Evaluation of a Sustainable Pyrotechnic Tracer ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10657-10661

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Evaluation of a Sustainable Pyrotechnic Tracer Composition for Small Caliber Ammunition Jared D. Moretti,*,† Christopher M. Csernica,† Jay C. Poret,† Anthony P. Shaw,† Nicholas E. Carlucci,† Martin G. Seiz,‡ and John D. Conway‡ †

Pyrotechnics Technology Division and ‡Small Caliber Munitions Division, US Army (RDECOM-ARDEC), Picatinny Arsenal, New Jersey 07806-5000, United States ABSTRACT: Recent efforts have aimed to replace the near-infrared (NIR) tracer composition (R-440) currently specified for some types of 7.62 × 51 mm NATO ammunition with a more environmentally sustainable composition (R-680). This report summarizes the two main efforts aimed to evaluate the suitability of such a replacement: (1) establishing a charge weight window for R-680 that meets the tracer depth requirement for 7.62 mm ammunition and (2) comparative characterization of the emission spectra of R-440 and R-680. To this end, the 7.62 mm tracer depth requirement was met only for rounds with net R-680 charge weights ranging from 665 to 745 mg. Static burn testing of pellets derived from R-440 and R-680 showed that both compositions gave similar burning times (∼5 s) and substantially identical NIR output (0.20 W/sr). KEYWORDS: Sustainable pyrotechnics, Tracer, Ammunition, Near-infrared emission

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INTRODUCTION A pyrotechnic tracer is often loaded into ammunition rounds to provide a trail of visible and/or infrared radiation as the fired projectile travels.1 This “trace” is useful to the gunner for adjustment of fire or marking an impact point. Also useful is the ability to easily adjust the electromagnetic signature of the tracer; the output can be tuned to emit in either the visible2 or near-infrared (NIR)3 bandwidths, so that a wide range of signatures can be made available to the gunner for the various combat scenarios he or she may face. Such tuning can be accomplished by changing the fuel-oxidizer pairing and/or the stoichiometric relationship between them.4 In the case of the 7.62 mm M276 round, the “dim tracer” composition is an NIR illuminant named R-4405 and consists of barium and strontium peroxides (oxidizers), magnesium carbonate (burn retardant), and calcium resinate (fuel). Figure 1 shows a partially assembled M276 round. Normally, a round is fabricated by consolidating two successive increments of R440 powder into a copper metal jacket, followed by consolidation of a single increment of an igniter6 (I-568A). The igniter is used to light the tracer composition after it is lit by the hot propellant gases during gunfire. Table 1 shows the constitution of both R-440 and I-568A. Both of these compositions have a long service history, but in recent years R-440 has been targeted for replacement due to growing environmental concerns. More specifically, barium compounds have been cited as objectionably toxic.7 Barium peroxide, in particular, is regulated by the US Occupational Safety and Health Administration (OSHA) and is cited by numerous other federal and state agencies in the US.8−10 Aside from the toxicity issues associated with barium compounds, R-440 also has problems associated with calcium resinate. The supply of this uncommon natural product is often This article not subject to U.S. Copyright. Published 2017 by the American Chemical Society

Figure 1. Long view of finished M276 rounds, showing the uppermost igniter layer dyed red.

limited to few, or even only one vendor, which renders quality control of calcium resinate complicated. Furthermore, calcium resinate has the tendency to self-heat which poses additional concerns for safety in handling and transportation.11,12 In response to the life cycle risks posed to the M276 round by R-440, a program was initiated to evaluate a more environmentally benign alternative material. The result of this unpublished formulation development effort was R-680, shown in Table 1, which uses only strontium peroxide as the principal oxidizer. Also, the calcium resinate fuel has been replaced by polyurethane cured with methylene diphenyl diisocyanate Received: July 31, 2017 Revised: September 14, 2017 Published: October 3, 2017 10657

DOI: 10.1021/acssuschemeng.7b02623 ACS Sustainable Chem. Eng. 2017, 5, 10657−10661

ACS Sustainable Chemistry & Engineering

Research Article

Table 1. Chemical Makeup of Tracer Compositions R-440 and R-680 and Igniter I-568A Percent composition by weight Compositiona

Barium peroxide (BaO2)

Strontium peroxide (SrO2)

Magnesium carbonate (MgCO3)

Calcium resinate

R-440 I-568A R-680

40.0

40.0 86.0 77.4

10.0

10.0

a

MDI-based polyurethane 14.0 12.6

10.0

For identification, a dye such as toluidine red may be added to I-568A as 0.5% of total batch weight.

