Codes are Shown in chain of IDP cause higher mobility, and thus provide better
Table II) stress transfer.
DINP and DOA show similar mechanical behavior in explosive
P-1 P-2 P-3 P-4
formulations. DINP is the only plasticizer candidate with aro-
K 4024 5950 4254 2948 matic ring in the structure. Although it has the highest molecu-
n 0.5262 0.3914 0.5263 0.6113 lar weight among other plasticizers, because of its bulky
structure, it creates higher free volume than others. That would
be the explanation for having lower Young’s modulus.
equal to flow behavior index (measure of deviation from New- Explosives in the munitions are exposed either mechanical and
tonian behavior) minus one (n 2 1) and intercept refers to log thermal stresses or shocks during their transportation, usage,
of consistency coefficient (K). and storage. Therewithal, aging is another mechanism which
In our case, uncured explosive shows pseudoplastic behavior. K lowers the mechanical properties by the time. Higher elongation
and n values calculated from the equations obtained from Fig- and stress values are critical to resist that kind of conditions. If
ure 2 are shown in Table VI. mechanical properties of explosives are not sufficient to com-
pensate these stresses, some cracks may occur in the explosives.
Degree of pseudoplasticity can be measured by flow behavior Cracks in the explosive promote an increase in sensitivity of the
index. As n increases pseudoplasticity decreases so IDP contain- munition. All of the explosives containing different type of plas-
ing explosive (P-2) shows high pseudoplasticity among others. ticizers show excellent mechanical properties and fulfill all
Flow behavior index of two adipate plasticizer containing explo- mechanical requirements defined by military standard.26
sives (P-1 and P-3) are almost the same. All four explosive for-
mulations containing different plasticizers achieved 3 h pot life Glass transition temperature was calculated from the first deriv-
requirement with less than 6 kP viscosity which ensures enough ative of elongation versus temperature graph in dilatometer. Tg
time for the filling process. values of explosives can be put in order as follows: P-2 < P-
1 < P-3 < P-4. Viscosity data of plasticizers in corresponding
Filler size, filler geometry, and adhesion between filler and the explosive formulations are in the same order with explosives:
polymer are some of the factors that affect the final mechanical IDP < DOA < DINA < DINP as expected. According to the
properties. It was argued that sharp edges of standard RDX results obtained, all explosive formulations are suitable for mili-
crystals may act as stress concentrators causing a reduction in tary applications where 254 C is the minimum temperature at
strain at maximum load.38 Both improved RDX and RS-RDX which munitions could be used or stored. Dilatometer curve of
from Chemring Nobel Company has better roundness than explosive sample with DOA plasticizer can be given as
standard Type II RDX. Aluminum powder used in all formula- Supporting Information.
tions has also spherical shape. It is also known that shape of the
Direct relationship between density and thermal conductivity is
fillers have important role on elastic or tensile modulus of com-
one of the well-known basic principles. One may easily con-
posites. Modulus is increasing with the aspect ratio of the fillers.
clude that thermal conductivity of explosives is increasing with
Short fibers are the most appropriate choice for better tensile
an increase in explosive densities as shown in Figure 3.
properties. Flakes and irregular shaped particles also improve
mechanical strength of composites more than spherical ones. Peak decomposition temperatures of all explosive formulations
Spherical inert and energetic filers are the most prominent rea- were around 215–219 C as shown in Figure 4. TGA results
son for low tensile modulus of explosives with RS-RDX. show that plasticizer type has no significant effect on decompo-
sition temperature of the explosives.
Explosive containing IDP (P-2) plasticizer has superior tensile
properties among other explosives containing different plasticiz- Upper limit for compatibility of PBXN-109 at 100 C for 48 h is
ers. In fact, linear structure, low molecular weight, and long 0.5 mL per 1 g sample. All compositions were under this limit
and therefore can be accepted as stable.

Figure 3. Thermal conductivity (䊊) and density (䊏) relationship (sym- Figure 4. Decomposition temperatures of explosives (symbols are shown
bols are shown in Table II). in Table II).

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Figure 5. SEM image of explosive containing DOA as a plasticizer. Figure 7. SEM image of explosive containing DINA as a plasticizer.

