Environmentally Friendly Boron-Based Pyrotechnic Delays: An

Subscriber access provided by TULANE UNIVERSITY

Article

Environmentally Friendly Boron-Based Pyrotechnic Delays: An Additive Manufacturing Approach Ian Walters, and Lori J Groven ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06204 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Environmentally Friendly Boron-Based Pyrotechnic Delays: An Additive Manufacturing Approach Ian T. Walters*, Lori J. Groven* Department of Chemical and Biological Engineering South Dakota School of Mines and Technology 501 E. St. Joseph Street Rapid City, SD 57701, USA Email: [email protected], [email protected] Abstract Pyrotechnic time delays are reactive systems that burn for a desired period of time at a specified rate and are commonly used in military applications such as grenades and hand-held signals. Typically, they are manufactured by pressing the reactive powder into a metal housing. This rigid design inherently limits configurability and they have had reliability issues. Additionally, there has been advocacy for environmentally friendly pyrotechnic ignition delays, by removal of barium chromate (BaCrO4). This study has two distinct objectives. First, determine the viability of two possible replacements, strontium molybdate (SrMoO4) and barium molybdate (BaMoO4), for the harmful component, BaCrO4, in the traditional T-10 delay. This includes combustion characteristics such as burning rate, combustion temperature, and gas generation. Furthermore, thermal characteristics are determined through differential scanning calorimetry (DSC), and combustion products are analyzed with X-ray diffraction (XRD). Second, by using a novel approach, these reactive delay systems are integrated into printable ink formulations and deposited onto soapstone substrates. This demonstrates a high degree of configurability and an effort towards a universal delay. Keywords: pyrotechnic ignition delay, additive manufacturing, combustion, energetics, 3D printing Introduction Pyrotechnic time delays are self-sustaining reactive systems consisting of a fuel and oxidizer that burn at a prescribed rate to sequentially ignite another material. Traditionally, military delays have consisted of tungsten, manganese, or boron as the fuel with an oxidizer such as barium chromate, lead chromate, or potassium perchlorate.1-5 Of these compositions, boron/barium chromate, also known as T-10, is widely used as a pyrotechnic ignition delay in various devices.6 It has shown to be a reliable ignition delay in providing a consistent burning rate that can be tailored by the B/BaCrO4 ratio and is considered to be a gasless reaction.7 However, barium chromate has been shown to be highly carcinogenic.8 Current studies to replace BaCrO4 with alternate oxidizers have primarily focused on tungsten and manganese delay systems while insufficient research has been done on boron delay systems. A thorough outline of these systems that include B/Fe2O3,9 Mn/MnO2,10 and W/MnO211 were presented in a review on gasless pyrotechnic delays by Focke et al.12 They concluded that significant progress has been made with these environmentally friendly delay systems, but the ability to use additive manufacturing to make these delays has not been addressed. Typically, pyrotechnic delays consist of a compressed delay powder formulation that is encased in a metal tubing. This is a rigid design that offers very little flexibility and limits configurability. A universal delay would, ideally, not be limited by a set architecture and could be fabricated in almost any configuration to result in the required burning time. 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

In recent years, Poret et al. and Reimer et al. developed B4C/NaIO4/PTFE and B/SrMoO4 systems, respectively, in efforts to replace BaCrO4, specifically in boron-based systems.13,14 In both cases, they were successful in showing a viable replacement to the B/BaCrO4 system, but the B4C/NaIO4/PTFE reactive system generates gaseous products and neither system demonstrated the ability to be used in a universal configuration. In other studies, using non-boron-based systems, Mn/MnO2, W/MnO2, Ti/C-3Ni/Al systems were also developed to replace environmentally hazardous time delays.10,11,15,16 Again, the reactive systems were demonstrated to be viable replacements, but configurability was not explored. In the case of the Ti/C-3Ni/Al system, Miklaszewski et al. utilized a four-component system, two sets of reactive systems that were combined. A system consisting of more than two components has proven to be a viable option, but the ability to assess the role of the oxidizer becomes fundamentally less complicated in a twocomponent system. Therefore, B/BaCrO4, the traditional T-10 system, is an ideal choice to explore because the carcinogenic component, BaCrO4, can be isolated and replaced and the fundamental role of the oxidizer can be determined. Additionally, manufacturing of delay systems can be problematic due to the dispersion of harmful powders into the air. Additive manufacturing of delay ink formulations would be an immediate improvement to health and safety by providing containment and limiting exposure. Additive manufacturing has not only shown to be an effective technique in the fields of bioengineering and electronics,17-20 but it has also shown to be an attractive and beneficial technique in the field of energetics. In 2010, Puszynski et al. demonstrated the ability to efficiently and precisely load percussion primers with nanothermite mixtures using a basic direct printing platform.21 Recently, the ability to safely print reactive thermite material by mixing inert pre-thermite inks in-situ was demonstrated by Durban et al.22 Although much of the research focus has been on thermite, the configurability that additive manufacturing can bring has only started to be explored. The two major challenges to additive manufacturing with energetic inks are formulation and processing. In the case of ignition delays, a reactive system consisting of micron-sized powders requires a solvent, surfactant or gelling agent to aid in suspension, and suitable shear viscosity, elasticity, and interfacial tension for the selected printing technology. Furthermore, processing of the energetic formulations requires specific parameters to achieve a desired outcome. These parameters include but are not limited to printing parameters, postprocessing methods such as curing or drying, and properties of the final product.23 The objective of this study was to investigate two possible replacements for barium chromate in the T-10 delay and translate them into printable inks to demonstrate configurability. Specifically, the viability of strontium molybdate (SrMoO4) and barium molybdate (BaMoO4) as alternative oxidizers to barium chromate was demonstrated by burning rate measurements, differential scanning calorimetry (DSC), gas generation, combustion products, and combustion temperature. In addition, the secondary objectives of this study were to determine a target apparent viscosity for printable ink formulations for the specific printing technology and explore the possibility of working towards a universal delay configuration through additive manufacturing. This could provide an efficient alternative to the current technique of packing loose powders, with a high potential of applications. Experimental Materials Boron (B, APS < 5 micron, 94-96%), barium molybdate (BaMoO4, -100 mesh, 99.9%), and strontium molybdate (SrMoO4, -200 mesh, 99.9%) were purchased from Alfa Aesar. Barium chromate 2 ACS Paragon Plus Environment

