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Novel Enhanced Blast Explosives; Aluminized Enhanced Blast Explosive Based on Polysiloxane Binder

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Abstract and Figures

Drastic measures are taken in order to improve the properties of enhanced blast explosives. First, a binder with high reactivity as reducing agent is selected. The binder, platinum cure silicone, generates pyrophoric decomposition products in situ during the detonation. Silicon dioxide, the result of binder pyrolysis, plays the role of a catalyst for the aluminum combustion. Arising problems with curing inhibition of the polymer are solved, as nanosized SiO2 is employed for hardening catalyst carrier. Quantum calculations show specific interactions of the binder and aluminum, forming the backbone of the composition. Furthermore, particle size distribution of the main fuel, aluminum powder, is selected so the Al will have enough time to heat, ignite and burn in the atmosphere, creating the thermobaric effect. Potassium perchlorate is employed as an oxidizer, as it reacts with detonation products faster than the widely used ammonium perchlorate. The resulting enhanced blast explosive is denoted as H-TBX. High speed camera results show similar shock wave velocity 2.5 m from 2.7 kg TNT charge (2 ms after initiation) and 3.5 m from 2.65 kg H-TBX charge (3 ms after initiation). In the open field, the H-TBX generates 1.83 times higher peak pressure and 2 times higher impulse than TNT. These findings are compared with data from the literature for Tritonal, PBXN-109 and AFX-757. H-TBX has improved parameters over the former generations of enhanced blast explosives and is currently used in multiple weapons systems.
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Novel Enhanced Blast Explosives;
Aluminized Enhanced Blast Explosive Based on
Polysiloxane Binder
Stefan K. Kolev1*, Tsvetomir T. Tsonev2
1”E. Djakov” Institute of Electronics- Bulgarian Academy of Sciences, 72
Tzarigradsko Chausee Blvd., 1784 Sofia, Bulgaria
2SURT Technologies LTD, 6A Pastar Svyat Str., 1700 Sofia, Bulgaria
*Correspondence to kolevskk@abv.bg
Keywords: enhanced blast, thermobaric, fuel-air, polymer bonded explosive
Extended Abstract
The practical need for an enhanced blast explosive, brisant enough and violent in the
aerobic phase, insensitive and loadable in any warhead shape, is the driving force behind the
present work. Also, there is a specific need in the industry - for the replacement of the
hygroscopic and thermally unstable ammonium perchlorate.
Active polymers with oxidizer groups are too sensitive, so we chose an organosilicon
binder, that acts as a powerful reducing agent. The binder generates pyrophoric decomposition
products in situ during the detonation. Silicon dioxide, the result of its pyrolysis, plays the role of
a catalyst for the aluminum combustion. Silicone interacts with the oxide layer of Al powder in a
specific manner, to form the mechanical matrix of the composition. Last but not least, the
organosilicon, used for the first time in thermobaric explosives, has better oxygen balance than
HTPB. Particle size distribution of the main fuel, aluminum powder, is selected so the Al will
have enough time to heat, ignite and burn in the atmosphere, creating the thermobaric effect.
Keeping in mind the drawbacks of AP, potassium perchlorate is chosen as oxidizer. KP reacts
faster with reducing agents, especially at high pressure, because it has much higher burning rate
pressure exponent. The important role of the oxidizer is to provide enough energy in the initial
phase of the explosion, that the majority of the Al powder can heat to 2050 K and ignite. All
implemented innovations led to a composition with improved properties over the former
generations of enhanced blast explosives. Patent for the present formulation, which will be
denoted as “H-TBX”, was granted to the authors, Kolev and Tsonev, on 16.05.2018. After
3
extensive laboratory and field testing, the H-TBX was approved for mass production and is
currently used in multiple weapons systems, such as the TB-22M grenade (RPG-22 type) for the
Bulspike-TB launcher. The H-TBX formulation is produced in Bulgaria under the abbreviation
“PTBS”.
a), b) and c) Detonation of 2.5 kg H-TBX + 0.15 kg A-IX-1, 3 ms after initiation, d) Detonation
of 2.7 kg TNT, 2 ms after initiation. In all frames, the blast wave is situated on the surface of the
fireballs; a), b) and d) are recorded with Olympus i-speed 3 camera and c) with Phantom V7.3
camera; The contrast is increased and brightness decreased (b) in order to measure the
dimensions of the fireball; Distance between poles in a), b) and d) is 10 m.
4
Abstract
Drastic measures are taken in order to improve the properties of enhanced blast
explosives. First, a binder with high reactivity as reducing agent is selected. The binder, platinum
cure silicone, generates pyrophoric decomposition products in situ during the detonation. Silicon
dioxide, the result of binder pyrolysis, plays the role of a catalyst for the aluminum combustion.
Arising problems with curing inhibition of the polymer are solved, as nanosized SiO2 is
employed for hardening catalyst carrier. Quantum calculations show specific interactions of the
binder and aluminum, forming the backbone of the composition. Furthermore, particle size
distribution of the main fuel, aluminum powder, is selected so the Al will have enough time to
heat, ignite and burn in the atmosphere, creating the thermobaric effect. Potassium perchlorate is
employed as an oxidizer, as it reacts with detonation products faster than the widely used
ammonium perchlorate. The resulting enhanced blast explosive is denoted as H-TBX. High
speed camera results show similar shock wave velocity 2.5 m from 2.7 kg TNT charge (2 ms
after initiation) and 3.5 m from 2.65 kg H-TBX charge (3 ms after initiation). In the open field,
the H-TBX generates 1.83 times higher peak pressure and 2 times higher impulse than TNT.
These findings are compared with data from the literature for Tritonal, PBXN-109 and AFX-757.
