Military still sees red: Chlorine-free flares

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  • Published: Sep 1, 2015
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Military still sees red: Chlorine-free flares

Fashionable flares

The carmine colour of pyrotechnic flares used by the military and in civilian application is usually generated by strontium chlorides. However, there is an urgent need to reduce chlorine use and now military research has developed a chlorine-free formulation for such flares that in some ways overturns the received wisdom that there was ever a need for a chlorine source in the first place.

The carmine colour of pyrotechnic flares used by the military and in maritime and civilian applications is usually generated by strontium monochloride. However, there is an urgent need to reduce chlorine use and now military research has developed a chlorine-free formulation for such flares that in some ways overturns the received wisdom that there was ever a need for a chlorine source in the first place.

Ernst-Christian Koch of the Lutradyn–Energetic Materials Science & Technology in Kaiserslautern, Germany, Jesse Sabatini of the US Army Research Laboratory, Energetics Technology Branch, in Maryland, Jay Poret and Jared Moretti of the US Army RDECOM-ARDEC Pyrotechnics Technology & Prototyping Division Picatinny Arsenal, in New Jersey, and Seth Harbol of US Army, 22nd Chemical Battalion, (Technical Escort) Edgewood Arsenal, also in Maryland, USA, describe details of their work which aims to avoid the formation of carcinogenic compounds in the journal Angewandte Chemie.

Deep red

The familiar deep, red burning flame of fireworks and military and civilian distress signal lights is commonly generated by the formation of gaseous strontium monochloride. Many such flares (fusees) and fireworks have perchlorate oxidizers too as well as chlorine-containing organic compounds such as polyvinyl chloride (PVC). The researchers, including defence scientists Sabatini and Koch wanted to preclude the need for such chloroorganics. "These formulations typically contain a metallic fuel such as magnesium, strontium nitrate, as the oxidizer, an organochlorine compound such as PVC, an organic binder, and sometimes auxiliary chlorine sources," the team explains. It is a problem for those who need to see red, that simply removing the chloroorganics from the formulation simply gives a washed-out, orange flame rather than the emergency red.

No washout

The team has now found that if they use less magnesium in one formulation but add a nitrogen-containing organic ingredient (5-aminotetrazole, 5-AT) they get a dominant wavelength in the red region of the visible spectrum and so a stronger red flame and spectral purity. 5-AT is known as a gas-generation compound used for auto-ignition purposes such as in vehicle air bags or for pyrotechnics purposes. According to the US Army Public Health Command it is a relatively benign compound. Interestingly, a second nitrogen organic compound, hexamine, also gave similar results for the flame colour. Although the resulting emission spectrum of the flame lacked the bands typically observed with strontium monochloride, this loss was well compensated for by the strong emission peaks of the strontium monohydroxide and strontium hydride that formed. The key to colour success was the lowering of incandescent and orange-emitting strontium oxide levels, which was done by adding the reducing and concomitantly explosive compounds including the nitrogen- and hydrogen-containing organocarbons.

"The present study has shown that it is feasible to obtain both high intensity and highly saturated red flames exclusively based on strontium monohydroxide with concomitant strontium hydride emission," the authors say. "Avoiding chlorine in these formulations eliminates the risk of the formation of PCBs (polychlorinated biphenyls), PCDDs (polychlorinated dibenzo-p-dioxins), and PCDFs (polychlorinated dibenzo-p-difurans). This finding, hence, will have a great impact on both military pyrotechnics and commercial firework sectors," the team concludes.

"The next step in this research is to begin examining ways to remove the chlorine/halogens from other light-emitting pyrotechnics (i.e. green- and blue-light-emitters, for example)," Sabatini told SpectroscopyNOW. "We have reported on some of this in the past already, but the experiments have only been done on a small-scale, and need to be scaled-up before they can be deemed to be practical. But this is what makes the chlorine-free red pyrotechnics so exciting to us. The successful chlorine-free red formulations disclosed in the Angewandte paper have already been scaled-up and proven [themselves] on a large-scale.  So we hope that this will pique the interest of fireworks companies, and further hope that the chlorine-free red formulations will be considered further by the military for future use.

Related Links

Angew Chem Int Ed 2015, 54, online: "Chlorine-Free Red-Burning Pyrotechnics"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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