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(BaCrO4, -325 mesh, MIL-B-550A, Grade A) was purchased from Hummel Croton and Methocel (K4M) was purchased from the Dow Chemical Company. All materials were used as-is. Formulation Three dry powder reactive systems were formulated: B/BaCrO4, B/SrMoO4, and B/BaMoO4. The B/BaCrO4 system was formulated with 14.6 wt-% boron and 85.4 wt-% barium chromate (4:1 molar ratio). Per outlined in published work by Joseph McLain, the given ratio was shown to have the highest heat of reaction and burning rate.24,25 The B/SrMoO4 system was formulated with 12.5 wt-% boron and 87.5 wt-% strontium molybdate. It was selected based on research by Reimer and Mangum who had observed comparable results to an existing B/BaCrO4 formulation.5 The B/BaMoO4 system was formulated using the same ratio as the B/SrMoO4 system. Each dry powder formulation was mixed using a Resodyn LabRAM at 60 g intensity for one minute for three total times with a one-minute pause between each segment. Additionally, the three reactive systems were integrated into an aqueous 2.5 wt-% Methocel (K4M) solution to produce printable inks. The gel matrix generated by the Methocel solution provided stability while promoting homogeneity to the suspended particles during mixing and deposition of the inks. Based on comparable viscosity, 60 wt-% solids was chosen for the B/BaCrO4 system and 70 wt% solids was chosen for the B/SrMoO4 and B/BaMoO4 systems. Mixing was performed stepwise with the Methocel gel and each powder in a Thinky Planetary Mixer. First, the previously prepared Methocel gel was added to a 150 mL high-density polyethylene (HDPE) Thinky container. Next, the boron was added to the container and the container was placed in the Thinky Mixer at 2000 rpm for one minute. The oxidizer was then added to the container and, again, placed in the Thinky Mixer at 2000 rpm for one minute. An additional mixing step was then performed with a spatula and then in the Thinky Mixer at 2000 rpm for one minute. The three reactive systems were also formulated with an equivalent amount of dry powder Methocel using the Resodyn LabRAM and same method as the dry powder mixtures that did not contain Methocel. Material Characterization As-received powder materials were imaged with a Zeiss Supra 40VP Scanning Electron Microscope (SEM) to determine particle morphology and general size. Apparent viscosities of the printable ink formulations were measured at shear rates from 2.2 to 55 s-1 with a Brookfield DV3T Rheometer equipped with an SC4-25Z spindle. Differential scanning calorimetry (DSC) was performed with 90 µm alumina pans in UHP argon at a heating rate of 10 °C/min with an SDT Q600 from TA instruments. Measurements were performed on the dry powder mixtures and the dry ink formulations to determine thermal and reaction characteristics. Reaction products from dry powder mixtures were characterized using a Rigaku Ultima Plus powder X-ray diffractometer (XRD) with a scan rate of 1 deg/min over a 2θ range from 10 to 80. Reference intensity ratio (RIR) values from the International Centre for Diffraction Data (ICDD) were used to characterize peak positions. Deposition of Printable Ignition Delays Printable inks were deposited onto flat soapstone pieces (127 x 12.7 x 4.76 mm) and soapstone rods (127 x 6.35 mm) using a Janome JR2000N series robot equipped with a Techcon auger valve (Figure S1, Supporting Information). Soapstone substrates were selected due to their extremely high melting temperature, non-reactive nature, low thermal conductivity, ease of manufacturing, and low cost.26 In the case of the flat soapstone substrates, inks were deposited in a linear trace of approximately 100 mm 3 ACS Paragon Plus Environment


