Thermodynamic Modeling of Pyrotechnic Smoke Compositions - ACS

Mar 11, 2016 - The FactSage 6.4 software package was used to model the thermodynamics of pyrotechnic smoke compositions based on boron carbide, ...
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Thermodynamic Modeling of Pyrotechnic Smoke Compositions Anthony P. Shaw,*,† Jason S. Brusnahan,‡ Jay C. Poret,† and Lauren A. Morris† †

Armament Research, Development and Engineering Center, U.S. Army RDECOM-ARDEC, Picatinny Arsenal, New Jersey 07806, United States ‡ Defence Science and Technology Group, Edinburgh, South Australia 5111, Australia S Supporting Information *

ABSTRACT: Some of the most effective visible obscurants for military applications are toxic or incendiary or present serious logistical complications. Sustainable alternatives are needed to mitigate the risks of human exposure and environmental contamination. The FactSage 6.4 software package was used to model the thermodynamics of pyrotechnic smoke compositions based on boron carbide, hexachloroethane, and phosphorus. The computational results are shown to be relevant in light of prior experimental observations. Boron phosphide is proposed as a benign source of phosphorus for next-generation pyrotechnic smoke compositions. The thermodynamics of the BP−KNO3 system have been studied computationally. The results indicate that certain stoichiometries should produce elemental phosphorus upon combustion. The properties of the BP−KNO3 system are examined considering the functional requirements of smoke munitions. KEYWORDS: Smoke, Obscurants, Pyrotechnics, Thermodynamics, Modeling

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INTRODUCTION Smoke-producing compositions and devices are used extensively by armies around the world for a number of purposes. On the battlefield and in training exercises, smoke clouds are used for signaling, marking targets, and screening troop movements. Historically, visible obscurants have been characterized by a hazard/performance trade-off. Some of the most effective options are toxic or incendiary or possess other problematic characteristics. Safer alternatives generally have not provided the desired level of obscuration performance. Although white phosphorus (WP) offers the greatest visible obscuration performance of any known substance, it presents serious hazards and logistical complications.1−3 WP is typically dispersed aerially by bursting mortar or artillery projectiles. Combustion upon contact with the atmosphere produces phosphorus oxides that are extremely hygroscopic. These oxides rapidly absorb atmospheric moisture resulting in the formation of a large and highly effective smoke screen.4 However, scattered WP particles do not combust instantaneously and therefore can cause collateral damage in combat as well as the contamination of domestic training ranges.5−7 Due to the pyrophoric nature of WP, smoke munitions containing this substance must be stored near a source of water so that they may be submerged if damaged.8 To address some of the hazards associated with WP, smoke compositions containing red phosphorus (RP) were developed although this approach has introduced other logistical issues. RP-based smoke compositions slowly degrade in the presence of air and trace moisture, producing acids that can corrode © XXXX American Chemical Society

munition components and phosphine gas that is highly toxic and flammable.9 The use of microencapsulated and stabilized RP slows the aging process but does not completely inhibit it.10 Additionally, RP-based smoke compositions are generally very sensitive to unintended ignition by impact, friction, and electrostatic discharge.11 Notably, the most problematic characteristics of phosphorus-based smoke munitions are caused by the white or red phosphorus itself, not the resulting phosphoric acid aerosol.12−14 Smoke compositions containing hexachloroethane (HC), zinc oxide, and aluminum were developed in the 1940s.15 The zinc chloride formed and aerosolized upon combustion is deliquescent making the resulting cloud exceptionally large, dense, and effective for screening. Unlike most phosphorusbased smoke compositions, which burn in direct contact with the atmosphere, HC compositions are usually pressed into steel canisters. The smoke is released through small vent holes at one or both ends. HC smoke grenades are no longer used by the U.S. Army due to the acute toxicity of the smoke, which has caused accidental inhalation-related injuries and deaths.16 Despite a number of efforts to address this problem, less toxic alternatives with comparable efficacy remain elusive.17−19 Boron carbide-based smoke compositions were developed in our laboratories to fill a gap in the hazard/performance continuum.20−22 Experimental “BC” smoke grenades aerosolize Received: December 22, 2015 Revised: March 1, 2016

