Title of the lnvention
A novel EASAl method for preparing nanoparticles of high energetic compounds.
Field of the invention
This invention relates to a novel evaporation assisted solvent antisolvent interaction
(EASAI) method for preparing nanoparticles of high energetic compounds (HECs).
Nanosized particles of high energetic compounds such as RDX, HMX ets. can be
prepared using EASAl method. The method is suitable for preparation of HEC
nanoparticles with average particles size below 100 nm. The same method can also
be used to prepare the nanoparticles of other organic compounds including
pharmaceutical drugs.
Background of the Invention
Formation of small solid particles of micro or nano-meter size with controlled particle
size distribution is of great interest in app!ications involving polymers, drugs and
energetic materials [I]In. g eneral, the aim of research and development activities in
the area of high energy materials (HEMS) is focused on developing energetic
materials with improved performance but with reduced sensitivity. The focus of
. .
research in high explosives has been mainly to synthesize new explosive molecules
wth the above stated benign properties. nowever, development through molecular
modeling and organic synthesis of novel energetic materials has been very slow and
conventional nitramine explosives such as RDX and HMX continue to be used as the
workhorse explosives across.the globe [2]. Powerful explosives such as CL-20 and
octanitrocubane have higher energetic performance than HMX [3]. But, sensitivity to
accidental stimuli is a matterof concern. The sensitivity of explosives is related to
their chemical as well as physical characteristics. Physical properties such as crystal
size, shape, morphology, purity, inclusions, defects in the crystal and the
microstructure of inter-crystalline voids can be altered to improve the performance of
existing explosives [4]. It is reported in the literature that naneTATB crystals have
higher mass loss than the micro-TATB crystals [5]. Thus, by size reduction of the
crystals, sensitivity can be reduced and reactivity can be improved. However, only
limited production strategies are available for making organic nanoparticles in
general when compared to the large number of methods,that are available for the
" I .
2
preparation of inorganic nanoparticles. Methods used for the preparation of organic
nanoparticles are primarily based on either precipitation, milling or through chemical
reaction techniques. Sub micrometer-sized particles of energetic compounds have
been prepared using various techniques such as sol-gel method [2], supercritical
fluid methods [6], mechanical milling [7], ultra sonication [8, 91, spray. assisted
precipitation [lo] etc. There are relative advantages and disadvantages for these
methods. For example, milling techniques rely on applying extremely high
mechanical energy on the system leading to structural changes of the crystal. Milling
energetic materials can be hazardous also. Sol-gel methods are multi-step and time
consuming synthetic routes. As a consequence, methods which rely on solubility
changes to induce nanoparticle formation are more attractive. Among these, reprecipitation
is a simple and convenient technique, which proved to be particularly
effective for the preparation of organic nanoparticle dispersions. In this method, an
organic solution containing an active substance is injected to the antisolvent (e.g.
water) that is solvent-miscible under rapid mixing, which generates high super
saturation leading to fast nucleation rates [ I l l . The instantaneous precipitation
., occurs by a rapid desolvation of the hydrophobic active in-gradient in the antisolvent
medium. The precipitation rate can be enhanced by elevated temperatures during
mixing [12]. The antisolvent may contain hydrophilic stabilizers such as polymers or
surfactants. The hydrophilic stabilizer in the antisolvent is absorbed on the particle
surface to inhibit particle gr.o wt.h . Herein, a simple eavporation assisted solvent
' antisolvent interaction method for the preparation of nano-HECs is. developed and
claimed for.
State of the art
Particle size is .a major parameter that can influence the performance and safety of
high energetic compounds, viz, propellants and explosives. Particle shape also can
affect these parameters. The preparation of high energetic material nanoparticles
with particles size below 100 nm is a big challenge. We have developed the EASAl
method for preparing the nano-HECs well below 100 nm. The parameters affecting
particles size and morpholagy were thoroughly optimized for the EASAI method.
Particle size can be controlled by varying parameters such as: solvent, antisolvent,
temperature of antisolvent, ratio of solvent to antisolvent, volume of solvent injected.
This method was successfully used for nano sizing of the HECs like RDX and HMX
with particles size below 100 nm and spherical morphology.
Objective of the invention
The main objective of the invention is'the development of a novel method for the
preparation of nanoparticles of HECs. Further object of the invention is preparing the
nano-HECs well below 100 nm.
Brief Description of the Drawings
Figure 1- Effect of solvent to antisolvent ration (vh) on the site of the
Nanoparticles of RDX.