Page 4 of 27

in length, but trace diameter was varied by utilizing multiple dispensing tip sizes (1.60, 1.19, and 0.58 mm ID) and changing robot movement speed. A Leica Z16 APO zoom system was used to capture magnified images of the printed traces and measure effective diameter. These traces were later used to measure burning rate vs. average effective diameter. The soapstone rods were manually spun by hand as the inks were slowly dispensed to configure traces in an evenly spaced helix formation. Although these prints were not used to measure burning rate, they were essential in showing that the inks can be oriented in almost any configuration. Burning Rate Effective diameter measurements were performed at five different positions along each linear trace and averaged to get an average effective diameter. Combustion was initiated from one end with 28-gauge Nichrome wire connected to a DC power supply. Burning rates were then measured with a Chronos high-speed camera (1057 fps, f-stop at 4) equipped with a UV filter lens. Combustion Temperature To measure combustion temperature, the reactive delay powder that did not contain Methocel was gently packed into a custom ceramic boat that was lined with carbon felt for insulation. An Omega Type-C thermocouple (0.25 mm diameter) was inserted into the packed powder from the side and the powder was ignited with A 28-gauge Nichrome wire connected to a DC power supply. Data was collected using DASYLab v. 13.0 software until the temperature decayed to at least 600°C. Gas Generation Gas generation was measured with a custom setup using a volumetric displacement method. The setup consisted of a 50 mL Chemglass buret for gas evolution measurement that was connected to a 1000 mL round bottom flask as a water reserve on one side and a 2 mL combustion chamber on the other side. For each measurement, approximately 0.1 to 0.5 grams of reactive powder mixture was first hand-pressed into a 316 stainless-steel tube (15 mm length, 5 mm outer diameter, 0.25 mm wall thickness, McMasterCarr #50415K22) to be exposed on the ignition side. The encasing was then carefully inserted into the combustion chamber which was positioned to gently rest on the electric feed-through (Figure S2, Supporting Information). Combustion was initiated with a 28-gauge Nichrome wire connected to a DC power supply. The generated gaseous species were then measured by the water displacement in the buret. Results and Discussion Morphology Considerations Prior to extensive experimentation, scanning electron microscopy (SEM) was conducted on the as-received materials (Figure S3, Supporting Information). In all cases, boron was used as the fuel for the delay mixtures, but the oxidizer was either barium chromate, strontium molybdate, or barium molybdate. When assessing the morphology, all of the as-received powders had particles with primarily a spherical shape. However, the strontium molybdate contained noticeable impurities in the form of rodlike particles. The boron powder was comprised of particles ranging from 100 to 500 nanometers with agglomerations ranging from 1 to 5 microns. Barium chromate, strontium molybdate, and barium molybdate powders were generally coarser, ranging from a few hundred nanometers to a few microns, 4 ACS Paragon Plus Environment

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with agglomerations as large as 10 microns. They all appeared to have a very similar agglomerate and primary structure, indicating the rheology of prepared mixtures would likely be similar. Apparent Viscosity Published research on the ability to print T-10, or any other pyrotechnic delay, is very minimal or nonexistent, so determining apparent viscosity was an essential initial step in formulating printable inks. While the printing process includes several inherent parameters to adjust printing quality, they are still highly dependent on the viscosity of the formulation. Apparent viscosity profiles with respect to shear rate of desired printable formulations were selected through an iterative process using the B/BaCrO4 formulation. Using the B/BaCrO4 formulation with 55 wt-% solids, print traces were relatively uniform and consistent from trace to trace. However, air pockets and channels developed inside the material as it dried, creating an anticipated phenomenon during combustion (Figure 1). The air pockets and channels allowed for superheated gases to rapidly travel through the material, igniting it along the way in a violent manner. In Figure 2, the moment the combustion front erupted through the cracks along the path of the printed trace is shown as captured by a high-speed camera. It was later determined that this phenomenon increased the apparent burning rate by approximately three times. This clearly demonstrates the importance of the ink properties on final combustion characteristics and, ultimately, reliability. When the solids loading of the B/BaCrO4 formulation was increased to 60 wt-%, the apparent viscosity profile that was generated went from 217,100 cP at 2.2 s-1 to 11,630 cP at 55 s-1 (Figure 3). This was an overall increase in viscosity that allowed printing to be more uniform when compared with the 55 wt-% solids formulation. The B/SrMoO4 and B/BaMoO4 formulations were then prepared to parallel the viscosity profile of the B/BaCrO4 formulation with 60 wt-% solids. It was determined that 70 wt-% solids, in both cases, matched the apparent viscosity profiles relatively well. The viscosity profile of the B/SrMoO4 formulation went from 135,200 cP at 2.2 s-1 to 20,150 cP at 55 s-1, whereas the B/BaMoO4 formulation went from 221,200 cP at 2.2 s-1 to 8,192 cP at 55 s-1. Using a Leica Z16 APO zoom system, images of the 60 wt-% solids B/BaCrO4 formulation were captured from multiple angles to validate uniformity of each trace and confirm the elimination of air pockets and channels (Figure 4). Additional images of the printed 70 wt-% solids B/SrMoO4 and B/BaMoO4 formulations are available in the Supporting Information.

Figure 1. Channels and air pockets that developed on the underside of a printed trace when using the B/BaCrO4 formulation with 55 wt-% solids.

