Issue Date: June 30, 2008
Pyrotechnics For The Planet
WHETHER THEY'RE lighting up Roman candles or basking in the glow of a fireworks extravaganza this Fourth of July, chemists are entitled to feel a certain amount of professional pride along with their patriotism. After all, it's chemistry that gives a humble bottle rocket its pop and makes a chrysanthemum shell bloom into a crowd-pleasing explosion of colored sparks.
Even so, when it comes to pyrotechnics, students of chemistry would be wise to bear in mind this old adage from physics: What goes up must come down. The complex brew of oxidizers, propellants, fuels, binders, and coloring agents is what makes each firework's burst brilliant. But it leaves behind a smoky ghost of combustion products and particulate matter, which waft their way into the nearby soil and water.
The same thing happens when real rockets give off their red glare. Military pyrotechnics, which encompass everything from missile propellants to handheld flares, release a plume of smoke and potentially toxic products that pose a health hazard to the men and women of the armed forces who may breathe them in.
Consequently, chemists have been working to make new pyrotechnic compounds and formulations so that bombs bursting in air do so more benignly.
Typical pyrotechnics function by burning, so their basic chemical components consist of an oxidant and a fuel. Black powder, the original pyrotechnic, blends potassium nitrate oxidizer with charcoal and sulfur fuel. Set this witch's brew alight, and in a flash the nitrate oxidizes the charcoal and sulfur, producing glowing solids and a vast volume of hot gases. Other components, such as colorants, binders, and propellants, can be added to the mix, depending on the task the pyrotechnic has to perform.
Over the years, perchlorate has become the oxidizer of choice for most pyrotechnic applications, supplanting less stable chlorate oxidants that were the cause of numerous deadly explosions. "Potassium perchlorate is the ideal oxygen donor to use in pyrotechnics in terms of safety, cost, and reproducibility," says John A. Conkling, a pyrotechnics expert and adjunct professor of chemistry at Washington College, in Chestertown, Md.
Unfortunately, perchlorate has also been identified as a potential human health hazard. Studies suggest that it inhibits the thyroid's ability to take up iodine from the bloodstream and can reduce the production of thyroid hormone. And because the anion is highly water soluble, it readily slips into groundwater. "The major effort in most areas of environmentally friendly pyrotechnics research is to find perchlorate replacement materials," Conkling says.
Conkling points out that in a working pyrotechnic—for example, a firework or a roadside flare—combustion should consume the majority of the perchlorate. In practice, however, that doesn't always happen. Pyrotechnics can be loaded with excess perchlorate to ensure burning; burning can snuff out prematurely; and, inevitably, there are duds that don't burn at all.
A team led by the Environmental Protection Agency's Richard T. Wilkin studied the concentration of perchlorate in the surface waters of a small lake in Ada, Okla., where there is an annual Fourth of July fireworks show (Environ. Sci. Technol. 2007, 41, 3966). Within 14 hours of the pyrotechnic display, the perchlorate level in the lake spiked as high as 1,000 times its baseline value. The researchers found that it took anywhere from 20 to 80 days for the perchlorate level to come down to its background level.
"THE PRESSURE is on to eliminate future perchlorate contamination by eliminating the perchlorate ingredient from as many rocket propellant and pyrotechnic compositions as possible," says Robert G. Shortridge, a scientist in the Pyrotechnic Operations Branch at the Crane Division of the Naval Surface Warfare Center.
Pyrotechnic flares have numerous roles in military operations, Shortridge notes. Aviators routinely carry red flares in their flight suits and life rafts to use as distress signals if their aircraft go down. Ground-based troops frequently use green and yellow flares to mark their locations, and all different types of flares are used on training grounds so soldiers become accustomed to the frequent explosions they're likely to encounter on the battlefield.
Aircraft also use decoy flares to thwart heat-seeking missiles. Such pyrotechnics give off an infrared signal that mimics the aircraft engine's infrared signature, so the missile goes after the flare rather than the plane or helicopter. With pyrotechnics being so vital to military operations, the Department of Defense's Strategic Environmental Research & Development Program and Environmental Security Technology Certification Program sponsor an extensive series of efforts to make pyrotechnic materials friendlier to the environment.
For example, Shortridge and his colleagues have been working to replace the perchlorate in colored signal flares. So far, they've had the most success with red signal flares that use strontium-based oxidants. Their perchlorate-free formulation is about to undergo safety testing, as well as tests in which the flares will be loaded into the signal hardware and subjected to the environmental rigors they would experience in service. "We intend to pass all of them while making the environment a little safer too," Shortridge says of the tests.