A

DOI: 10.1021/acssuschemeng.5b01762 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering potassium borates and potassium chloride with remarkable efficiency but these materials are not particularly hygroscopic, limiting obscuration performance. New hygroscopic smokes with acceptable hazard profiles are needed as HC replacements and as plausible alternatives to WP- and RP-based systems. Thermodynamic modeling can be used to reduce the cost and duration of energetic materials development. Despite the common use of such modeling in propellants and explosives development, the technique has been less frequently applied to pyrotechnics. Thermodynamics software packages such as CHEETAH,23 NASA CEA,24 and the ICT Thermodynamic Code25 compute equilibrium products using a finite database of possible chemical species, typically under adiabatic conditions. Several limitations of this approach should be taken into consideration. The combustion of slow-burning pyrotechnics does not occur adiabatically because there is ample time for heat to be lost to the surroundings. Pyrotechnics often contain metals and other elements that may produce products not present in the software database. The actual combustion reactions do not necessarily produce equilibrium products. It is also difficult to account for the possible participation of atmospheric components in the combustion reactions. Nonetheless, in certain cases these issues can be managed and the computational results can provide meaningful insights. In this article, we demonstrate that thermodynamic modeling is applicable to certain types of pyrotechnic smoke compositions. The FactSage 6.4 software package,26 containing thermodynamic data for an extensive collection of inorganic species, has been used to model the combustion of BC, HC, and phosphorus-based smoke compositions. A critical assessment of the results in the context of prior experimental work is presented. Boron phosphide (BP) is proposed as a new pyrotechnic fuel for effective, yet sustainable, smoke compositions. The BP−KNO3 system is evaluated computationally as an initial step toward this goal.

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Figure 1. Calculated equilibrium product phase distribution as a function of reaction temperature for BC-1a. The adiabatic temperature, corresponding to 100%, is 1927 K: gases (solid red), liquids (long-dashed blue), solids (short-dashed black).

EXPERIMENTAL SECTION

Figure 2. Calculated equilibrium gas products as a function of reaction temperature for BC-1a. The adiabatic temperature, corresponding to 100%, is 1927 K: all gas products (solid red), permanent gases (dotdash green), difference (dotted purple). Permanent gases include CO, N2, and H2.

Thermodynamic equilibrium calculations were performed with FactSage 6.4 (Thermfact/CRCT and GTT-Technologies).26 The particular databases used were FactPS, FToxid, and a custom database containing calcium stearate monohydrate (C36H72CaO5), polyvinyl acetate (C4H6O2), epoxy (C21H24O4), hexachloroethane (C2Cl6), Pwhite, Pred, BP, P2B12, and BPO4. The custom database was professionally built by The Spencer Group, Inc. using available literature data.27 All simulations were conducted at a constant pressure of 1 atm with the reactants initially at 298.15 K. The initial analyses were performed in adiabatic mode (ΔH = 0). The results consist of predicted adiabatic reaction temperatures (Tad) and the thermodynamic products at those temperatures. Where indicated, calculations were performed at fixed temperatures less than the predicted Tad. These calculations simulate heat loss to the surroundings (ΔH < 0) and give the thermodynamic products at the temperatures that are specified. In such cases, the calculations were performed at temperatures as low as 70% of Tad and selected products are plotted as a function of the percentage of Tad (Figures 1, 2, S1, and S3). Other details pertaining to the calculations are contained within the figure captions, table footnotes, and Supporting Information.