Table 2. Initial Build Matrix for 7.62 mm Tracers Based on R-440 and R-680 Charge Weight (mg) Lot

Tracer Mix

first incr. (510 MPa)

second incr. (540 MPa)

I-568A (590 MPa)

Net Wt. (mg)

Depth (mm)

1 2 3 4 5 6 7 8 9 Production Production

R-440 R-680 R-680 R-680 R-680 R-680 R-680 R-680 R-680 R-440 R-680

530 410 440 470 500 530 560 590 620 -----

350 350 350 350 350 350 350 350 350 -----

80 80 80 80 80 80 80 80 80 -----

890 770 800 810 820 850 860 880 900 -----

1.5113 2.0879 1.7577 1.5951 1.4707 1.2954 1.1862 1.0541 0.9576 2.7584 2.7534

increments constant at 350 and 80 mg. This initial exercise failed, however, as the measured tracer depths of lots 1−9 did not meet the acceptable tolerance for the M276 dim trace round (2.286−2.997 mm). While this was disappointing, the tracer depth measurements did establish the expected trend of reduced depths with increased charge weight. In order to establish the desired charge weight range, therefore, an additional iteration of tracer fabrication would be needed, this time covering a lower charge weight range. Despite the initial lack of success, we next sought any useful information that may be gained by the burn performance of lots 1−9. Although the geometry of these tracers (i.e., weight and depth) was nonrepresentative of those in production, the burn testing would still provide a valid comparison of the emissive properties of R-440 and R-680. Normally, the best practice for tracer burn testing is to spin the burning samples to best simulate the axial rotational effects experienced during bullet flight. However, the ARDEC spin apparatus proved unable to sustain a single axis when loaded with 7.62 mm rounds. The apparatus simply showed too much wobble to produce rotation about a single axis that would be amenable to any accurate radiation measurements. As a result, we proceeded to test the rounds upright and statically using standard hotwire ignition. The burn test results for ARDEC-produced lots 1−9 and normal production rounds are shown in Table 3. Two measurement systemsone silicon detector with a photometric filter and one silicon detector with NIR filterswere used to capture the visible and NIR emissive properties of all tested tracers. The intensities and burn times measured by each detector are given in different columns for comparison. As expected, the burn times measured from both detectors are in close agreement with each other for all nine lots. This round of static burn testing gave very mixed results. Most noticeable was the erratic burning of all tested tracers, presumably caused by excessive slag clogging the tracer jacket cavity. This slag is quite common during the normal burning of any pyrotechnic that produces a large percentage of solid products. However, due to the thin diameter and relatively low

(MDI). The new dim tracer composition has undergone extensive gunfire testing in the actual bullet hardware as a direct replacement for R-440;3 the results of these gunfire tests have shown that R-680 is capable of meeting the system requirements, but some additional producibility evaluation is needed before full deployment to the native production environment. We report here on our producibility studies linking charge weight to tracer column height for the R-680 tracer composition. We also compare the emissive properties of R680 to R-440.

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RESULTS AND DISCUSSION To evaluate the feasibility of replacing R-440 with R-680 in production, it was critical to establish the manufacturing parameters for R-680 that would meet the quality control requirements of the M276 round. One of the most critical of these requirements for the M276 is tracer depth, defined as the below-flush distance between the rim of the jacket and the top of the tracer column (typically 2.286−2.997 mm for 7.62 mm ammunition). Meeting the tracer depth requirement is critical, as this is related to the amount of composition pressed into the jacket. Greater depth values will result in diminished tracer display time, while lower values will make it geometrically impossible for the propellant cartridge to fit properly around the jacket. In production, tracer depth is primarily controlled by a single parameterthe amount of tracer composition loaded into each jacket, hereafter referred to as charge weight. Due to this intrinsic relationship between tracer depth and charge weight, the initial task was to assemble experimental tracer lots across a range of R-680 charge weights to establish tracer depth tolerance. For the first tracer build, we chose 530, 350, and 80 mg as the charge weights of the first, second, and third increment weights for a control lot based on the currently specified tracer and igniter mixes, R-440 and I-568A (lot 1 in Table 2). Then, a range of experimental lots based on R-680 and I-568A (lots 2−9) were assembled, across which the weight of R-680 used in the first increment was ramped up from 410− 620 mg, while holding the weights of the second and third 10658



Research Article

Table 3. Photometric (visible) and Radiometric (NIR) Data for Tracer Burn Tests Visible Detector

Lot No.