Figure 2- Particles size variation of RDX Nanoparticles with temperature of
antisolvent.
Figure 3- Effect of concentration of RDX and HMX in acetone solution on particles
size prepared nanoparticles'of RDX and HMX using EASAl method.'
Figure 4- FESEM images of RDX nanoparticles that were prepared using
EASAl method. Experimental conditions of preparation are being inscribed on
each image.
Figure 5- FESEM images of HMX nanoparticles that were prepared using
EASAl method. Experimental conditions of preparation are inscribed on each
image.
Figure 6- Particles size measured from FESEM images of nano-HEC's that
were precipitated from different solvents.Some physical properties of the solvents
are also given.
Figure 7- FESEM images of RDX nanoparticles that were prepared using
EASAI' method by changing the solvent. The experimental conditions were:
Temperature (70 OC), solvent to antisolvent (water) ratio (1 :250) and
concentration (5 mM).
Figure 8- FESEM images of HMX nanoparticles that were prepared using EASAl,
method by changing the solvent. The experimental conditions were: Temperature
(70 OC), solvent to antisolvent (water) ratio (1:250) and concentration (5 mM).
Figure 9- Graph of Transmittance and Wavenumber of RDX.
Figure 10- Graph of Transmittance and Wavenumber of HMX.
Figure 11- Graph of lntensity and 2Theta of RDX.
Figure 12- Graph of lntensity and 2Theta of HMX.
Figure 13- Graph of TG and Temperature of RDX.
Figure 14- Graph of TG and Temperature of HMX.
Figure 15- Graph of DSC and ~emperatureo f RDX.
Figure 16- Graph.of DSC and Temperature of RDX.
Detailed description of the preferred embodiments
A novel EASAl .method using by which ' nanosized particles of high energetic
compounds such as RDX, HMX etc. can be prepared. The method involved the
injection of a solution of the energetic compound to an antisolvent at elevated
temperatures leading to the rapid precipitation of nanoparticles of the energetic
compound. The parameters such as solvent, antisolvent, temperature of antisolvent
and ratio of solvent to anti solvent, and concentration of the HEC in the solvent can
be tuned to modify the particle size and shape. The EASAl method is suitable for
preparation of HEC nanoparticles with particles size below 100 nm. Choice of a
suitable low boiling solvent, appropriate antisolvent and its temperature is the key for
this invention. This method can produce nanoparticles of HECs even at higher
I concentration.
I Examples
I Materials
I
RDX (98.2 %) and HMX (99.1 %) were prepared by using Bachmann process. All
organic solvents: dimethylsulfoxide (DMSO), dimethyl forrnamide (DMF), ethyl
acetate (EA), N-methyl-2-pyrrolidone (NMP), methanol (MN) and ethanol (EN) were
I purchased from sigma Aldrich and used as received. HPLC micro syringe was
purchased from Hainilton (Reno, NV, USA). Syringe filter 0.22 pm' pore size and
Whatman Anodisc 25 filter of pore size 0.02 pm was purchased from ~ i l l i ~ o r e
(Billerica, MA, USA). Ultra-pure water (18.2 Mf2-cm) from double stage water purifier
(ELGA PURELAB Option-R7, WVS Ltd., UK) was used throughout.
Preparation of HECs nanoparticles
Solutions of HECs with known concentration in different solvents were prepared by
adding accurately weighted amount of HECs. A syringe filter of pore size 0.22 pm
could be used to filter the solution to ensure that no particle are present in it. The
solution (100 pl) was quickly injected into antisolvent (25 ml) at 70 "C under
magnetic stirring using an HPLC micro-syringe .to precipitate nanoparticles.
Nanoparticles of HECs were precipitated immediately on the in/e$ion of their
respective solution into antisolvent (water). Smaller particles with narrow size
distribution were formed when stirring was continued up to few minutes after the
injection to complete the mixing. The nanoparticles can be collected by various
methods such as vacuum drying, free drying, flitration using whatman anodisd3 25
filter membrane (diameter = 25 mm and pore.size 20 nm) etc.