H-TBX has improved parameters over the former generations of enhanced blast explosives and
is currently used in multiple weapons systems.
5
1. Introduction
The idea behind enhanced blast explosives is to use energetic fuel (reducing agent) to
react with the detonating substance/oxidizer, and even more so, with the atmospheric oxygen.
Retrospectively, complex combinations of fuels have been used.
1
-
4
Usually, enhanced blast
explosives (EBX) with more than 20-25% reducing agent are labeled as thermobaric (TBX).
Those are the true cases, where the aerobic reaction participates in the generation of blast wave.
The first thermobaric weapon, the RPO-A Schmel was fielded in Afghanistan in 1988. It
contains a suspension of isopropyl nitrate (IPN) and magnesium (Mg).3 Experimental details on
the performance of IPN/Mg and IPN/Mg/Al mixtures were recently published by Iorga et al.
5
Later variations of the formulation use IPN, hexogen, aluminum (Al), and fumed silica as
thickener.2,
6
Suspensions based on IPN generally suffer from the toxicity and volatility of the
monopropellant. The lack of mechanical strength renders the compositions useless in spinning
munitions that have to endure acceleration, such as mortar and artillery.
In order to avoid problems with monopropellants, the next generations of enhanced blast
explosives, developed in USA, use rubbery polymer binders, hydroxyl-terminated polybutadiene
(HTPB) in most cases.
7
They can be described as homogenous mixtures of binder (HTPB with
plasticizers), brisant explosive and reducing agent, usually a metal powder. Example of such
explosives is the PBXIH-135, containing octogen, Al and HTPB binder,7 loaded in the SMAW-
D NE grenade.
8
Despite its good mechanical properties and high brisance, PBXIH-135 performs
no better than 10% aluminized EBX in impulse testing based on floating roof.
9
Generally,
aluminized EBX based on inert binders cannot burn all the metal fuel in the air and therefore, the
potential energy content is not fully converted to blast impulse.
10
,
11
Obvious reason for these
drawbacks is the nature of the binder. HTPB pyrolyzes first to butadiene monomer and dimer
and then to lower molecular weight hydrocarbons.
12
At the high temperature of detonation
(>3000 K), hydrocarbons can additionally decompose, forming soot and hydrogen.
13
The listed
decomposition products of HTPB are not pyrophoric and do not have catalytical activity on the
Al combustion, they only hinder the access of oxygen to the Al particles.
Addressing these drawbacks, additional oxidizer is added to the formulation, to support
the burning of the reducing agent. In almost all cases, this oxidizer is ammonium perchlorate
(AP).2-4,
14
,
15
An example of such explosive is the AFX-757,15 used in the JASSM missile and
similar weapons.
16
It contains hexogen, HTPB, Al powder and AP. The improved formulation
allows a good air blast equivalent to be obtained, namely 1.39 (compared to Composition B) for
the AFX-757.
17
Similar energetic materials, however, have drawbacks that do not allow them to
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be used in wide variety of munitions. Ammonium perchlorate, apart from being hygroscopic and
unstable at high temperature, decomposes relatively slowly, it cannot fully react in the detonation
front.
18
As a consequence, such explosives are effective only in high quantities and strong
confinement, because they depend heavily on the post detonation mixing and the reaction of
oxidizer with the detonation products. The smallest munition loaded with AFX-757, we are
aware of, is the GBU-39 - 190 mm SDB aviation bomb.
19
,
20
AFX-757 has rather low brisance,
with only 25% hexogen it generates about PCJ = 10 GPa pressure, far too low for decent metal
fragmentation and acceleration effect. Very similar to the AFX-757 is the Russian analogue LP-
30T,
21
used in the TBG-7V - RPG-7 munition (marked on the warhead). In LP-30T, the HTPB is
substituted by LD-70, a plastigel binder containing polyacrylate and dinitrodiethyleneglycol
(70%), dinitrotriethyleneglycol (30%) monopropellants.
22
Adding magnesium to similar
mixtures generally increases the explosive power,
23
unfortunately, magnesium is incompatible
with ammonium perchlorate.
24
Actually, magnesium is incompatible with pretty much any
oxidizer used in pyrotechnics, in wet atmosphere.24
From the more exotic compositions, the fluorinated aluminum, used in the Hellfire
missile AGM-114N Metal Augmented Charge,
25
,
26
is worth discussing. So far, this is the only
annular design, used in ordnance. The Metal Augmented Charge is pressed around a powerful
HMX booster. Drawbacks of this composition include the toxicity of produced aluminum
fluoride and the bad mechanical properties.25 If the warhead of AGM-114N wasn’t perfectly
cylindrical, it would be almost impossible to load. Compositions based on exotic reducing agents
like boron, nanomaterials or core-shell particles have also been studied,1,
27
-
32
but for now, they
remain laboratory curiosity without practical use.
The practical need for an enhanced blast explosive, brisant enough and violent in the
aerobic phase, insensitive and loadable in any warhead shape, is the driving force behind the
present work. Also, there is a specific need in the industry - for the replacement of the
hygroscopic and thermally unstable ammonium perchlorate.
Active polymers with oxidizer groups are too sensitive, so we chose an organosilicon
binder, that acts as a powerful reducing agent.
33
,
34
The binder generates pyrophoric
decomposition products in situ during the detonation. Silicon dioxide, the result of its pyrolysis,
plays the role of a catalyst for the aluminum combustion. Silicone interacts with the oxide layer
of Al powder in a specific manner, to form the mechanical matrix of the composition. Last but
not least, the organosilicon, used for the first time in thermobaric explosives, has better oxygen
balance than HTPB. Particle size distribution of the main fuel, aluminum powder, is selected so
7
the Al will have enough time to heat, ignite and burn in the atmosphere, creating the thermobaric
effect. Keeping in mind the drawbacks of AP, potassium perchlorate is chosen as oxidizer. KP
reacts faster with reducing agents, especially at high pressure, because it has much higher
burning rate pressure exponent.