organic dyes require low combustion temperatures and nonequilibrium conditions to achieve the desired effect sublimation of the dyes without excessive oxidation which can degrade color quality.28 Although it is possible to model the combustion of the sugar−KClO3 pair, the intended pyrotechnic reaction, the relevance of the results with respect to the behavior of the actual smoke compositions is not obvious. In contrast, smoke compositions that volatilize combustion products, such as salts or oxides, operate at higher temperatures and are much more likely to approach equilibrium. BC Smoke Compositions. At a minimum, the subject compositions contain a fuel−oxidizer pair (B4C−KNO3) and an inert diluent to moderate combustion temperature. In appropriate proportions, ternary B4C−KNO3−KCl mixtures produce thick white smoke clouds upon combustion but tend to burn very rapidly. Small amounts of calcium stearate may be added to reduce the burning rate. Additionally, polymeric binders such as polyvinyl acetate (PVAc) may be used for composition granulation and to reduce nuisance dust during processing. Table 1 shows the effect of varying amounts of calcium stearate and PVAc on the calculated adiabatic reaction

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RESULTS AND DISCUSSION General Considerations. Not all pyrotechnic compositions are suitable candidates for thermodynamic modeling. Chemical equilibrium calculations are only relevant for systems that reach, or closely approach, equilibrium. For example, colored smoke compositions containing sugar, KClO3, and B

DOI: 10.1021/acssuschemeng.5b01762 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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adiabatic reaction temperatures (Tad ≈ 2160−2350 K).32 One of the BC smoke compositions, BC-1a, was selected for more detailed analysis. Figure 1 shows the product phase distribution as a function of temperature in the range spanning 70−100% of Tad (1349−1927 K). As the reaction temperature drops below 85% of Tad (approximately 1640 K), liquid potassium borates and chloride begin to take the place of gaseous KBO2 and KCl. The phase of the products produced by a smoke composition is just as important as their chemical identity. Products formed in the gas phase readily leave the combustion zone, eventually condensing and forming smoke, provided they are not permanent gases. Products formed in the liquid or solid phases are more likely to remain in the immediate vicinity as slag, especially salts and oxides. Figure 2 shows a breakdown of the predicted gas phase products. In this case, the smoke is not particularly hygroscopic so the aerosol yield may be approximated by eq 1.

Table 1. BC Smoke Compositions and Calculated Adiabatic Reaction Temperatures composition

Tad (K)

B4C (wt %)

KNO3 (wt %)

KCl (wt %)

BC-1a BC-2a BC-3a BC-1b BC-2b BC-3b

1927 1970 2013 1982 2025 2068

13 13 13 13 13 13

58 59 60 60 61 62

25 25 25 25 25 25

a

Ca stearatea (wt %) 2 1

PVAcb (wt %) 2 2 2

2 1

Calcium stearate monohydrate. bPolyvinyl acetate.

temperature. As these materials are not energetic, their addition causes a temperature decrease with a 141 K difference noted between BC-3b and BC-1a. All of the compositions in Table 1 were previously tested in fully assembled smoke grenades with the exception of BC-3b, which was evaluated on a laboratory scale.20−22 Table 2 shows that the calculated adiabatic equilibrium products do not vary considerably across the series, as expected