5 ACS Paragon Plus Environment


Page 6 of 27

Figure 2. Combustion of the B/BaCrO4 formulation with 55 wt-% solids deposited on a soapstone substrate.

Figure 3. Apparent viscosity vs. shear rate of printable ink formulations.

Figure 4. Images of the B/BaCrO4 formulation with 60 wt-% solids from the (a) cross-section, (b) bottom, (c) side, and (d) top to measure effective diameter (de).


Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Burning Rate Linear combustion was investigated for two specific purposes: measuring burning rate and confirming the capability to fully propagate. However, the objective did not include formulating the printable inks to have equivalent burning rates. Additional efforts are required to achieve burning rates that are analogous to T-10. To determine burning rates, the printable ink formulations were deposited in a linear trace on flat soapstone pieces (Figure 5a). Combustion was initiated at one end and captured with a high-speed camera. The B/SrMoO4 and B/BaMoO4 formulations had average burning rates of 10 (± 0.4) and 16 (± 0.4) mm s-1, respectively. However, the B/BaCrO4 formulation burned at a much faster rate, 68 (± 1.4) mm s-1. This was within the typical burning rate range for the T-10 delay, but a primary concern was how burning rate was influenced by effective diameter. In all three cases, there was a relatively constant burning rate as effective diameter decreased (Figure 6). All traces fully propagated upon combustion, so a critical effective diameter never became apparent. It is also worth noting that the grouping of some data points in Figure 6 can be attributed to the usage of different dispensing tips. When observing combustion, a green gaseous product was more visible with the B/BaCrO4 formulation (Figure 7). The other formulations also had gaseous products, but they were less visible, which could suggest less gaseous products being generated. This was further investigated with a gas generation study. Videos showing combustion of the linear traces are available online as Web Enhanced Objects. In addition, to demonstrate universal configurability, the printable ink formulations were uniformly deposited around soapstone rods in a helix formation (Figure 5b). They were then ignited from one end while being recorded with a high-speed camera. The burning rates of these traces were not measured, but qualitatively, they demonstrated a high degree of configurability and stability during deposition and burning. In Figure 8, the images show a time-stepped burning of the B/SrMoO4 formulation deposited around a soapstone rod. Combustion uniformly propagated fully through the entire trace. Although they are not shown, the configurability of the B/BaCrO4 and B/BaMoO4 formulations were also verified in the same manner with an identical result. Videos demonstrating combustion around soapstone rods are available online as Web Enhanced Objects.

Figure 5. Soapstone substrates with traces of the B/BaMoO4 printable ink formulation.



Figure 6. Burning rate vs. average effective diameter of (▲) B/BaCrO4, (♦) B/SrMoO4, and (●) B/BaMoO4 printable ink formulations deposited on soapstone substrates.

Figure 7. Time-stepped combustion of B/BaCrO4 (left column), B/SrMoO4 (middle column), and B/BaMoO4 (right column) printable ink formulations deposited on soapstone substrates. Note the luminous green combustion plume indicating gas.


Page 8 of 27

Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Time-stepped combustion of the B/SrMoO4 printable ink formulation deposited around a soapstone rod.

Thermal Characteristics Differential scanning calorimetry (DSC) was utilized to quantify and compare thermal outputs of printable ink formulations and loose dry powder formulations. There was concern that the addition of Methocel as a binder could negatively affect combustion behavior. When comparing the printable ink formulations to the dry powder formulations, generally, the inks had much sharper and higher thermal outputs (Figure 9). It was particularly apparent in the B/BaCrO4 formulations where the ink formulation generated approximately five times more heat flow upon reaction. This behavior can be attributed to the tighter packing of fuel and oxidizer. The Methocel binder promotes contact between fuel and oxidizer, whereas the loose dry powder formulations inherently do not. When comparing different formulations, a similar trend was observed as with the burning rates. The increase in heat flow during reaction of the B/BaCrO4 formulation, 25.5 W g-1, was much higher than the B/SrMoO4 and B/BaMoO4 formulations, 1.22 and 2.49 W g-1, respectively. As BaCrO4 is heated to 1400°C, it rapidly decomposes, releasing its available oxygen. Therefore, when the exothermic reaction between fuel and oxidizer is initiated, heat is generated to promote decomposition. The decomposition then further progresses the reaction, generating more heat. In the case of the replacement systems, SrMoO4 and BaMoO4 both must undergo a phase change when they melt at 1040 and 1480°C, respectively, before decomposition occurs. This endothermic phenomenon greatly hinders the reaction because energy is required for the phase change. Also, the reactive system 9 ACS Paragon Plus Environment


Page 10 of 27

becomes a relatively slower solid-liquid reaction when compared with a solid-gas reaction due to diffusion limitations.

Figure 9. DSC of B/BaCrO4 (top), B/SrMoO4 (middle), and B/BaMoO4 (bottom) printable ink and dry powder formulations.