35
The important role of the oxidizer is to provide enough energy
in the initial phase of the explosion, that the majority of the Al powder can heat to 2050 K and
ignite. All implemented innovations led to a composition with improved properties over the
former generations of enhanced blast explosives. Patent for the present formulation, which will
be denoted as “H-TBX”,
36
,
37
was granted to the authors, Kolev and Tsonev, on 16.05.2018.34
After extensive laboratory and field testing, the H-TBX was approved for mass production and is
currently used in multiple weapons systems, such as the TB-22M grenade (RPG-22 type) for the
Bulspike-TB launcher. The H-TBX formulation is produced in Bulgaria under the abbreviation
“PTBS”.
2. Materials and methods
Temperature controlled chambers (heat and cool) used in the MIL-STD-2105D, 5.1.1 28-
day temperature and humidity (T&H) test are ILKA type KTK-3000, ILKA type KTK-800,
ILKA type TBV-2000 and Nuve KD 200. High speed photo cameras are Phantom v7.3 at 3000
fps and Olympus i-Speed 3 at 2000 fps. Software packages: i-Speed Viewer V. 3.1.0.7 and
Phantom Camera Control V. 9.2.675.2-C are used to create the detonation snapshots and
measure the shock front velocities. The impact sensitivity tests are performed using 2.5 kg drop
weight machine, assembled on site. Sensitivity to electric discharge is measured using capacitor
banks, charged to 20 kV, DC. AVL B250 IPG piezoelectric high pressure transducers are used
for the blast parameters measurements. AISI 1008 carbon steel sheets are used for the metal
acceleration assessment.
Aluminum powder and potassium perchlorate are purchased from Sofiyahim (Bulgaria);
hexogen is purchased from Dunarit (Bulgaria). The used platinum cure silicon has the following
properties: molecular mass 20000±10% g/mol; initial viscosity when mixed 3000 cps; Shore
hardness 00-30; density 1.07 g/cm3; pot life 2 hours at 15°C; elongation at break >500%;
shrinkage after curing < 0.0005 cm/cm; fumed silica content as catalyst carrier 0.1%. The
polymer consists of polydimethylsiloxane (PDMS) chains and the crosslinking is realized by
(methylvinyl)siloxane (MVS) and (methylhydrogen)siloxane (MHS) groups.
It should be noted that platinum cure silicone is sensitive to poisoning of the catalyst.
Certain substances containing sulfur or phosphorus, as well as acids and bases, can inhibit the
8
curing of the polymer. In clean laboratory conditions, the chance of such substances contacting
the samples is negligible. However, on industrial scale, there is a demand that the polymer is as
resistant to cure inhibition as possible. Our experience shows that using fumed silica (nanosized
SiO2) as a catalyst carrier (instead of just dispersing the catalyst into the polymer), dramatically
increases the resistance to cure inhibition. Measuring the extent of the effect remains a task for
future work.
All quantum chemical calculations are performed using the CP2K/Quickstep package.
38
,
39
The DFT is applied within the generalized gradient approximation (GGA), using Perdew-Burke-
Ernzerhof (PBE) functional.
40
Double-zeta basis set DZVP-MOLOPT-SR-GTH, optimized for
calculating molecular properties in gas and condensed phase, is applied for all atoms in the
studied systems.
41
For reducing the computational cost, Gaussian and Plane-Wave (GPW)
expansion sets are used for expanding the electronic wavefunctions.
42
,
43
. Only the valence
electrons are explicitly considered. Their interaction with the remaining ions is described using
the pseudopotentials of Goedecker-Teter-Hutter (GTH).
44
,
45
The charge density cutoff of the
finest grid level is equal to 400 Ry. The number of used multigrids is 5. Dispersion interactions
(for the PBE functional) are taken into account for all studied complexes. DFT + D approach,
with D3 set, recommended for use with electro neutral and charged complexes is used.
46
3. Results and discussion
3.1 Formulation
H-TBX is a cast-cured polymer bonded explosive. It is mixed, loaded (extruded) into
ordnance, and then cured to a solid mass with Shore A = 40 hardness. H-TBX contains the next
components in mass proportions:
12-20% Silicone polymer (both components)
15-20% Potassium perchlorate
25-35% Aluminum powder 8 micron
35-50% White crystalline hexogen powder
Potassium perchlorate is milled on site to less than 20 micron (sieve 20 micron) and used
directly to prepare the H-TBX. Potassium perchlorate must be free of chlorates. Aluminum
9
powder is standard spherical, average particle size D50 = 8 micron, uncoated. Hexogen is pure,
white crystalline without any additives, coatings or phlegmatizers, particle size < 100 micron.
There is no need to use bimodal hexogen.
3.2 Stability and mechanical properties
For the composition to be stable, every ingredient must be compatible with the other
ones. Before the experimental proving, we did a literature survey on the subject. The next
combinations are discussed:
- potassium perchlorate and aluminum powder
- silicone and hexogen
- silicone and potassium perchlorate
- hexogen and aluminum powder
- hexogen and potassium perchlorate
- silicone and aluminum powder
The combination potassium perchlorate - aluminum powder has been studied by
Shimizu,24 concluding that the system is completely storage stable. Pourmortazavi et al.
measured the temperature of decomposition of the mixture, which is higher than 673 K.
47
Ignition temperature has been measured in the interval of 853.15-888.15 K. The system silicone
hexogen has been studied by Elbeih et al.
48
,
49
Silicone has stabilizing influence on the hexogen.