Tracer Mix

Burn Time (s)

1 2 3 4 5 6 7 8 9 Production Production

R-440 R-680 R-680 R-680 R-680 R-680 R-680 R-680 R-680 R-440 R-680

8.465 3.053 3.408 3.630 3.817 3.692 4.015 4.348 4.023 No ignition 3.77

NIR Detector Burn Time (s)

Average Radiant Intensity (W/sr)

1.090 0.960 2.027 2.304 2.009 2.633 1.893 2.100 2.540

8.985 3.077 3.497 3.650 4.018 3.710 4.031 4.361 4.037

0.039 0.027 0.052 0.063 0.056 0.070 0.048 0.056 0.061

3.94

2.45

0.07

Average Luminous Intensity (cd)

burning temperature of the dim tracer composition, combustion propagation of R-440 and R-680 mandates adequate spinning to eject the condensed phase combustion products from the jacket. In the absence of spin, slag is not centrifuged to the rim of the tracer. This leads to a concave burning front, as opposed to the well-established convex front13−19 of a spinning tracer. The concave front has less surface area than a convex front and thus transfers less heat to the adjacent, unreacted material. This explains the intermittent burning observed in testing of lots 1−9. From this, it is quite clear that any future iteration of static burn testing of dim tracers should involve spinning to better approximate the in-flight environment of 7.62 mm projectiles. In light of these difficulties, the first iteration of testing was still informative of the comparative tracer performance of R-440 and R-680. First, both compositions had comparable static visible output over the burn time. This low visible signature is important to ensure the gunner’s position is not revealed during covert operations. The other key observation was that the R680 tracers exhibited lower average NIR output relative to their R-440 analogs. The reduced NIR performance of R-680 is an especially important parameter for considering an engineering change as it relates directly to the proper function of the overall system. Because of the lack of an unambiguous comparison between R-440 and R-680 in actual bullet hardware, we turned to static burn testing of bare pellets derived from each dim tracer composition. Some caution is advised when trying to directly compare the performance of both compositions when tested in this manner; this simplified format is not exactly representative of combustion propagation in the tracer configuration as the pellets are expected to burn radially (i.e., nondirectionally from top-down and outside-inward), while the tracer is expected to have a more linear (top-down) burning mode. Also unlike the pellets, the actual tracers are jacketed with a copper alloy, a thermally conductive material that can alter burn propagation by wicking heat away from the reactive zone. Keeping in mind the stark difference between the two form factors, static testing of bare pellets gave more straightforward results. One trend consistent with the earlier testing in tracer jackets was the difficulty of lighting the R-440 composition, requiring more resistive heating from the hotwire than R-680. Also consistent is the visual comparison of R-440 and R-680 after pellet testing (Figure 2). It is clear that the residue of R-

Figure 2. Images of R-440 (top) and R-680 (bottom) pellet burn residues.

680 exhibits more expansion upon burning than R-440. A possible explanation for this is the higher proportion of R-680 combustion products in the gas phase, which results in greater separation between consecutive layers of solid combustion products. Other differences from the testing of tracer lots were noticed upon quantitative emissive measurements of pellets from both compositions (Table 4). Throughout all pellet testing, we Table 4. Visible and NIR Measurements from Pellet Burn Tests Pellet R440 R680

Visible Burn Time (s)

NIR Burn Time (s)

Visible Intensity (cd)

NIR Intensity (W/sr)

5.81

5.95

6.77

0.20

4.52

5.22

8.72

0.20

observed smooth continuous burning, instead of the punctuated, erratic burning. Also, in pellet format, R-680 did not show a drastically different visible range than R-440; although the burn time for R-680 was ∼20% shorter as measured by the visible detector, its intensity was ∼25% higher. This suggests that the integrated luminous intensities for both are essentially the same. More interesting was the comparison of the NIR detector measurements. In essence, both compositions exhibited nearly identical NIR signatures, burning for almost the same duration and at the same intensity. This provides evidence of the interchangeability of the two compositions and 10659



Research Article

prompts further testing at the tracer level using adequate sample spinning. Having gathered enough comparative emissive data for both compositions, we chose to revisit the producibility studies aimed to link charge weight with tracer depth. Last time we were clearly overloading the tracers, so for the next iteration we targeted a charge weight range below that of the last iteration. Table 5 shows the second iteration of tracer assembly,

charge weights correspond with a sharp drop in tracer depth, well outside the specified range. Moreover, the second iteration re-established the expected trend between increased charge weights and decreased tracer depths. Lastly, the mechanical losses to the tooling in the “Losses” column of Table 6 nearly double with every other 30 mg increase in the charge weight of the first increment.