Combustion Temperature Combustion temperature plays a critical role in the evolution of gaseous products and free metals. At temperatures above 2000°C, BO, a gaseous product, becomes more thermodynamically favorable than B2O3, a solid product.25 Furthermore, it may substantiate the variance in burning rates and thermal outputs because there is often a strong correlation between the three combustion characteristics. A plot of combustion temperatures with respect to time is given in Figure 10. In the case of the B/BaCrO4 system, the combustion temperature peaked at 1798°C in 1.21 seconds but rapidly decreased to 600°C over 121 seconds. The system with the replacement oxidizers peaked at lower temperatures but the temperature decay was at a much slower rate. Combustion temperature with the B/SrMoO4 system peaked at 1194°C in 13.6 seconds and decreased to 600°C over 319 seconds. Similar to the T-10 system, combustion temperature with the B/BaMoO4 system peaked at 1508°C in only 1.39 seconds. However, it gradually decreased to 600°C over 263 seconds. 10 ACS Paragon Plus Environment

Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


These results correlated very well with the differences in burning rates and measured heat flows with all three reactive systems. As combustion temperature increased, burning rate and heat flow also increased. Additionally, the slow temperature decay observed with the replacement systems could be associated with the broad thermal output shown with the DSC results. It is also worth noting that the peak temperatures with the replacement systems were only slightly higher than the melting temperatures of the oxidizers in each system. This supports the aforementioned idea that the combustion temperatures are limited by the reaction mechanism.

Figure 10. Combustion temperature profiles of dry powder formulations.

Gas Generation Gas generation measurements of the reactive formulations in this study were essential in quantifying the significance of replacing BaCrO4, the oxidizer, with either SrMoO4 or BaMoO4. Additionally, the role of Methocel with respect to gas generation needed to be investigated (Table 1, Figure 11). In the case of the dry powder formulations that did not contain Methocel, the B/BaCrO4 system generated 13.6 (± 0.5) mL g-1. The replacement systems, B/SrMoO4 and B/BaMoO4, generated 14.7 (± 0.5) and 9.5 (± 0.6) mL g-1, respectively. When powder Methocel was added to these reactive systems, a comparable increase in gas generation was observed with all three reactive systems. The B/BaCrO4 system increased to 27.6 (± 3.5) mL g-1 while the B/SrMo4 and B/BaMoO4 systems increased to 26.9 (± 1.2) and 24.3 (± 3.4) mL g-1, respectively. However, when the reactive systems were integrated into the printable inks, the replacement systems compared well with the powder mixtures containing powder Methocel, but a large increase in gas generation was observed with the B/BaCrO4 system. The B/SrMoO4 and B/BaMoO4 systems generated 26.5 (± 2.3) and 22.7 (± 1.8) mL g-1, respectively, and the B/BaCrO4 system generated 37.0 (± 3.0) mL g-1. This confirms the observation during combustion of possibly more gaseous products generated from the B/BaCrO4 system, but it doesn’t explain why significantly more gas was generated when formulated into a printable ink. It was determined that the small quantity of Methocel included in each system had the potential to produce large amounts of gaseous products due to the large carbon content. For example, 0.5 g of the dry B/BaMoO4 ink formulation contains 5.5 mg of Methocel (1.1 wt-% of total dry solids). Methocel, or methyl cellulose, has an approximate chemical formula of C17H32O11, and assuming complete combustion, that translates to 17 moles of CO2 for every mole of Methocel (Equation 1). Using the ideal gas law (PV=nRT), this then further equates to 5.5 mL of CO2 at room temperature and 33 mL of CO2 at 1508°C, the peak combustion temperature of B/BaMoO4. This would most likely be minimized by using a Methocel with a higher molecular weight. In the case of the B/BaCrO4 system, the discrepancy between the powder mixture containing dry powder Methocel and the ink formulation can be attributed to the water-based processing and is likely due to the adsorption of water and hydroxyl groups. 11 ACS Paragon Plus Environment


Page 12 of 27

(1)

2𝐶17𝐻32𝑂11 + 39𝑂2 (𝑔)→34𝐶𝑂2 (𝑔) + 32𝐻2𝑂 (𝑔)

Table 1. Gas generation of reactive systems formulated as powder mixtures, powder mixtures with powder Methocel, and dry inks deposited on soapstone substrate.

Gas Generation (mL g-1)

Reactive System

Powder Mixture

Powder Mixture with Powder Methocel

Dry Ink Deposited on Soapstone

B/BaCrO

13.6 (± 0.5)

27.6 (± 3.5)*

37.0 (± 3.0)*

14.7 (± 0.5)

26.9 (± 1.2)**

26.5 (± 2.3)**

9.5 (± 0.6)

24.3 (± 3.4)**

22.7 (± 1.8)**

4

B/SrMoO

4

B/BaMoO

4

* 1.6 wt-% Methocel

** 1.1 wt-% Methocel

Figure 11. Gas generation of reactive systems formulated as powder mixtures, powder mixtures with powder Methocel, and dry inks deposited on soapstone substrate.

