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Novel Enhanced Blast Explosives; Aluminized Enhanced Blast Explosive Based

on Polysiloxane Binder

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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.

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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

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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

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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 Interest52 an explosive HEX-24 is described, consisting of hexogen,

potassium perchlorate, aluminum powder and petroleum based binding agent. According to the

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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.

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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.

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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.54 As expected, the coordination bond between

Al3+ center and oxygen atom from the silicone is observed, Figure 4a, with a bond length of 189

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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 PB–alumina system is 215 kJ/mol for 1500 pm long

PB chain.

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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 2.062 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 H–TBX (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 relations64 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 lighting.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).

𝑡+ = 1.2√m6

√r (3)

vs = 343.2√1 + 0,86p (4)

p = 0.9869(1.02m1∕3

r+ 4.36

m2∕3

r2+ 14

m

r3) (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 Sedov–Taylor blast model,

equations (6-9).73

𝑅𝑠(𝑡) = 𝑎𝑡𝑏 (6)

𝑏 = (𝑠 + 2) (𝑛 + 2)⁄ (7)

𝑎 = (𝐸𝑑∕(𝜏0

𝑠𝑙03−𝑛)

𝜌)

1/(𝑛+2)

(8)

𝑙0 = (3𝑚

2𝜋𝜌)

1∕3

(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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32

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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