Table 5. Build Matrix for Second Iteration of R-440/R-680 Tracer Fabrication

Assembly of 7.62 MM Rounds. The R-440 and R-680 tracer mixes were transported to ARDEC along with empty 7.62 mm jackets from Orbital-Alliant Techsystems, Inc. (OATK); the tracer mixes were used “as is” with no further purification or processing. For all tracer lots, the three separate increments were carefully weighed on an analytical balance in accordance with the charge weights given in the build tables (±0.005 g). To pack one round, an empty copper alloy jacket was placed into the tooling and each increment was pressed separately on a Carver press according to the consolidation pressures also given in the build tables (see Figure 3 below). The powder losses

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Charge Weight (mg) Lot

Mix

first incr. (510 MPa)

second incr. (540 MPa)

I-568A (590 MPa)

10 11 12 13 14 15 16 17 18

R-440 R-680 R-680 R-680 R-680 R-680 R-680 R-680 R-680

415 295 325 355 385 415 445 475 505

330 330 330 330 330 330 330 330 330

85 85 85 85 85 85 85 85 85

MATERIALS AND METHODS

particularly the nominal weights to be measured for the first, second, and third increments. This time, the second and third increments were held at 330 mg of R-680 and 85 mg of I-568A, while the first increment started at 295 mg and was ramped up to 505 mg in 30 mg increments across lots 11−18. The consolidation pressures used in this second iteration were identical to those from the first iteration. Once the second iteration rounds were fabricated, the tracer depths were measured in the same manner as those from the first iteration (see Table 6). It was clear after packing the Table 6. Build Measurements from 2nd Iteration Lot

Mix

Nominal Mass (mg)

Losses (mg)

Net Wt. (mg)

Tracer Depth (in.)

10 11 12 13 14 15 16 17 18

R-440 R-680 R-680 R-680 R-680 R-680 R-680 R-680 R-680

830 710 740 770 800 830 860 890 920

103 15 27 41 55 82 102 140 162

727 695 713 729 745 748 758 750 758

0.1107 0.1182 0.1014 0.0959 0.0942 0.0930 0.0789 0.0812 0.0691

Figure 3. Finished 7.62 mm tracer round nested in tooling. were collected and weighed separately; this weight was subtracted from the nominal weight of each round to calculate the net weight of each round. Tracer depths were measured from the side of the tracer column to the edge of the round and recorded in mm. Characterization. Static ignition testing was performed in the ARDEC flare tunnel at ambient temperature and pressure. The samples are ignited with a heated nickel−chromium wire energized with a 1000W power supply that generated approximately 15−20 A of current in the wire. Optical emissive properties of these formulations were characterized using both a single element photopic light detector along with a near-infrared detector. Both light detectors were manufactured by International Light Technologies, Peabody, MA, and are composed of a SED 033 silicon detector (33 mm2 area silicon detector with quartz window) coupled to either a photopic filter or a filter combination that generates the 600−900 nm passband. The field of view of both detectors was limited by use of a hood (H-hood). The detector’s current output was converted to voltage using a DL Instruments 1211 transimpedance amplifier. The amplifier’s voltage output was collected using a NI-9215 National Instruments data card and analyzed with in-house developed LabVIEW-based data acquisition and analysis software.

control lot comprised of R-440 that this new approach was sound since lot 10 was measured at 2.8118 mm, which is within the acceptable range of 2.286−2.997 mm. From the final column in Table 6, it is clear that lots 2−5 also fell within this range and their net weights ranged from 713−748 mg. This nominal range corresponds to a net R-680 charge weight range of 665−745 mg, since none of the I-568A igniter was lost to the tooling during round fabrication. This net charge weight range can thus be recommended for full-scale production. Notice also from Table 6 the almost linear relationship that decomposes after lot 15; net charge weights clearly trend with lower tracer depths measuring across lots 11−15. Beyond 745 mg, however, this trend appears to temporarily reverse with lot 17 having a slightly higher tracer depth than 16. Still higher

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CONCLUSION The R-680 dim tracer composition has been established as a viable alternative to the R-440 composition currently specified for the M276 dim tracer round. A robust charge weight range 10660