The measured temperature of pure hexogen decomposition (peak of thermogravimetric study) is
486.25 K, while for the mixture silicone hexogen it shifts with 0.5 K to 486.75 K.48 Silicone is
used in non-polymerized state as a phlegmatizer for hexogen.49 In polymerized state silicone
decreases the impact sensitivity of hexogen about 3 times (XTX 8004).
50
Silicone and potassium
perchlorate both have high temperatures of decomposition (silicone over 573 K, potassium
perchlorate over 673 K). Both materials do not give acidic or alkali reactions. This is the reason
the combination of silicone and potassium perchlorate is used in gas generators.
51
The system
hexogen aluminum powder is historically proven to be stable and finds use in variety of
energetic materials.7 In the book Engineering Design Handbook; Explosives Series Properties of
Explosives of Military Interest
52
an explosive HEX-24 is described, consisting of hexogen,
potassium perchlorate, aluminum powder and petroleum based binding agent. According to the
10
studies it has stability up to 373.15 K heating, despite the low stability binding agent used. H-
TBX on the other hand contains extremely stable binding agent, which is the silicone. The
stability of the system, silicone aluminum powder, used in pyrotechnics, is proven by Eisele et
al.
53
The authors come to conclusion that silicone has higher thermal stability and thermal
stabilizing action on rocket propellants than the modern binders, for example - HTPB.
Knowing what to expect, first we tested the behavior of 20 grams H-TBX samples at low
and high temperature. The low temperature value was selected to represent the coldest weather
conditions that an H-TBX warhead could be exposed to. The high temperature value was
selected, so the compatibility of the components can be tested, without decomposing the
hexogen. Tests were conducted from 213.15 K to 393.15 K.36,37 No physical changes were
observed and no mass change was detected. Next, stability under vacuum test is performed
according to NFT 70-517 French standards. The objective of this test is to measure the volume of
gas released from the heated material. It is done at temperature of 353.15 K for 193 hours. For all
samples (polymer, polymer+perchlorate, polymer+hexogen, H-TBX), released gas volume was
less than 0.1 cm3/g. We conclude that the stability of the composition at high temperature is only
dependent on the decomposition of hexogen. At temperatures approaching hexogen
decomposition, the other ingredients behaved like an inert matrix, so we propose that the
temperature stability can be improved if hexogen is substituted by octogen. Sensitivity to impact
of the composition is found to be in the range of 60-80 cm for 2.5 kg falling weight (hexogen =
20-30 cm). H-TBX is insensitive to static electricity, up to 350 mJ. Consequently, TBG-7V type
of warhead is loaded in order to perform stability assessment in a practical situation, Figure 1.
11
Figure 1. Three TBG-7V type warheads, loaded with 2.5 kg H-TBX each, ready for testing to be
performed.
Stability of the loaded composition to temperature fluctuations is tested according to the
MIL-STD-2105D, 5.1.1 28-day temperature and humidity (T&H) procedure. Very hard regime is
chosen with temperature variations between 213.15 K (night time) and 348.15 K (day time),
Figure 2a, b, c. The loaded warheads passed all criteria of the test, Figure 3a, b. No extrusion of
polymer from the composition and no mechanical damage as cracking was observed. Thermal
expansion and contraction of the loaded explosive are measured equal to ±1 mm, Figure 2d, for
the 270 mm long warhead. Expansion and contraction are measured for the temperature intervals
of 288.15 K - 213.15 K and 288.15 K - 348.15 K. Six TBG-7V type warheads, loaded with H-
TBX, were kept at temperatures equal to 213.15 K, 288.15 K and 348.15 K (two at each
temperature) until thermal equilibrium is reached. The warheads were then detonated in the open
to see if the low and high temperatures have effects on the explosive train (detonator, booster and
the main charge). In all cases full detonations were recorded. Shore hardness is measured equal
to 40A at 293.15 K. Although the theoretical maximum density of H-TBX is 1.84 g/cm3, the
TBG-7V is loaded to 1.75 g/cm3 without vacuum. Vacuum loading will certainly improve
density. Finally the H-TBX is tested to acceleration by firing (>20 shells) in 122 mm shell for D-
30 howitzer (about 15000 G), again passing the test.
12
Figure 2. a), b) and c) Opened and enclosed TBG-7V type warheads as well as H-TBX and
polymer (binder) samples in the temperature controlled chamber during the MIL-STD-2105D,
5.1.1 28-day temperature and humidity (T&H) test. Ice forms during the low temperature period
(c); d) Measuring the thermal expansion of H-TBX in the TBG-7V type warhead.
13
Figure 3 TBG-7V type warheads, loaded with H-TBX; a) Before MIL-STD-2105D, 5.1.1 28-day
temperature and humidity (T&H) test and b) After the test.
The good mechanical properties of H-TBX can be explained by the binder interactions
with the solid fillers. Every silicone monomer {-Si(R2)O-} has an oxygen atom with partial
negative charge and free electron pairs, available for complexation. These numerous oxygen
atoms can form hydrogen bonds with the CH2 groups of hexogen, electrostatic complexes with
K+ ions of KClO4 or Al3+ ions of the alumina (Al2O3, the passivation layer of Al powder).
Aluminum ion, having 3+ electric charge and free 3-level orbitals, forms coordination complexes
with multiple electronegative centers, thus we expect a strong coordination between oxygen
atoms from the silicone and the available Al3+ at the alumina surface. Binding between silicone
and the large surface of Al powder is expected to form the mechanical matrix of the composition.