∑ (all gases) − ∑ (permanent gases) ≈ aerosol

The predicted aerosol yield (labeled “difference” in Figure 2) remains above 70 wt % until the temperature drops to 85% of Tad. Smoke chamber tests of fully assembled grenades have shown that the average yield is 75 wt %.22 This suggests that the actual grenades burn at a temperature greater than 1640 K (about 85% of Tad), in a range that reasonably accounts for heat lost to the grenade hardware/surroundings and the other factors mentioned previously. HC Smoke Compositions. Thermodynamic equilibrium calculations predict that HC smoke compositions produce large amounts of gaseous ZnCl2, in agreement with experimental observations (Supporting Information). The aluminum content of the compositions controls their exothermicity and burning rate. Greater aluminum content and correspondingly higher combustion temperatures may also cause the formation of AlCl3(g) and Zn(g). A slow-burning composition, reportedly suitable for use in smoke grenades, was predicted to form 79.5 wt % ZnCl2(g) at an adiabatic temperature of 1214 K. Calculations at lower temperatures revealed that the ZnCl2 is increasingly formed in the liquid phase below 1000 K (about 80% of Tad). This temperature represents a lower limit for the systembelow it, the compositions would not be effective. The analogous limiting temperature for BC smoke compositions is far higher (1640 K). As far as salts and oxides are concerned, ZnCl2 is remarkably volatile. This metal chloride is also vigorously hygroscopic. The effectiveness of HC smoke is humidity-dependent, and the resulting aerosol mass is often greater than that of the initial composition.22 Such secondary reactivity, occurring af ter the primary combustion event, is not easily modeled. Phosphorus-Based Smoke Compositions. WP and RP are extremely effective as smoke-producing substances because of the large mass increase that occurs when they burn and as the resulting deliquescent oxides rapidly absorb atmospheric moisture. The overall mass increase can exceed 500% and depends on relative humidity.4 The initial atmospheric combustion can be modeled when air is added as a reactant. Three systems were consideredpure WP, pure RP, and the M819 composition. Figure 3 shows how the calculated adiabatic reaction temperatures vary as a function of air content. For WP and RP, the peak predicted temperatures and corresponding reaction products are nearly identical (Table 3). Subtle differences arise because RP is slightly more stable than

Table 2. Calculated Adiabatic Equilibrium Products (wt %) for BC Smoke Compositionsa composition product (phase)

BC-1a

BC-2a

BC-3a

BC-1b

BC-2b

BC-3b

KBO2 (g) KCl (g) CO (g) N2 (g) B2O3 (g) (BO)2 (g) BN (s)

48.5 22.6 12.4 4.7 2.2

49.3 22.8 10.8 5.6 3.4

49.7 23.3 9.2 6.5 4.8

50.2 22.6 9.8 5.9 3.5

5.9

4.6

3.2

4.2

50.6 23.1 8.2 6.9 4.9 1.0 2.8

50.5 24.1 6.6 7.9 6.9 1.6 1.3

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a

Products occurring in amounts of 1.0 wt % or greater are shown. The full output is provided in the Supporting Information.

given the similarity of the compositions. The major predicted products are gaseous KBO2 and KCl. Once formed, volatilized, and dispersed, rapid cooling of these products produces particulates that make up the smoke cloud. These results are in good agreement with previous XRD and XRF analyses which indicated that the smoke is composed of crystalline KCl and amorphous potassium borates.22 KBO2 is just one possible crystalline phase in the K2O·B2O3 system; such materials may have varying stoichiometries and are often glassy.29,30 While most of the boron is allocated to KBO2, small amounts of boron oxides are also predicted. The prediction of CO as a reaction product instead of CO2 reflects the fact that boron− oxygen bonds are generally stronger than the OCO double bond.31 The CO is reasonably expected to burn upon contact with the air, forming CO2, although this has not been confirmed experimentally. The nitrogen originating from KNO3 is predicted to form N2 and BN, the latter in the solid phase. Several factors can cause significant differences between actual combustion temperatures (Tc) and predicted adiabatic reaction temperatures (Tad) including heat lost to the container/surroundings, mixture inhomogeneity, and component impurities. For example, the measured combustion temperatures of Ti/C-3Ni/Al delay compositions (Tc ≈ 1500−1700 K) were approximately 30% less than the predicted C