Research Article

peroxide; https://www.cdc.gov/niosh/ipcsneng/neng0381.html (accessed May 2017). (10) NJ Department of Health and Senior Services. Hazardous Substance Fact Sheet, Revised August 2001, Barium peroxide; http:// www.nj.gov/health/eoh/rtkweb/documents/fs/0190.pdf (accessed May 2017). (11) Hash, M. I. Handbook of Fillers, Extenders, and Diluents, 2nd ed.; Synapse Information Resources, Inc.: Endicott, NY, 2007; pp 242. (12) Babrauskas, V. Calcium resinate. In Ignition Handbook: Principles and Applications to Fire Safety Engineering, Fire Investigation, Risk Management and Forensic Science; Fire Science Publishers: 2003; pp 702. (13) Briere, P. The Effect of High Spin on the Performance of Tracer Compositions. Propellants, Explos., Pyrotech. 1989, 14, 250−254. (14) Messina, N. A.; Ingram, L. S.; Summerfield, M. Design and Fabrication of a Prototype Tracer Surveillance Tester; Contractor Report ARPAD-CR-83003; 1983; Princeton Combustion Research Laboratories Inc.: Princeton, N.J., USA. (15) Schulman, R. S. Factors Affecting Small Arms Tracer Burning; Report R-1287; 1955; Pitman-Dunn Laboratories, Frankford Arsenal: Philadelphia, PA, USA. (16) Izod, D. C. A.; Eather, R. E. Improved Red Tracer Flares; 4th International Pyrotechnics Seminar, Steamboat Village, 22−26 July 1974; Denver Research Institute, University of Denver: Denver, CO, 1974. (17) Puchalski, W. J. The Effect of Angular Velocity on Pyrotechnic Performance; 4th International Pyrotechnics Seminar, Steamboat Village, 22−26 July 1974; Denver Research Institute, University of Denver: Denver, CO, 1974. (18) Barton, T. J.; Ribby, M. J. Studies of Factors Affecting Tracer Performance; 7th International Pyrotechnics Seminar, Vail, Colorado, 14−18 July 1980; IIT Research Institute: Chicago, 1980. (19) Schulman, R. S. Effect of Variation of Cavity Geometry Upon Small Arm Tracer Burning; Report R-1421; 1957; Pitman-Dunn Laboratories, Frankford Arsenal, Philadelphia, PA, USA.

for R-680 has been established in the M276 form factor. Both compositions have similar performance in terms of burning time and visible/NIR signature. The interchangeable use of the two is thus expected to be transparent to the soldier while meeting functional system requirements. The burn testing of actual tracer rounds was limited in this study due to the inability to spin the samples when burn testing. For future evaluation of small caliber ammunition larger than 5.56 mm, the development of a more robust tracer spinning apparatus is needed. This will enable spinning about a single axis when testing 7.62 mm and wider caliber ammunition, which will be more representative of the actual rounds in flight. This, in turn, would enable more accurate spectroscopic analysis of a broad range of tracers at ARDEC. In addition to the spin apparatus, additional formulation development would be very beneficial. There are other possible ARDEC illuminants previously developed in flare form factors that could offer enhanced performance in the tracer format beyond that of R-680. Evaluation of these alternate compositions in small caliber form factors is currently under way and will be the subject of future manuscripts from this laboratory.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.D.M.). ORCID

Jared D. Moretti: 0000-0003-3313-4788 Notes

This manuscript has been approved for unlimited public release. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the U.S. Army for funding. REFERENCES

(1) Shidlovskiy, A. A. Principles of Pyrotechnics; accession number ADB005105; Defense Technical Information Center (DTIC): Fort Belvoir, VA, 1975; pp 303−327. (2) Doris, T. A., Jr.; Martin, R. Tracer Mixture for Laser Hardened Optics. U.S. Patent no. 5,472,536, 1995. (3) Guindon, L.; Jalbert, C.; Lepage, D. Non-Toxic Heavy-Metal Free-Zinc Peroxide-Containing IR Tracer Compositions and IR Tracer Projectiles Containing Same For Generating a Dim Visibility IR Trace. 2011, U.S. Patent no. US 7,985,311 B2. (4) Conkling, J. A.; Mocella, C. M. Chemistry of Pyrotechnics: Basic Principles and Theory, 2nd ed.; Taylor & Francis: Boca Raton, 2010; pp 59−96. (5) Doris, T. A., Jr. Low Light Level Tracer Mix. U.S. Patent no. 3,677,842, 1972. (6) Doris, T. A., Jr.; Vest, K. D. Igniter Composition. U.S. Patent no. 6,036,794, 2000. (7) US Department of Health and Human Services (HHS). Public Health Service, Agency for Toxic Substances and Disease Registry, Toxicological Profile for Barium and Barium Compounds; updated August 2007: https://www.atsdr.cdc.gov/ToxProfiles/tp24.pdf (accessed May 2017). (8) National Primary Drinking Water Regulations − Barium; document number EPA 811-F-95-002b-T; Environmental Protection Agency (EPA): https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey= 9100PO1Y.PDF (accessed September 13, 2017). (9) The National Institute for Occupational Safety and Health (NIOSH), International Chemical Safety Cards (ICSC), Barium 10661


Evaluation of a Sustainable Pyrotechnic Tracer ... - ACS Publications

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