The other area in which pyrotechnics could improve from an environmental standpoint is their use of coloring agents. To achieve colored fireworks and flares, pyrotechnic makers employ metals or metal compounds that emit light in the visible spectrum. Red hues come from strontium, sodium glows yellow, barium burns green, and blues and greens come from copper.
At one time, mercury and lead compounds were used as colorants, but they were phased out long ago. Ironically, the modern pyrotechnic components that could use some "greening" are the barium compounds that give fireworks and flares their green color.
While tromping through a fresh snowfall in Vienna on New Year's Eve, Georg Steinhauser decided to find out just how much of these metal combustion products make their way into the environment from a typical fireworks display. Steinhauser, a licensed pyrotechnician and chemistry postdoc at Vienna University of Technology, scooped samples of snow in the city and countryside before and after the holiday pyrotechnics show and tested them for combustion products.
Postpyrotechnics snow from the city had significant concentrations of barium and strontium, indicating that the fireworks had left a chemical signature behind. By comparison, snow from the countryside, which was well out of fireworks range, remained clean after the show.
Before you break out the plastic sheeting and duct tape in anticipation of this year's Independence Day pyrotechnics extravaganza, you should know that most experts think the level of pollution from shooting off fireworks outdoors a couple of times per year is actually pretty small. Steinhauser is quick to point out that with the exception of barium, the metallic combustion products he found are harmless. The particulate matter and combustion products from fireworks quickly disperse, and the amount of perchlorate that fireworks give off is relatively little.
Pollution from fireworks becomes more problematic when the pyrotechnics are being used indoors, such as at concerts or sporting events, or when they're set off in the same spot night after night. "In places where you're doing displays multiple days per week, pollution can definitely become a major issue," says David E. Chavez, a chemist at Los Alamos National Laboratory (LANL). "If you have pyrotechnic devices being used day after day, show after show, then you'll gradually build up a certain amount of toxic metals."
The problem came to LANL's attention about a decade ago, when the national lab was approached by Walt Disney Co. Disneyland's neighbors in Anaheim, Calif., were complaining about pollution from the amusement park's nightly fireworks show, and the company wondered whether LANL's explosives experts could develop environmentally friendlier fireworks.
"Smoke was essentially the main issue," Chavez explains. The black powder used to propel the fireworks skyward left a trail of smoke, as did pyrotechnic combustion products, such as potassium chloride from the potassium perchlorate oxidant, and metal oxides from metallic fuels, such as magnesium.
Disney was able to solve the black powder problem with an engineering solution. The company built a system that uses compressed air to send pyrotechnics aloft, eliminating the need for black powder. "The other problem is a chemistry problem, which is eliminating the smoke once it gets up there," says Mike Hiskey, an energetic materials expert who was part of the LANL team and now runs his own pyrotechnics company, DMD Systems.
In the late 1990s, the LANL team tried to address the smoke problem while making compounds with high nitrogen content as potential explosives and propellants. "We took one of our high-nitrogen materials and mixed it with a little strontium nitrate just to see what it would look like, and we got a very quickly burning, beautifully colored flame with absolutely no smoke," Hiskey recalls. "We thought, 'We're on to something here.' "
Unlike traditional pyrotechnics, which get their energy from oxidizing carbon or metal fuels, high-nitrogen materials store their energy in their N–N and N–H bonds. "When they give off their energy, it's not an oxidizing process," Chavez explains. Instead, the molecules release energy as they break up into N2 and H2. Very little carbon is present in these nitrogen-rich molecules, he continues, so much smaller amounts of oxidizers, such as perchlorate, are needed.
Less carbon and less perchlorate also mean less smoke, Chavez adds. With less smoke to obscure color, pyrotechnic makers can cut down on the amount of coloring agent they need in a firework or flare. "You can reduce the amount of metal pretty dramatically," Chavez says. In a traditional pyrotechnic blend, the coloring agent can account for as much as 20–30% of the mixture by weight. In a high-nitrogen pyrotechnic formulation, the colorant makes up just 2–5 wt % of the blend.
Nitrogen-rich pyrotechnics also offer chemists the opportunity to combine the metal coloring agent and fuel in a single compound. The high-nitrogen compounds readily associate with popular colorant metals, such as strontium or copper.
"In a regular pyrotechnic mixture, you just have a metal compound that has to be vaporized somehow—generally through heat generated in the oxidizing process," Chavez explains. "We thought if you could actually vaporize individual atoms of metals using metal complexes of bistetrazole or bistetrazoleamine, for example, it would make for a much more efficient method of getting the color-producing metals in the gaseous form."