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product and the Tad is just 941 K (Table 3). Similar Tad values have been calculated for fuel-rich RP-KNO3 mixtures.35 Boron Phosphide as a Benign Source of Phosphorus. The use of phosphide compounds, instead of elemental phosphorus, is an alternative strategy for incorporating this element into pyrotechnic compositions. However, transition metal phosphides have a relatively low phosphorus content and ionic phosphides such as Na3P and Ca3P2 are susceptible to hydrolysis. Boron phosphide (BP) appears to be one of the only phosphides suitable for pyrotechnic applications. BP contains 74 wt % phosphorus and has a crystalline density of 2.97 g/cm3.36 Boron phosphide crystals actually contain more phosphorus per cubic centimeter than pure white phosphorus (2.20 g/cm3 versus 1.82 g/cm3).33 BP is an indirect-band gap III−V semiconductor that may find use in high-temperature applications due to its favorable electronic properties and chemical inertness.37,38 Unlike ionic phosphides, BP is resistant to hydrolysis. A recently reported bulk-synthetic method calls for a purification step involving boiling concentrated hydrochloric acid.39 The ability of boron phosphide to resist chemical attack may impart favorable aging characteristics to BP-based compositions. In contrast to red phosphorus, boron phosphide is unlikely to produce phosphine upon aging. Although, the long-term stability of mixtures containing BP would still need to be verified through accelerated aging experiments. In some respects, BP resembles B4C. Both are remarkably inert and are only attacked under extreme conditions, by hot molten oxidizers such as potassium nitrate or sodium peroxide.40 In a very early report concerning the synthesis and properties of BP, Henri Moissan noted that the compound incandesced and deflagrated when “projected onto a bath of molten alkali nitrate.”41 As of late 2015, boron phosphide was not available from commercial sources. Pyrotechnic prototyping can require hundreds of grams of materials. This makes BP-oxidizer systems all the more suitable for initial characterization by computational methods. Comparison of the B4C−KNO3 and BP−KNO3 Systems. Figure 4 shows the Tad profiles for the binary B4C−KNO3 and BP−KNO3 systems. In the former, a composition containing 16 wt % B4C is predicted to produce mainly KBO2(g) at a peak Tad of 2956 K. The BP−KNO3 system appears to operate at

Figure 3. Calculated adiabatic reaction temperatures for phosphorus− air systems: WP (solid blue), RP (long-dashed red), M819 (dotted green). The M819 composition is 78.7 wt % RP, 13.9 wt % NaNO3, 7.4 wt % epoxy. Air at 25 °C and 50% relative humidity is approximated as 74.76 wt % N2, 22.97 wt % O2, 1.29 wt % Ar, 0.98 wt % H2O.

Table 3. Calculated Adiabatic Reaction Temperatures and Equilibrium Products (wt %) for Phosphorus−Air Systemsa composition

WP

RP

c

d

d

air (wt %) Tad (K)

M819b d

M819b

77

77

73

0

2460

2447

2409

941

57.54 15.87 15.62 9.23 0.99 0.34 0.24

57.54 14.84 17.35 8.55 0.99 0.34 0.22

55.17 12.09 19.23 6.48 0.94 0.95 0.15 2.95 0.98 0.79 0.11

2.29

product (phase) N2 (g) PO2 (g) (P2O3)2 (g) PO (g) Ar (g) H2O (g) P2 (g) CO (g) Na (g) CO2 (g) H2 (g) P4 (g) CH4 (g) Na3PO4 (s) C (gr, s)

13.18

0.21

0.48 69.37 0.18 8.94 5.35

a

Products occurring in amounts of 0.10 wt % or greater are shown. 78.7 wt % RP, 13.9 wt % NaNO3, 7.4 wt % epoxy. cAir at 25 °C and 50% relative humidity approximated as 74.76 wt % N2, 22.97 wt % O2, 1.29 wt % Ar, 0.98 wt % H2O. dStoichiometry producing peak Tad. b

WP (their ΔfH° values differ by 17.6 kJ/mol).33 The M819 mortar projectile contains an expelling charge that ignites and ejects multiple smoke pellets. In addition to red phosphorus, the M819 composition contains 13.9 wt % NaNO3 and 7.4 wt % epoxy binder.34 Atmospheric burning of excess phosphorus is the primary reaction; the purpose of the substoichiometric NaNO3 is to promote rapid combustion. Without a pyrotechnic oxidizer, red phosphorus burns rather slowly unless it is shocked and preheated by a detonation.1 The Tad profile for the M819 composition is remarkably similar to that of pure RP because the composition is so phosphorus-rich. In the absence of air, white phosphorus vapor (P4) is the major predicted