Particle size and morphology
Particle size (z-average diameter, dlnm) of the precipitated nanoparticles could. be
measured using dynamic light scattering (DLS) method (Zetasizer Nano ZS, Malvem
Instrument Ltd., UK) at .25 OC. ~ccuratep' articles size and morphology of the
nanoparticles can be confirmed by using an FESEM (FEI Quanta FEG 450 and
supra55 VP model, ZElSS Instrument, Pvt. Ltd). The suspension of HEC
nanoparticles in water was drop coated on a.glass slide of 1 cm2 and dried. Samples
were subsequently sputter-coated with gold at 20' mA for 180 s before the FESEM
observations. Particle size of more than 300 nanoparticles from different FESEM '
images that were taken from different regions of the samples were calculated in each
experimental condition using "Image J" software and average values are reported.
Smaller particles with narrow size distribution can be obtained by using the following
conditions: (a). create a high degree of supersaturation, (b) uniform spatial
concentration distributions in solutions, and (c) the negligible growth of all crystals.
The rate of supersaturation and nucleation are the more important factors which
affect the size and shape of particles [13]. If the rate of supersaturation is higher'than
the rate of nucleation, then smaller size nanoparticles with narrow size distribution
could be obtained. EASAl method can be used to tune the particle properties
effectively. One of the ways .to tune particle size is to change the volume ratio of
solvent (acetone) to antisolvent (water), as presented in Figure 1. Yet another way to
I-PO DELHE a3 - 1.-2.2-. 01 5 1 $ : 4.4
tune the particle size is to vary temperature of the antisolvent as presented in Figure
2: Particle size decreases with increasing antisolvent temperature due to the
enhancement in rate of evaporation of the solvent. Boiling point of acetone is 56 'C
and hence it evaporates at faster rates when the temperature of the antisolvent is
higher than 56' OC. Hence the method is called evaporation assisted solvent
antisolvent interaction (EASAI) method. Particle size of HECs can also be tuned by
varying the concentration of HEC solutions as shown in Figure 3. Variation in particle
size of RDX and HMX nanoparticles under different condltisns is also shown in
Figures 4, 5 & 6. The HEC nanoparticles were mostly spherical when they are
formed using EASAI method.
The EASAI method also can be used. to vary the size and shape of the HEC
nanoparticles by changing the solvent-antisolvent pair as shown' in Figure 6. FESEM
images of the nano-RDX and nano-HMX particles are shown in Figure 7 and 8. It is
evident from these figures that spherical nanoparticles with almost uniform particle
sizes were formed in most of 'the conditions. However, cubical and rod shaped
morphologies were formed when ethyl acetate (EA) was used as the solvent. From
I
Figure 6, it is clearly evident that the particle size depends on the nature of the
I
solvent. Chung et al [I41 studied the effect of solvents on the particle size of low
molecular weight organic compounds at 298 K. They concluded that both solubility of
the compound and polarity of the solvent are the key factors determining particle
size. Lower boiling point helps solvents with high miscibility with water in evaporation
assisted precipitation that leads to enhanced nucleation rate and formation of large
number of nuclei. Further growth of the nuclei into large particles is not possible as
substantial depletion of the HEC molecules in the solution takes place during
nucleation.
Chemical, crystallographic and thermal properties
FTlR spectroscopy was performed using the Perkin ' Elmer FTlR emission
spectrometer (Spectrum Two). The FTlR spectrum of raw and nanoparticles of HECs
were recorded from 4000 to 600 cm-' frequency range. Pure KBr Pellet was used for
background correction. The samples were properly grounded with KBr powder and
then pressed to obtain a suitably sized pellet for FTlR spectrum measurement. XRD
measurements were performed on a Smart ~ a bX-R ay Diffractometer (Rigaku,
Japan) using Cu Ka radiation as X-ray source (A = 0.15418 nm) at room
temperature. Samples were placed on a sample holder and scanned in the 28 range
of 10" to 70" at a scan rate of 2" mine' with a step size of 0.02" by-applying 45 kV
voltage and 100 mA current. TGA-DSC analyses were carried out by using a
Netzsch 'STA 449 F1 Jupiter instrument. The sample (2-5 mg) was taken in' a .
standard alumina pan with an alumina lid with a pin hole at the middle. An empty
1 crucible was used as reference. The samples were heated from 20 to 500 "C at a
I
I . heating rate of 5 "C min-' usirig nitrogen gas of 60 ml min-' protective and 40 ml min"
purge flow rates.