In order to perform investigation in atomistic resolution, quantum-chemical calculations
of the binder interactions with the alumina surface are performed. Computational details are
available in the Materials and methods section. The initial structure contains the elementary
hexagonal cell of α-alumina (corundum, 60 atoms) and 1500 pm long polydimethylsiloxane
(PDMS) chain. After geometry optimization, the alumina cluster differs from crystallinity
because of the interactions with the polymer, Figure 4a. Thus obtained amorphous alumina
represents exactly the surface of Al particle. It is known that low temperature oxidation
(passivation) of Al yields amorphous oxide layer.
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As expected, the coordination bond between
Al3+ center and oxygen atom from the silicone is observed, Figure 4a, with a bond length of 189
14
pm. Radial distribution function for the Al-O distances of the alumina cluster, Figure 4c, starts at
165 pm, peaking at 175 pm and ends at 210 pm. The coordination Al-O bond has length within
this range. Oxygen atoms of the alumina can also form weak hydrogen bonds (HBs) with CH3
hydrogen atoms from the silicone. Similar weak HBs, between CH3 groups and electronegative
centers, were already observed and studied in model systems.
55
In the case of silicon, the HB
lengths start from 210 pm, Figure 4d, up to the accepted HB cutoff, 400 pm.
56
For comparison,
the interactions of polybutadiene (PB) (1500 pm long chain) and alumina are also studied, Figure
4b. Hydroxyl groups are not added to the PB chain as they exist in negligible quantities in the
commercial HTPB binder. Similarly, PB can interact with alumina in two ways: via π Al3+
bonding and hydrogen bonds. The first type of interaction is realized when π electron density of
a double bond (C=C) interacts with Al3+ ion, Figure 4b, with shortest C-Al3+ distance of 220 pm.
In the case of PB, the HB lengths start from 243 pm, up to 400 pm, Figure 4e. In the case of
silicone, HBs are shorter because of the inductive effect of oxygen atoms and the CH3 groups,
pointing out of the main Si-O chain, that easily make contact with available O atoms of alumina.
Calculated binding energy between silicone and alumina is equal to 268 kJ/mol for 1500 pm long
silicone chain. The corresponding value for PBalumina system is 215 kJ/mol for 1500 pm long
PB chain.
15
Figure 4 a) Geometry optimized complex of PDMS and alumina, only one hydrogen bond is
shown - as dotted blue line, Al3+-O(PDMS) bond is shown with solid line. Hydrogen atoms are
represented in cyan, carbon in grey, oxygen in red, silicon in green and aluminum in light purple
b) Geometry optimized complex of PIB and alumina, only one hydrogen bond is shown for
clarity, Al3+-C bond is shown with solid line; c) Radial distribution function for the Al-O
distances of the alumina cluster (PDMS-alumina system); d) Radial distribution function for the
O-H distances (hydrogen bond lengths of PDMS-alumina system); e) Radial distribution
function for the O-H distances (hydrogen bond lengths of PIB-alumina system).
3.3 Performance
3.3.1 Brisance, metal fragmentation and acceleration
Brisance characteristics of heterogeneous explosives depend mainly on the fast
detonating energetic materials used.18 Despite the current advances in the science of energetic
molecules and crystals,
57
-
61
hexogen (and octogen) still remain the best choices as a combination
of performance, reasonable sensitivity and price. Calculations with CHEETAH 2.0
62
are
16
performed in order to assess the detonation velocity and pressure of H-TBX, under the
assumption that 20% of the Al powder reacts initially. Results indicate PCJ = 17 GPa, at
detonation velocity of 7204 m/s. Temperature of the detonation products is equal to 3406 K.
Evaluation of the thermochemical code, performed by Lu,
63
shows that it produces results with
only 1.1% error for an explosive (ARX-2010) with the same hexogen content as H-TBX.
However, the H-TBX is expected to have higher detonation velocity than the ARX-2010 as it
uses denser oxidizer and binder, and contains more hexogen per volume.
Figure 5 Graphical representation showing the detonation wave movement as a function of time
for the 90 mm H-TBX charge. Slope of the trend line is equal to the detonation velocity.
Detonation velocity of H-TBX is experimentally measured using piezo probes at the said
distances (90 mm diameter charge). Graphical representation Distance=f(Time) gives a
detonation velocity of 7242 m/s as the slope of the trend line, Figure 5. Data for the plot in
Figure 5 is presented in the Supporting Information - Table S1. With detonation pressure and
velocity similar to TNT, we expect similar metal fragmentation capability. Indeed, the H-TBX
loaded in 130 mm shell for M-46 howitzer produced fragments with mass distribution similar to
the original pressed TNT shell.36 Metal acceleration is assessed based on 57 mm prefragmented
S-5 warheads (for the S-5 air-to-ground missile) loaded with HTBX (650 g + 50 g A-IX-1)
and 700g A-IX-1. Both warheads provide 360 fragment elements, 2 g each. The targets, used in
this experiment, are sheet metal panels with thickness varying from 2 to 4 mm respectively,
positioned in spiral pattern (from 3 to 7 meters) around the charge, Figure 6a and Figure S1. The
RDX (A-IX-1) loaded warhead produced 302 hits, which resulted in 302 penetrations. In the
same configuration, the H-TBX warhead produced 248 penetrations from 248 hits, but also
17
managed to drop down few of the steel targets used. All hits from the fragment elements resulted
in penetrations for both warheads, their distribution as a function of the steel sheets thickness and
distance to the target is presented in Figure 6b. The exact count of penetrations for all sheets is
presented in Table S2. Graphical solution of the Gurney relations
64
gives medium fragments
velocity of about 1800 m/s for the H-TBX warhead (assuming that H-TBX has similar metal
acceleration abilities to TNT and the mass of H-TBX is equal to the mass of fragments, 360x2g).
Figure 6 a) H-TBX loaded 57 mm prefragmented S-5 warhead in front of the steel sheets before
initiation; b) Graphical representation of the penetrations count as a function of the steel sheets
thickness (l in mm) and distance to the target (s in m) for both tested explosives.