Figure 4. Calculated adiabatic reaction temperatures for the B4C− KNO3 (solid blue) and BP−KNO3 (long-dashed red) systems. D

DOI: 10.1021/acssuschemeng.5b01762 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering lower temperatures and has a flatter temperature profile. Two maxima of 1848 and 1860 K correspond to fuel-lean and fuelrich stoichiometries. These points occur at 26 and 52 wt % BP; the predicted products are shown in Table 4. The fuel-rich Table 4. Calculated Adiabatic Reaction Temperatures and Equilibrium Products (wt %) for B4C−KNO3 and BP−KNO3 Systemsa compound

B4C (16 wt %)b

BP (26 wt %)b

BP (52 wt %)b

Tad (K)

2956

1848

1860

67.00 11.58 8.71 7.04 1.97 1.68 1.12 0.50 0.12 0.11

34.92 10.25

38.51

product (phase) KBO2 (g) N2 (g) B2O3 (g) CO (g) BO2 (g) CO2 (g) BO (g) K (g) NO (g) O (g) BPO4 (g) (P2O3)2 (g) P2 (g) P4 (g) (BO)2 (g) B2O3 (l) BN (s) a b

0.65

11.93

0.18

20.72 20.45 1.63

1.68

Figure 5. Calculated phosphorus-containing gas products of the binary BP−KNO3 system (adiabatic conditions, equilibrium products): POx (solid red), BPO4 (short-dashed blue), Px (dotted green). POx is the sum of (P2O5)2, (P2O3)2, and PO2. Px is the sum of P2 and P4.

36.49 1.55 0.37 8.78 11.75

Products occurring in amounts of 0.10 wt % or greater are shown. Stoichiometry producing a maximum in Tad.

composition is predicted to produce a large amount of hot elemental phosphorus, mainly in the form of P2(g). The P4(g) ⇌ 2P2(g) equilibrium becomes relevant at temperatures above 1100 K.42 Both P4(g) and P2(g) are highly flammable and will oxidize immediately upon contact with the air, forming hygroscopic phosphorus oxides, and ultimately phosphoric acid. The ability of the BP−KNO3 system to produce elemental phosphorus partly depends on preferential formation of KBO2 instead of BPO4 and phosphorus oxides. Combustion of BP in an oxygen atmosphere is known to produce BPO4.43 The presence of potassium is therefore critical but it is not the only requirement. The compositions must also be oxygen-deficient. Several factors promote phosphorus formation under these conditions. Although it may seem counterintuitive, KBO2 formation should be thermodynamically favored even though BPO4 is more stable on a molar basis. The ΔfH° values for KBO 2 (g) and BPO 4 (g) are −674 and −976 kJ/mol, respectively.44,45 The reason is that formation of one BPO4 molecule would occur at the expense of two KBO2 molecules. Additionally, B−O bonds are more likely to form than P−O bonds when oxygen is limited because boron is generally more oxophilic than phosphorus.31 Equation 2 closely approximates the reaction stoichiometry and product distribution of the fuelrich composition in Table 4. The extent of oxygen deficiency is made apparent by the predicted formation of BN. 8BP + 3KNO3 → 3KBO2 + 4P2 + B2O3 + 3BN

Figure 6. Calculated boron- and potassium-containing gas products of the binary BP−KNO3 system (adiabatic conditions, equilibrium products): KBO2 (solid purple), BPO4 (short-dashed blue), K (dotted orange).