FTlR spectroscopy was used to determine the chemical composition before and after
the antisolvent precipitation of HECs. It is clearly evident from Figure 9 that nano-
RDX samples have similar IR bands as bulk-RDX. The major bands for the RDX
samples were as follows: 1592 cm-' (v. NO2), 1270 cm-' (v. NO2 + v N-N), 1039 cm-'
(ring stretching bands) 945 cm-' and 783 cm-' (6 NOz and, yN02) and .604 cm-' (T + y
NO2). Peaks in the range of 700-760 cm-' are characteristic of a-polymorphic form of
RDX and there are no peaks in this range for PRDX.[15]. Similarly, the major IR
bands observed for the nano-HMX samples in Figure 10 matched well with the raw-
HMX. The major IR bands for HMX samples were assigned as follows: 1564 cm-'
(vSNO2), 1145 cm-' (vs.NOz, v ring), 964 cm-' and 946 cm-' (ring stretching bands),
830 cm-' and 761 of (6 and yN02), 625 and 600 cm-' (t + y NO*). .No.peak was
observed in the FTlR spectrum of HMX in the range of 700-750 cm-' as marked in
~igure1 0. This indicates that all the HMX samples were in the p-polymorph form
[16]. Additionally, it can be concluded that EASAl .method does not lead to any
change in chemical structure of RDX and HMX.
XRD was performed to ascertain the crystalline state of the nano-HECS. The XRD
pattern of raw-HECs matched well with the nano-HECs. This clearly indicates that
HEC nanoparticles made by EASAl method are also crystalline. XRD patterns of
RDX and HMX respectively are shown in Figures 11 and 12. The XRD patterns of all
RDX and HMX samples showed similar patterns corresponding to a-RDX and f3-
HMX respectively [17, 181. The peaks at 28 for all RDX samples at: 13.1°, 13.4",
15.4", 17.4", 17.8", 20.4", 22.0°, 25.4" and 29.3" confirmed that the RDX samples
were in a-polymorphic form [17, 181. The peaks for all HMX samples at 28: 14.7",
I 16.0°, 23.0°, 26.1°, 29.6" and 31.9" confirmed that the HMX samples were in
polymorphic form [I 7, 181.
Thermal properties of energetic compounds are important as the initiation of
HECs is intrinsically related to thermal properties. The TGA thermal.curves of
RDX and HMX samples are presented in Figures 13 and 14 respectively.
Similarly DSC thermal curves of RDX andHMX samples are presented in
I
I Figures 15 and 16 respectively. RDX melts initially and then decompose to
give gaseous products. Melting of RDX was registered as an endothermic
event. in.the DSC thermal curves at around 202 "C. Rapid mass loss follows
after melting and almost 100 % mass loss was observed in TGA thermal
curves with a simultaneous exothermic in DSC thermal curves. These thermal
events are typical of raw-RDX that is recorded under similar experimental
conditions. The DSC thermal curves for raw-HMX and the nano-HMX
samples showed an endothermic at around 190 'C. This corresponds to solidsolid
state phase transition from @ to 6 phase [I911 Exothermic thermal
decomposition leading to almost complete mass loss was observed for all
HMX samples after 270 "C. This result is also similar to the characteristic
thermal response of HMX which is widely reported in the literature.
LPQ DELMf O3-12- 2015 17 144
Claims
Claim 1 A novel EASAl process for preparing nanosized particles of high energetic
compounds such asRDX, HMX etc. by the injection of a solutionof the energetic
compound to an antisolvent at elevated temperatures leading to the precipitation of
nanoparticles of the energetic compound.
Claim 2 The process as in claim 1 encompasses the rapid injection of a solution of
the energetic compound in a suitable solvent to an antisolvent at elevated
temperatures leading to the precipitation of nanoparticles of the energetic compound.
Claim 3 Tuning of the particle properties such as size and shape by varying the
parameter such as solvent, antisolvent, temperature of antisolvent and ratio of
solvent to antisolvent, rate of stirring and concentration of the HECs by using the
process as in claim 1.
Claim 4 Achieving particles having average particle size below 100 nm using the
proess mentioned in claim 1.
Claim 5 Preparation of Nanoparticles of RDX and HMX with average particle size
below 100 nm using acetone as solvent and water as the antisolvent at temperature
above 60 degree centigrade using the process as in klaim 1.
Claim 6 Preparation of non-spherical nanoparticles of RDX and HMX using ethyl
acetate as the solvent and water as the antisolvent at temperature above 60 degree
centigrade using the process as in claim 1.
Claim 7 Preparation of Nanoparticles having average particle size below 100 nm of
other organic compounds such as pharmaceutical drugs using the method
mentioned in Claim 1.