18
3.3.2 Aerobic combustion
The use of organometallic compounds for incendiary or enhanced blast purposes is not
new, for example the American weapon M202 Flash uses triethylaluminum (TEA).
65
Obviously,
the high reactivity of organometallics makes them very suitable for that purposes, far superior
than the inert HTPB. Anyway, room temperature pyrophoric compounds like TEA have safety
issues, they will burst in flames at the slightest decapsulation of the container. This is the reason
their practical implementation is extremely unwanted. So, for a practical application, we need an
organometallic that combines stability to at least 573 K, with very high, even pyrophoric
reactivity far above that temperature. Our idea was to use an organoelemental compound with
some of the chemical bonds at the element (metal) center already oxidized, to reduce the
reactivity. Such compound is found to be the organic silicone (PDMS for example). With a
formula {-Si(R2)O-}n , it has half (two) of the chemical bonds at the silicon atom formed
(oxidized) with O atoms, but the other (two) remaining electrons participate in bonds with
organic groups (R). We expect the bonds Si-R to be easily attacked by oxidizing species at the
high temperature of detonation. Scission of the Si-R bond will produce radicals (Si and R
containing). Scission at the main chain will also produce reactive O(R2)Si radicals. Atoms
regrouping will produce pyrophoric silanes. Theoretical study of the PDMS decomposition at
3500 K finds products like C6H11Si, C3H9Si and SiH4,
66
from the silane homologous series,
which have pyrophoric properties. This way, the otherwise thermally stable silicone produces in
situ reactive pyrophoric products. Organic silicone is oxidized by oxygen to produce silicon
dioxide (SiO2), carbon dioxide and water, releasing 25 MJ/kg, the same amount of energy as
magnesium. Inorganic silicone is also readily oxidized in air, it is even suspected in participating
in electrical phenomenon such as ball lightning.
67
Silicon dioxide, on the other hand, is an effective catalyst for the aluminum combustion.
The mechanism of catalysis is: disturbing the passivation Al2O3 film, thus creating spots where
metallic Al is available for oxidation.
68
This is the reason for the fast burning rate of rocket
propellants based on organic silicone binder and Al fuel.53 For the discussed catalytical process
to take place, temperature over 823 K is needed, as this is the heat resistance point of silicone
film on aluminum.68 This process is going to be beneficial for the aerobic combustion of Al in H-
TBX, without increasing its sensitivity.
19
For the burned fuel to participate in blast enhancement, the burning must take place
during the positive phase of the pressure wave (t+).64 With other words, during t+, the Al particles
must heat to 2050 K (ignition temperature) and burn completely.
Equations (1) and (2) give practical approximation for the heating at 3000 K (th) and
burning time (tb)64
th = 0.0025(d1.95) (1)
tb = 0.003(d1.99) (2)
where d is the diameter of the particles in μm; th and tb in ms.
For the 8 μm Al particles th + tb = 0.33 ms. The positive phase duration (t+) depends on
the TNT equivalent of the explosion and the distance from the charge. We were unable to find in
the literature at what distance t+ should be calculated. Assigning this variable is another novelty
of the present work.
Experimental work follows theory. The loaded TBG-7V type warheads (Figure 1) are
detonated in the open (20 cm from the ground) and the events studied by high speed camera,
Figure 7a, b, c. Comparative testing is done with 2700 g trinitrotoluene (TNT) charge, Figure 7d.
Charge diameter for H-TBX and TNT is 100 mm; drawings of model 2.5 kg and 1 kg H-TBX
charges are available in the Supporting Information, Figures S2 and S3. During detonation,
neither H-TBX, nor TNT behave like ideal explosives. H-TBX is loaded with dense solid Al
particles and TNT forms carbon soot, also in the form of solid particles. For the TNT explosion,
we see the blast wave detaching from the glowing surface of detonation products at the 2nd ms
from the initiation. The longest radius of the glowing spheroid is 2.5 meters (2nd ms). For the H-
TBX, the blast wave detaches from the glowing sphere at the 3rd ms from the initiation, 3.5
meters from the charge initial position. So, the time Al particles have, to burn and enhance the
blast, should be equal to the time needed to reach the point - where the blast wave detaches from
the glowing products, plus the duration of the positive phase at that point. So, for the 2500 g H-
TBX (plus 150 grams A-IX-1 booster) we have 3 ms + 2.9 ms (duration of the positive phase
calculated for 7.4 kg TNT, see equation (3)
69
and below), or 5.9 ms in total. If oxygen is
available, there is more than enough time for the main fraction of 8 μm Al particles to participate
in blast wave enhancement as th + tb = 0.33 ms.
Further analysis of the H-TBX and TNT explosions is performed using data from the high
speed camera. The blast wave detaches from the detonation products in a very special point. This
is the last point from the initiation, where the velocities of expanding products and blast wave
20
(shock front) are equal. For the TNT charge, the shock front velocity vs (TNT) (at that point, at 2.5
m) is equal to 733 m/s, measured with the high speed camera. The calculated vs (TNT) for the same
charge at the same point, using Rankine-Hugoniot equation,
70
-
72
is 742 m/s, equation (4). The
needed overpressure is calculated in equation (5).
 
(3)
   (4)
  
  

) (5)
Where vs is the the shock front velocity in m/s; p is the overpressure in atm (ambient
pressure = 1 atm); m is the explosive (TNT) mass in kg and r is the distance from the charge in
m.