XRD Analysis of Combustion Products XRD analysis of combustions products is an effective method in determining the environmental implications of each reactive system and also explaining the differences in the amount of gaseous products. In the case of the traditional T-10 system, BaB2O4 (60.3% BaB2O4, 26.9% α-BaB2O4), CrB (6.5%), and Cr (6.3%) were the detected products (Figure S6a, Supporting Information). However, the reactants, boron and barium chromate, were at a 4:1 molar ratio so Equation (2) was the expected reaction, assuming it was gasless. With CrB and Cr being two of the products, it was assumed that the theoretical product, CrB2, reacted with ambient oxygen to produce CrB and gaseous boron oxide species (BOX) (Equation (3)). Some of the CrB then further reacted with ambient oxygen to produce elemental chromium and more gaseous boron oxide (Equation (4)). At this time, the exact composition of the gaseous species was unknown, but the green color indicated the presence of boron oxide gas. Therefore, BOX denotes the possible gaseous boron oxide species generated by the reactions, most likely BO and BO2. (2)

4𝐵 + 𝐵𝑎𝐶𝑟𝑂4→𝐵𝑎𝐵2𝑂4 +𝐶𝑟𝐵2 (3)

2𝐶𝑟𝐵2 + 𝑋𝑂2 (𝑔)→2𝐶𝑟𝐵 + 2𝐵𝑂𝑋 (𝑔)

(4)

2𝐶𝑟𝐵 + 𝑋𝑂2 (𝑔)→2𝐶𝑟 + 2𝐵𝑂𝑋 (𝑔) 12 ACS Paragon Plus Environment

Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Upon combustion, the B/SrMoO4 system generated large amounts of SrB2O4 (62.0%) along with MoB (22.5%), Mo (10.6%) and Mo2B (5.0%) (Figure S6b, Supporting Information). This was a reasonable result when balancing the two primary products in a reaction (Equation (5)). The reaction requires three moles of boron for every mole of strontium molybdate. When translated to weight percentage, that is 11.6 wt-% B and 88.4 wt-% SrMoO4, which is comparable to the initial stoichiometry of the mixture. Similar to the B/BaCrO4 system, some of the MoB further reacted with ambient oxygen to produce large amounts of elemental molybdenum and BOX gas (Equation (6)). Additional BOX gas was generated in Equation 7 where Mo2B was produced. 3𝐵 + 𝑆𝑟𝑀𝑜𝑂4→𝑆𝑟𝐵2𝑂4 +𝑀𝑜𝐵

(5)

2𝑀𝑜𝐵 + 𝑋𝑂2 (𝑔)→2𝑀𝑜 + 2𝐵𝑂𝑋 (𝑔)

(6)

4𝑀𝑜𝐵 + 𝑋𝑂2 (𝑔)→2𝑀𝑜2𝐵 + 2𝐵𝑂𝑋 (𝑔)

(7)

For the B/BaMoO4 system, the primary product detected, again, was BaB2O4 (59.5%) with MoB2 (19.1%) and MoB (17.8%) as the next highest products. There were only trace amounts of Mo (2.1%) detected (Figure S6c, Supporting Information). When developing two distinct reactions with the primary combustion products, this appeared to be an acceptable result (Equations (8) and (9)). Equation (8) has a stoichiometry of 12.7 wt-% B to 87.3 wt-% BaMoO4, and Equation (9) has a stoichiometry of 9.8 wt% B to 90.2 wt-% BaMoO4. These two reactions encompass the initial stoichiometry of the mixture, but MoB2 most likely reacted with ambient oxygen to produce some of the MoB and additional BOX gas (Equation 10). Since very little elemental molybdenum was generated (Equation (6)) and more boron was included in the solid reaction products, less BOX gas was created when compared with the other two systems. 4𝐵 + 𝐵𝑎𝑀𝑜𝑂4→𝐵𝑎𝐵2𝑂4 +𝑀𝑜𝐵2

(8)

3𝐵 + 𝐵𝑎𝑀𝑜𝑂4→𝐵𝑎𝐵2𝑂4 +𝑀𝑜𝐵

(9)

𝑀𝑜𝐵2 + 𝑋𝑂2 (𝑔)→𝑀𝑜 + 2𝐵𝑂𝑋 (𝑔)

(10)

Conclusions The molybdates of strontium and barium have been shown to be viable options as replacements for BaCrO4 in the traditional T-10 ignition delay. While the system that included BaCrO4 had a higher burning rate than the two systems with replacement oxidizers, they were all shown to have burning rates that were not dependent on effective diameter. Again, this study was not attempting to match the burning rate of T-10, but to demonstrate the feasibility of environmentally benign replacements. Thermal characteristics obtained by DSC and combustion temperatures followed the same trend as burning rates. The heat flow with the B/BaCrO4 system peaked very rapidly and was much higher than the replacement systems. A more gradual increase/decrease in heat flow was observed with the B/SrMoO4 and B/BaMoO4 systems. This behavior was then corroborated with combustion temperature measurements. The B/BaCrO4 system had a combustion temperature that peaked very quickly but also had a rapid decrease. Whereas, the B/SrMoO4 and B/BaMoO4 systems peaked at a slower rate but the decay in temperature was more gradual. Regarding environmental impact, XRD showed that the B/SrMoO4 and B/BaMoO4 systems generated relatively harmless reaction products upon combustion, but the traditional 13 ACS Paragon Plus Environment