this point the system becomes so oxygen deficient that an increasing quantity of BP is predicted to remain unreacted. BP−KNO3 System as a Possible Basis for NextGeneration Smoke Compositions. It is unlikely that boron phosphide could serve as a “drop-in” replacement for red phosphorus in existing RP-based formulations which are extremely fuel-rich and depend primarily on atmospheric combustion. Red phosphorus is considerably more flammable and autoignites in air at just 530 K.11 BP resists oxidation in air until at least 1000 K.46 The reaction is sluggish and reportedly produces BPO4.47 The properties of this phosphate as an aerosol are not known. The use of BP in compositions that undergo self-sustaining combustion within a canister (a steel grenade can, for example) appears to be a more sensible starting point. A canister configuration is versatile because subtle changes to the composition loading procedure may be used to alter the burning time. Core-burning grenades release smoke 3−4 times as rapidly as those that burn linearly.22 However, to avoid clogging the smoke vent holes with excessive slag, canister-

(2)

Figures 5 and 6 show the major predicted gas phase products of the BP−KNO3 system as a function of stoichiometry. The window for high phosphorus production occurs between about 40 and 70 wt % BP. The peak occurs at 52 wt % BP. Beyond E

DOI: 10.1021/acssuschemeng.5b01762 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS Zhaohua Luan, Christopher D. Haines, Rajendra K. Sadangi, and Jared D. Moretti are thanked for useful discussions. The U.S. Army is thanked for funding this work.

housed smoke compositions must produce enough heat to volatilize most of the combustion products. BP−KNO3 mixtures may not burn at temperatures great enough to efficiently aerosolize KBO2, especially once dilutive organic binders and burning rate modifiers are incorporated. It may be necessary to add a metallic fuel, such as aluminum, to increase the combustion temperature. A ternary mixture of 5 wt % Al, 45 wt % BP, and 50 wt % KNO3 has a Tad of 2085 K which is similar to BC-3b (Table 1) and 225 K greater than the maximum Tad in the binary BP−KNO3 system. Importantly, a moderate amount of aluminum and added organic material does not substantially alter the predicted product distribution (Supporting Information). Compositions like this, that could volatilize as much as 40 wt % KBO2 and 32 wt % phosphorus upon combustion, should provide excellent obscuration performance.

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ABBREVIATIONS ARDEC, Armament Research, Development and Engineering Center; BC, boron carbide-based smoke composition; HC, hexachloroethane-based smoke composition; PVAc, polyvinyl acetate; RDECOM, Research, Development and Engineering Command; RP, red phosphorus; Tad, calculated adiabatic reaction temperature; Tc, combustion temperature; WP, white phosphorus; XRD, X-ray diffraction; XRF, X-ray fluorescence

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01762. FactSage 6.4 data for the following systems: BC smoke compositions (Tables S1 and S2); HC smoke compositions (Tables S3−S5 and Figure S1); the M819−air system (Table S6); the BP−KNO3 system (Tables S7 and S8); the Al−BP−KNO3 system (Table S9 and Figure S2); candidate BP smoke compositions (Tables S10 and S11 and Figure S3). Water vapor concentration in the atmosphere as a function of relative humidity and temperature (Figure S4) (PDF)