The shock front velocity for the H-TBX charge vs (H-TBX) (at the discussed point, at 3.5 m)
is equal to 767 m/s, measured with the high speed camera. This result is very close to the TNT
shock front velocity at 2.5 m. As the shock front velocity is a function of the overpressure, vs =
f(p), equation (4), we should have equal overpressure 2.5 m from the TNT charge and 3.5 m
from the H-TBX charge. According to the revised Sadovsky formula, equation (5),69 the
overpressure of 2.7 kg TNT at 2.5 m is 4.28 atm. The same overpressure is generated by 7.4 kg
TNT at 3.5 m. Difference in fireball shapes on Figure 7a and 7c are caused by booster
placement, but it had no influence on detachment time or fireball radius. The frames on Figure
7a and 7b are identical with only difference in brightness and contrast adjustments.
21
Figure 7 a), b) and c) Detonation of 2.5 kg H-TBX + 0.15 kg A-IX-1, 3 ms after initiation, d)
Detonation of 2.7 kg TNT, 2 ms after initiation. In all frames, the blast wave is situated on the
surface of the fireballs; a), b) and d) are recorded with Olympus i-speed 3 camera and c) with
Phantom V7.3 camera; The contrast is increased and brightness decreased (b) in order to
measure the dimensions of the fireball; Distance between poles in a), b) and d) is 10 m.
Parameters of the TNT and H-TBX charges are fitted in the SedovTaylor blast model,
equations (6-9).
73
  (6)
        (7)
  

 (8)
 
 (9)
Where Rs is the radius of the shock front in m; t is the time from initiation in s; Ed is the
energy of detonation in J; τ0 - time scale of the energy release in s; ρ is atmospheric density in
kg/m3; m is the explosive mass in kg; n is the dimensionality of expansion: (n=1) planar
22
expansion, (n = 2) cylindrical expansion, and (n=3) spherical expansion; s is the energy release
factor, (s=0) instantaneous energy release and (s=1) constant-rate energy release; lo is the length
scale in m.
TNT detonation is characterized by cylindrical expansion, n=2, Figure 7d (shape of the
charge is also cylindrical), and instantaneous energy release, s=0. Energy release for TNT is
chosen as 4 MJ/kg. Calculations show that 2 ms after 2.7 kg TNT initiation, the blast wave has
traveled 2.43 m (Rs=2.43 m). For the H-TBX charge we chose spherical expansion, n=3 and
constant-rate energy release, s=1. These parameters are common for large aluminized charges.73
Energy release for H-TBX is considered as 4x2.79=11.16 MJ/kg. This is the energy released by
1 kg TNT x 7.4/2.65, where 7.4 is the amount of TNT needed to generate the same overpressure
as the 2.65 kg H-TBX warhead, see previous paragraph. For Rs=3.50 m (the experimental value
at 3 ms), τ0=1.24 ms. This value for τ0 corresponds to the period with the brightest light output,
seen using the high speed camera. Result of the fit show that the first 1.24 ms after initiation
contributes the most for the blast wave generation, with the release of energy equal to 11.16
MJ/kg.
To test if the difference in detachment times (from the detonation products) is influenced
by the 2 mm aluminum alloy casing of the H-TBX charge (TBG-7V type warhead has Al casing,
in the previous experiment TNT was detonated without a casing), we use the same S-5 warhead
as in metal acceleration, see 3.3.1. Prefragmented rings of S-5 are dismantled in this test. So, two
identical 1 mm Al alloy cylinders with 45 mm internal diameter are loaded with 700 g A-IX-1
and 50 g A-IX-1 + 650 g H-TBX. The blast waves detach from fireball at 1.3 ms (A-IX-1),
Figure 8a, and 1.7 ms (H-TBX), Figure 8b. This experiment proves that detachment times’
differences indeed depend on the types of explosives used.
23
Figure 8 a) Detonation of 700 g A-IX-1, 1.3 ms after initiation; b) Detonation of 50 g A-IX-1 +
650 g H-TBX, 1.7 ms after initiation. In both frames the blast wave is situated on the surface of
the fireballs.
Table 1. Measured blast wave parameters for the TBG-7V type warheads, loaded with 2.5 kg H-
TBX and 150 g A-IX-1.
Distance
m
Max Pressure
atm
Impulse
atm*ms
Duration
ms
2
9.5
4.9
2,4
3
3.7
3.0
2.9
4
1.7
1.2
3.1
5
1.5
1.2
3.8
6
1.1
1.2
4.7
7
0.7
1.1
4.7
8
0.6
0.8
4.7
10
0.5
0.7
7.0
Blast wave parameters (open field) for the 2.65 kg H-TBX warhead are measured using
piezoelectric pressure transducers Table 1, Figure S4. The sensors are placed 1 meter apart at
distances 2-10 meters. Three sensors were used at a time (placed at 2,3,4 ; 5,6,7 and 8,9,10 m)
and three identical warheads were detonated. Data for 9 m is not presented because of a sensor
malfunction. As expected, the peak (max) pressure drops quickly, from 9.5 atm at 2 m to 0.5 atm
at 10 m. The generated impulse also decreases with the distance, and the duration of the positive
phase increases. We can compare the blast wave parameters of H-TBX with those, obtained by
2.7 kg TNT charge at 6 meters. For the TNT (6 m from the charge) we have measured max
24
pressure of 0.6 atm, impulse 0.6 atm*ms and positive phase duration equal to 3.4 ms. The H-
TBX charge generated 1.83 times higher peak pressure and 2 times higher impulse than the TNT.
Positive pressure phase was 1.3 ms longer in the case of H-TBX. Using equation (5), we can
calculate that at 6 m, 6.2 kg TNT is needed to generate the same peak pressure as the H-TBX
(1.1 atm). This value is 16% lower than the result obtained with the high speed camera (7.4 kg
TNT). Obtained results are compared with literature data on the air blast performance of other
mass produced (cast) enhanced blast explosives. Nicolich and Niles17 give the following
equivalents: for Tritonal - 1.30, PBXN-109 - 1.42 and for AFX-757 - 1.65 (all compared to
TNT69). The authors do not specify if the equivalents are calculated by using data on impulse or
peak pressure.