IN THE PAST 10 years or so that chemists have been pursuing high-nitrogen pyrotechnics, they've managed to create a vast menagerie of these nitrogen-rich compounds. Thomas M. Klapötke, a chemistry professor at Germany's University of Munich, and Vienna University of Technology's Steinhauser recently published a comprehensive review on the topic (Angew. Chem. Int. Ed. 2008, 47, 3330).
Looking at these compounds—salts based on tetrazole, bistetrazole, bistetrazoleamine, dihydrazino tetrazine, and bistetrazolylamino tetrazine, to name a few—one would think that the goal is to string as many nitrogen atoms together in a compound as is possible. But much more goes into making a good high-nitrogen pyrotechnic, Klapötke says.
"You try to make a molecule with lots of nitrogen that is kinetically stable enough so that it can be handled in a safe way," Klapötke tells C&EN. "It's easy to make a compound with a lot of nitrogen that is friction, impact, or electrostatically sensitive. We don't want that for flares or civil fireworks. We want them to be safe to handle." To that end, the nitrogen atoms are often contained in an aromatic or pseudoaromatic ring system, so that delocalization lends the molecule some kinetic stability.
Klapötke's lab has been pursuing nitrogen-rich compounds for use as military flares. Such specialized military applications are probably the first place that high-nitrogen pyrotechnics will find a practical use, chemists working in the field agree. They may also make lovely fireworks, but they're simply not cost competitive with commercially available pyrotechnics, most of which come from China and are assembled from inexpensive starting materials with very low labor costs.
Darren Naud says he and Hiskey learned that lesson the hard way eight years ago when they founded DMD Systems as a weekend enterprise away from their full-time jobs at LANL. "We originally started the business thinking we could use bistetrazoleamine," Naud says, alluding to the work that he and Hiskey had done with the group at LANL.
To start playing around with commercial pyrotechnic formulations of bistetrazoleamine, however, they needed to get a drum of the stuff. The compound isn't commercially available, so Naud and Hiskey sought a contractor to make the material. Originally, they were given an estimate of $30 per lb—expensive but within reach. When the contractor came back with a revised estimate of $300 per lb, they knew they would have to look elsewhere.
"WE KNEW we had to use stuff that was commercially available and dirt cheap, so we settled on propellant-grade nitrocellulose, which is used by the military in millions of millions of tons every year as a gun propellant," Hiskey says.
"Nitrocellulose is probably one of the best low-smoke ingredients," Naud adds. "It burns with little smoke, and there's no fallout or residual combustion by-products that are nasty. There's just CO2, water, and nitrogen."
Nitrocellulose has most of the oxygen it needs for complete, clean combustion already in the molecule as nitrate esters, Hiskey explains. There's no need to load it up with perchlorate oxidizers. Because the material produces little smoke, only small amounts of coloring agents are required to get vividly hued pyrotechnic effects.
Two-and-a-half years ago, Hiskey and Naud left LANL to devote themselves to DMD full time. In the remote reaches of northern New Mexico, the company's five employees do both pyrotechnics R&D and production, frequently shooting off fireworks beneath their warehouse's 25-foot-high ceilings.
DMD's low-smoke pyrotechnics have found a nice niche with customers who want indoor fireworks. "We crank out mainly theatrical pyro stuff," Hiskey says. They supply fireworks for Las Vegas shows, rock bands, circuses, Disney, and the folks at World Wrestling Entertainment.
Hiskey estimates that a fireworks display from DMD costs about twice as much as a traditional show, but customers are willing to pay extra for pyrotechnics that are safer for indoor use. "How do we make a product that would be able to compete with the Chinese pyrotechnics?" Hiskey asks. "The answer is that we can't, unless the customer demands that it's perchlorate-free or low-smoke." And that demand is on the rise, Hiskey and Naud tell C&EN. Their clients are also starting to ask them for perchlorate-free pyrotechnics for outdoor shows.
It's going to take that kind of demand, combined with tougher regulation, if eco-friendly pyrotechnics are going to light up the skies, scientists say. Despite the competition from inexpensive, traditional pyrotechnic formulations, they think there is a future for greener products. "Anything that we do for the environment initially costs money, but you have to take into account that all the cleanup processes often cost even more," Klapötke notes. In the long run, he says, it could be cheaper to go with pyrotechnics that maintain their dazzle and glow while minimizing their environmental fizzle.
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