Page 14 of 27

T-10 delay generated large quantities of chromium containing compounds that are potentially hazardous and carcinogenic. There was no distinguishable correlation between gas generation and other combustion characteristics but the difference in gas generation was addressed with the reaction products; less elemental molybdenum detected in the reaction products of the B/BaMoO4 system translated into less generation of BOX gas. In addition, the relatively small amounts of Methocel in the printable ink formulations significantly contributed to the generation of gaseous products upon combustion. Furthermore, T-10 and the replacement systems were successfully formulated into printable inks and deposited onto soapstone substrates. Linear traces deposited onto flat soapstone substrates were confirmed to be uniform and visually free of air voids. In addition, traces deposited and combusted around soapstone rods demonstrated a high degree of configurability. This was a critical step towards a universal delay. Acknowledgments Funding for this work was provided by the Strategic Environmental Research and Development Program (SERDP), Project WP-2518, “Environmentally Sustainable Gasless Delay Compositions for Fuzes”. A special thanks to Dr. Robin Nissan, program manager. Supporting Information Linear trace printed with a Janome JR2000N series robot equipped with a Techcon auger valve (Figure S1); Combustion chamber used in gas generation setup (Figure S2); SEM images of as-received boron, barium chromate, strontium molybdate, and barium molybdate powders (Figure S3); Images of the B/SrMoO4 and B/BaMoO4 formulations with 70 wt% solids from the cross-section, bottom, side, and top (Figures S4 and S5); XRD spectra of B/BaCrO4, B/SrMoO4, and B/BaMoO4 reaction products (Figure S6); Combustion of B/BaCrO4 printed in a linear trace on a soapstone substrate (Video S1); Combustion of B/SrMoO4 printed in a linear trace on a soapstone substrate (Video S2); Combustion of B/BaMoO4 printed in a linear trace on a soapstone substrate (Video S3); Combustion of B/BaCrO4 printed in a helix trace around a soapstone substrate (Video S4); Combustion of B/SrMoO4 printed in a helix trace around a soapstone substrate (Video S5); Combustion of B/BaMoO4 printed in a helix trace around a soapstone substrate (Video S6). References (1) (2) (3) (4) (5) (6) (7)

W/BaCrO4/KClO4/Diatomaceous Earth: Tungsten Delay Composition, U.S. Military Specification MIL-T-23132A, June 16, 1972. Mn/BaCrO4/PbCrO4: Manganese Delay Composition, U.S. Military Specification MIL-M21383A, October 22, 1976. Zimmer-Galler, R. The Combustion of Tungsten and Manganese Delay Systems, accession number AD0879499, Defense Technical Information Center (DTIC), Fort Belvoir, VA, 1970. Shachar, E.; Gany, A. Investigation of slow-propagation tungsten delay mixtures. Propellants Explos. Pyrotech. 1997, 22, 207−211, DOI 10.1002/prep.19970220405. Zr-Ni/BaCrO4/KClO4: Delay Composition, U.S. Military Specification MIL-C-13739A, November 15, 1965. T-10 Delay Composition, U.S. Military Specification MIL-D85306A, November 7, 1991. Engineering Design Handbook, Military Pyrotechnics Series, Part One – Theory and Application, AMCP 706-185, U.S. Army Materiel Command, April 1967. 14 ACS Paragon Plus Environment

Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)