Average of four measurements was taken as the autoignition

literature after forming PBXN-109 compositions.38 While all
temperature. Explosive with DINP plasticizer shows slightly
explosives containing different kind of plasticizer have positive
higher autoignition temperature than the other compositions.
contribution to insensitivity of RDX, explosive containing DOA
That could be explained by higher thermal resistance of DINP
as plasticizer (P-1) somewhat had higher influence. Although
among other alternative plasticizers. Test results are consistent
2.0–3.5 J difference in impact sensitivity corresponds to 10–20%
with the PBXN-109 measurements done by Australian “Defence
higher insensitivity and cannot be ignored, all explosives con-
Science and Technology Organization.”38–40
taining different kind of plasticizer provide better insensitivity
Impact sensitivity results are given in terms of Joule. Uncoated than virgin RDX.
regular RDX impact sensitivity result was reported as 6.85 J in
While friction sensitivity of RS-RDX by itself was 156 N, after
the literature which is less than 9.80 J measured for RS-RDX in
coating with polymer matrix, there was a significant increase in
this study as expected.41 While RS-RDX without any coating
insensitivity. Ratio of sensitivity values before and after the
has 9.80 J impact sensitivity, after coating with polyurethane
coating process reported in several studies.35,38–40 All explosive
matrix almost 100% improvement (16.3–20.0 J) was gained in
formulations containing different plasticizers have equal or
impact sensitivity value. Similar amount of impact insensitivity
more than 360 N friction sensitivity.
improvements for different kind of RDX were reported in the
No ignition was observed from the explosive compositions sub-
jected to ESD sensitivity test. ESD sensitivity results were
pointed out an apparent unleashed odor from the explosive
samples. When RDX crystals were coated by polyurethane
matrix, ESD sensitivity were improved significantly. Null RDX
has 55 mJ ESD sensitivity according to AOP-07 (ED-2).24 There
was no big difference in between the sensitivity results of explo-
sives containing varied plasticizers.
As shown in SEM images, aluminum and RDX particles were
perfectly coated by polyurethane matrix containing DOA as
plasticizer (Figure 5). Cracks observed at the surface of powders
was due to the heating effect of high voltage (10 kV) applied
for a while on the surface of the samples during imaging.
Although surface of the P-2 explosive containing IDP plasticizer
had wrinkled surface, both aluminum and RDX powders were
seemed to be well coated by the polyurethane matrix (Figure 6).
Formation of the holes was due to the randomly formed wrin-
kles and not a result of debonding behavior of solid particles.
Polymer coating on the surface of solid particles of P-3 explo-
Figure 6. SEM image of explosive containing IDP as a plasticizer. sive was easily seen from Figure 7. There was no evidence for

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 All plasticizers were found as compatible with HTPB, RDX,

and aluminum.
 Density, hardness, tensile properties, and vacuum stability of
all explosive formulations with different type of plasticizers
fulfill the requirements defined in MIL-E-82886(OS).
 EOMV results show that molecules with aromatic rings and
branched chains create more free volume than linear chains
and lowers viscosity of the mixture.
 All alternative formulations have reasonable pot life and are
suitable for mass production.
 Thermal conductivity results are parallel to density of the
explosives.
 All thermal behaviors of explosives are suitable for the condi-
tions from 254 C to 171 C.
 A sharp decrease in sensitivity of RS-RDX has been observed
after coating with polymer matrix.
 New plasticizer candidates are suitable for aluminized cast
polymer bonded explosives.
According to the results, it cannot be concluded that the new
Figure 8. SEM image of explosive containing DINP as a plasticizer.
plasticizer candidates are superior to the former plasticizers on
the basis of performance but they are acceptable. On the other
debonding. The same type of cracks on the surface has been hand, especially DINP may improve aging characteristics of
observed as in the case of P-1 explosive. Surface of P-1 and P-3 PBXs because of its higher molecular weight, due to decrease in
explosives which have the same type of plasticizer (adipate) the migration of plasticizer. DINA and DINP were promising
have similar appearance. Coating of aluminum and RDX par- plasticizer alternatives not only because of their compatibility
ticles in P-4 explosive which contains DINP as plasticizer was with the main explosive ingredients, but also their considerable
also successful as shown in Figure 8. improvement action on tensile and insensitivity properties of
Except impact sensitivity test, all insensitivity measurements explosive.
showed that four explosive samples containing different plasti-
cizer have similar behavior against unpredictable stimuli. Coat- ACKNOWLEDGMENTS
ing of explosive particles has an important role on insensitivity We thank Turkish Scientific and Technological Research Council,
behavior of the cured explosive mix. Together with wetting of Defence Industry Research and Development Institute for their
particles by the polymer matrix, physical and chemical bonds full support on this study.
are important factors for appropriate coating process. In order
to improve interaction between solid particles and polymer
matrix DHE had been used as bonding agent in the formula- REFERENCES
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