We have to note that data from the pressure sensors can only be compared if a similar
warhead to the present one (Figure S2) is tested. Different factors can affect measurements with
piezo sensors including: the presence of fast burning, fast moving particles in the first
milliseconds of the explosion; pieces of the warhead body burning in the air and interacting with
the sensor; and the dependence of the pressure history vs the angle of the sensor towards the
warhead.
The high performance of H-TBX in air blast will only be realized if conditions for aerobic
burning are presented. If the explosive is detonated in inert atmosphere or underground, no
aerobic burning will occur. On the other hand, even better performance is expected, if the charge
is detonated in enclosed space. Confinement should allow greater part of the stored chemical
energy to be converted into blast impulse. At this point, we have not performed experiments or
calculations to determine the blast wave parameters of H-TBX in enclosed space. This task
remains to be completed in the future.
4. Conclusions
Novel enhanced blast explosive formulation (H-TBX) is created by using a silicon binder
that serves multiple roles. The binder generates in situ pyrophoric products and decomposes to
silicon dioxide, which has a catalytic effect on the aluminum combustion. Experimental and
theoretical results prove the good stability of the final product, with thermal stability only
depending on the used brisant explosive. Experimental results show that H-TBX can endure
heavy thermal cycling, namely MIL-STD-2105D, 5.1.1 28-day (T&H) test with temperature
variations between 213.15 K (night time) and 348.15 K (day time). Quantum calculations
25
explore the interactions between the binder and Al powder in the composition, shedding light on
coordination Al-O bonding and the formed weak hydrogen bonds. Binding energy between Al
and the binder is calculated, equal to 268 kJ/mol for 1500 pm long silicone chain, compared to
215 kJ/mol for 1500 pm long polybutadiene chain. The high density and relatively high content
of brisant explosive provide metal fragmentation and acceleration effects no worse than TNT. H-
TBX is primarily designed for air blast. Aluminum powder grains size is chosen so the fuel has
time to heat and burn with high yield during the positive pressure phase, creating the thermobaric
effect. High speed camera results show similar shock wave velocity 2.5 m from 2.7 kg TNT
charge (2 ms after initiation) and 3.5 m from 2.65 kg H-TBX charge (3 ms after initiation). In the
open field, the H-TBX generates 1.83 times higher peak pressure and 2 times higher impulse
than TNT. These results are compared with data from the literature for Tritonal, PBXN-109 and
AFX-757. With a combination of good stability, metal fragmentation and thermobaric effect, the
H-TBX was chosen as the thermobaric explosive for multiple weapons systems, currently
produced in Bulgaria. They include RPG-22 and RPG-7 types of warheads as well as 120 mm
mortar shells.
5. Data availability
The original video materials that support the findings of this study are available from the
corresponding author on request.
6. Author contribution statement
S.K. selected the polymer, its polymerization catalyst carrier and performed theoretical
estimations for the energetic and structural characteristics of the composition. T.T. selected the
oxidizer and worked on the ingredients’ theoretical compatibility assessment. Both authors
optimized the ingredients’ proportions and worked on the experimental part.
7. Competing interests:
The authors declare no competing interests.
26
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Supporting Information
Aluminized Enhanced Blast Explosive Based on
Polysiloxane Binder
Stefan K. Kolev1*, Tsvetomir T. Tsonev2
1”E. Djakov” Institute of Electronics- Bulgarian Academy of Sciences, 72
Tzarigradsko Chausee Blvd., 1784 Sofia, Bulgaria
2SURT Technologies LTD, 6A Pastar Svyat Str., 1700 Sofia, Bulgaria
*Correspondence to kolevskk@abv.bg
Keywords: enhanced blast, thermobaric, fuel-air, polymer bonded explosive
Table S1 Distances of the P2, P3, P4 and P5 piezo probes from the P1 probe; and the relative
times for the detonation wave arrival at each probe. All necessary data used to measure the
detonation velocity of H-TBX.
Probes
P1
P2
P3
P4
P5
Distance, mm
0
10.046
20.005
29.983
39.841
Time, μs
0
1.314
2.434
4.26
5.571
Table S2 The penetrations count as a function of the steel sheets thickness (in mm) and distance
to the target (in m) for both tested explosives.
Distance, m
Thickness, mm
A-IX-1
H-TBX
3
2
4
0
3
3
109
79
4
2
18
44
4
3
22
6
5
3
88
25
5
4
8
47
6
2
24
25
6
3
5
2
7
2
18
19
7
3
6
1
Sum
302
248
33
Figure S1 Steel sheets assembly used in the metal acceleration experiments with the S-5
warheads. Distances to the warheads are marked in red squares.
34
Figure S2 2.5 kg H-TBX charge. All dimensions in mm. H-TBX = 2.5 kg (at 1.75 g/cc), booster
= 106 g (at 1.5 g/cc). Booster of the original charge was A-IX-1 (pressed 95% RDX 5% wax) but
any modern RDX/HMX booster explosive can be used. Modern “insensitive” booster
formulation can and should be used.
35
Figure S3 1 kg H-TBX charge. All dimensions in mm. H-TBX = 1 kg (at 1.75 g/cc), booster =
71 g (at 1.5 g/cc). Booster of the original charge was A-IX-1 (pressed 95% RDX 5% wax) but
any modern RDX/HMX booster explosive can be used. Modern “insensitive” booster
formulation can and should be used.
36
Figure S4 View of a piezo sensor, used in the blast wave parameters measurements, and the
TBG-7V type warhead, loaded with H-TBX.
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