Guertin, J.; Jacobs, J. A.; Avakian, C. P. Chromium(VI) Handbook. CRC Press, Boca Raton, FL, 2004. Elischer, P. P.; Cleal, G.; Wilson, M. The Development of a Boron and Iron Oxide Delay System. Report MRL-R-994 - Materials Research Laboratories, Melbourne, Australia, 1986. Swanepoel, D.; Del Fabbro, O.; Focke, W. W. Manganese as fuel in slow-burning pyrotechnic time delay compositions. Propellants Explos. Pyrotech. 2010, 35, 105-113, DOI 10.1002/prep.200900005. Koenig, J. T.; Shaw, A. P.; Poret, J. C.; Eck, W. S.; Groven, L. J. Performance of W/MnO2 as an environmentally friendly energetic time delay compositions. ACS Sustainable Chem. Eng. 2017, 5, 9477-9484, DOI 10.1021/acssuschemeng.7b02579. Focke, W. W.; Tichapondwa, S. M.; Montgomery, Y. C.; Grobler, J. M.; Kalombo, M. L. Review of gasless pyrotechnic time delays. Propellants Explos. Pyrotech. 2018, 43, 1-40, DOI 10.1002/prep.201700311. Poret, J. C.; Shaw, A. P.; Csernica, C. M.; Oyler, K. D.; Vanatta, J. A.; Chen, G. Versatile boron carbide-based energetic time delay compositions. ACS Sustainable Chem. Eng. 2013, 1, 1333-1338, DOI 10.1021/sc400187h. Reimer, K.; Mangum, M. New "green" pyrotechnic time delays with strontium molybdate, JANNAF 59th Propulsion Meeting, San Antonio, TX, 2012. Miklaszewski, E. J.; Shaw, A. P.; Poret, J. C.; Son, S. F.; Groven, L. J. Performance and aging of Mn/MnO2 as an environmentally friendly energetic time delay composition. ACS Sustainable Chem. Eng. 2014, 2, 1312-1317, DOI 10.1021/sc500148k. Miklaszewski, E. J.; Poret, J. C.; Shaw, A. P.; Son, S. F.; Groven, L. J. Ti/C-3Ni/Al as a replacement time delay composition. Propellants Explos. Pyrotech. 2014, 39, 138-147, DOI 10.1002/prep.201300099. Murphy, S. V.; Atala, A. 3D bioprinting of tissues and organs. Nat. Biotechnol. 2014, 32, 773785, DOI 10.1038/nbt.2958. Ventola, C. L. Medical applications for 3D printing: current and projected issues. Pharm. Ther. 2014, 39, 704-711. Schubert, C.; van Langeveld, M. C.; Donoso, L. A. Innovations in 3D printing: a 3D overview from optics to organs. Br. J. Ophthalmol. 2014, 98, 159-161, DOI 10.1136/bjophthalmol-2013304446. Wang, H.; Sun, K.; Tao, F.; Stacchiola, D. J.; Hu, Y. H. 3D homeycomb-like structured graphene and its high efficiency as a counter-electrode catalyst for dye-sensitized solar cells. Angew. Chem. Int. Ed. 2013, 52, 9210-9214, DOI 10.1002/anie.201303497. Puszynski, J. A.; Bichay, M. M.; Swiatkiewicz, J. J. Wet processing and loading of percussion primers based on metastable nanoenergetic composites. US Patent 7,670,446 B2 (2010). Durban, M. M.; Golobic, A. M.; Bukovsky, E. V.; Gash, A. E.; Sullivan, K. T. Development and characterization of 3D printable thermite component materials. Adv. Mater. Technol. 2018, DOI 10.1002/admt.201800120. Mezger, M. J.; Tindle, K. J.; Pantoya, M.; Groven, L. J.; Kalyon, D. M. Energetic Materials: Advanced Processing Technologies for Next-Generation Materials; CRC Press, Boca Raton, FL, 2018, pp. 115-127. McLain, J. H. Heats of reaction plots as design criteria for pyrotechnic reactions. 1st International Pyrotechnics Society Seminar, Estes Park, CO, 1968, pp. 293-303. McLain, J. H. Pyrotechnics: From the Viewpoint of Solid State Chemistry; The Franklin Institute Press, Philadelphia, PA, 1980, pp. 46-62. Hall, S. Development of green extrudable delays for the navy. 42nd International Pyrotechnics Society Seminar, Grand Junction, CO, 2016, pp. 174-189. 15 ACS Paragon Plus Environment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 0022 nm 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 16 of 27

TOC/Abstract Graphic

a

Synopsis Using an additive manufacturing approach, SrMoO4 and BaMoO4 were demonstrated to be good replacements for BaCrO4, the harmful component in the T-10 pyrotechnic ignition delay.


Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Channels and air pockets that developed on the underside of a printed trace when using the B/BaCrO4 formulation with 55 wt% solids. 83x31mm (300 x 300 DPI)

ACS Paragon Plus Environment


Figure 2. Combustion of the B/BaCrO4 formulation with 55 wt% solids deposited on a soapstone substrate. 83x61mm (300 x 300 DPI)


Page 18 of 27

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Apparent viscosity vs. shear rate of printable ink formulations. 83x48mm (300 x 300 DPI)



Figure 4. Images of the B/BaCrO4 formulation with 60 wt% solids from the (a) cross-section, (b) bottom, (c) side, and (d) top to measure effective diameter (de). 83x61mm (300 x 300 DPI)


Page 20 of 27

Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Soapstone substrates with traces of the B/BaMoO4 printable ink formulation. 83x54mm (300 x 300 DPI)



Figure 6. Burning rate vs. average effective diameter of (▲) B/BaCrO4, (♦) B/SrMoO4, and (●) B/BaMoO4 printable ink formulations deposited on soapstone substrates. 83x59mm (300 x 300 DPI)


Page 22 of 27

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Time-stepped combustion of B/BaCrO4 (left column), B/SrMoO4 (middle column), and B/BaMoO4 (right column) printable ink formulations deposited on soapstone substrates. Note the luminous green combustion plume indicating gas. 83x70mm (300 x 300 DPI)



Figure 8. Time-stepped combustion of the B/SrMoO4 printable ink formulation deposited around a soapstone rod. 50x129mm (300 x 300 DPI)


Page 24 of 27

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. DSC of B/BaCrO4 (top), B/SrMoO4 (middle), and B/BaMoO4 (bottom) printable ink and dry powder formulations. 83x138mm (300 x 300 DPI)



Figure 10. Combustion temperature profiles of dry powder formulations. 83x50mm (300 x 300 DPI)


Page 26 of 27

Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11. Gas generation of reactive systems formulated as powder mixtures, powder mixtures with powder Methocel, and dry inks deposited on soapstone substrate. 83x46mm (300 x 300 DPI)


Environmentally Friendly Boron-Based Pyrotechnic Delays: An

Recommend Documents