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REFERENCES

(1) Koch, E.-C. Special Materials in Pyrotechnics: V. Military Applications of Phosphorus and its Compounds. Propellants, Explos., Pyrotech. 2008, 33, 165−176. (2) Toxicological Profile for White Phosphorus. Agency for Toxic Substances and Disease Registry, September, 1997; http://www.atsdr. cdc.gov/ToxProfiles/tp103.pdf (accessed November, 2015). (3) Yon, R. L.; Wentsel, R. S.; Bane, J. M. Programmatic Life Cycle Environmental Assessment for Smoke/Obscurants Vol. 2: Red, White, and Plasticized White Phosphorus; accession number ADA135910; Defense Technical Information Center (DTIC): Fort Belvoir, VA, 1983; pp 1− 73. (4) Rubel, G. O. Predicting the Droplet Size and Yield Factors of a Phosphorus Smoke as a Function of Droplet Composition and Ambient Relative Humidity Under Tactical Conditions; accession number ADA064076; Defense Technical Information Center (DTIC): Fort Belvoir, VA, 1978; pp 1−47. (5) Barillo, D. J.; Cancio, L. C.; Goodwin, C. W. Treatment of White Phosphorus and Other Chemical Burn Injuries at One Burn Center Over a 51-Year Period. Burns 2004, 30, 448−452. (6) Rivera, Y. B.; Olin, T.; Bricka, R. M. Summary and Evaluation for White Phosphorus Remediation: A Literature Review; accession number ADA317393; Defense Technical Information Center (DTIC): Fort Belvoir, VA, 1996; pp 1−72. (7) Walsh, M. R.; Walsh, M. E.; Collins, C. M. Enhanced Natural Remediation of White-Phosphorus-Contaminated Wetlands Through Controlled Pond Draining; accession number ADA373477; Defense Technical Information Center (DTIC): Fort Belvoir, VA, 1999; pp 1− 30. (8) International Ammunition Technical Guideline (IATG 06.50): Specific Safety Precautions (Storage and Operations), 1st ed.; United Nations, October 1, 2011; http://www.un.org/disarmament/unsaferguard/guide-lines/ (accessed November, 2015). (9) Manton, G.; Endsor, R. M.; Hammond, M. An Effective Mitigation for Phosphine Present in Ammunition Container Assemblies and in Munitions Containing Red Phosphorus. Propellants, Explos., Pyrotech. 2014, 39, 299−308. (10) Collins, P. J. D.; Smit, K. J.; Hubble, B. R. The Use of Red Phosphorus in Pyrotechnics−Results of an International Investigation; accession number CPIAC-2004-0319AR; Defense Technical Information Center (DTIC): Fort Belvoir, VA, 2004; pp 1−6. (11) Conkling, J. A.; Mocella, C. J. Chemistry of Pyrotechnics: Basic Principles and Theory; CRC Press: Boca Raton, FL, 2011; pp 83−84. (12) Toxicity of Military Smokes and Obscurants; Medinsky, M. A., Henderson, R. F., Eds.; National Academy Press: Washington, DC, 1997; Vol. 1. (13) Toxicity of Military Smokes and Obscurants; Medinsky, M. A., Walker, B., Jr., Eds.; National Academy Press: Washington, DC, 1999; Vol. 2. (14) Shinn, J. H.; Martins, S. A.; Cederwall, P. L.; Gratt, L. B. Smokes and Obscurants: A Health and Environmental Effects Data Base Assessment; accession number ADA185377; Defense Technical Information Center (DTIC): Fort Belvoir, VA, 1985; pp 1−121.

CONCLUSIONS Thermodynamic modeling is applicable to systems that operate at relatively high temperatures including BC, HC, and phosphorus-based smoke compositions. These compositions volatilize and disperse combustion products. The predicted product distributions are corroborated by what is already known about the chemistry of these systems. The trends in the computational results provide broad insights that would be difficult to ascertain experimentally. Slow-burning BC and HC smoke grenades appear burn at temperatures greater than 80% of T ad . The combustion characteristics of the M819 composition were shown to be heavily dependent on the amount of available air. Boron phosphide is one of the few phosphide compounds that may be suitable for pyrotechnic applications. As a chemically inert material under ordinary conditions, BP may be useful as a benign source of phosphorus. Fuel-rich mixtures of BP and KNO3 are expected to produce gaseous phosphorus upon combustion. In pyrotechnic smoke compositions, in situ phosphorus production could be used to achieve high obscuration performance. The BP−KNO3 system clearly merits experimental investigation and is a topic of ongoing research in our laboratories.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

This document has been approved by the U.S. Government for public release; distribution is unlimited. The authors declare no competing financial interest. F

DOI: 10.1021/acssuschemeng.5b01762 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.5b01762 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX