Chemistry of Pyrotechnics
Components of High-Energy Mixtures
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The desired pyrotechnic effect must be carefully considered A good fuel will react with oxygen (or a halogen like fluorine when a fuel is selected to pair with an oxidizer for a high-en-or chlorine) to form a stable compound, and substantial heat will ergy mixture. Both the flame temperature that will be produced be evolved. The considerable strength of the metal-oxygen and and the nature of the reaction products are important factors.
metal-halogen bonds in the reaction products accounts for the The requirements for some of the major pyrotechnic categories excellent fuel properties of many of the metallic elements.
are
A variety of materials can be used, and the choice of material will depend on a variety of factors - the amount of heat output required, rate of heat release needed, cost of the materials, sta-1. Propellants: A combination producing high temperature, a bility of the fuel and fuel /oxidizer pair, and amount of gaseous large volume of low molecular weight gas, and a rapid burn-product desired. Fuels can be divided into three main categor-ning rate is needed. Charcoal and organic compounds are ies. metals, non-metallic elements, and organic compounds.
often found in these compositions because of the gaseous products formed upon their combustion.
2. Illuminating compositions: A high reaction temperature is Metals
mandatory to achieve intense light emission, as is the pres-A good metallic fuel resists air oxidation and moisture, has a high ence in the flame of strong light-emitting species. Magne-heat output per gram, and is obtainable at moderate cost in fine sium is commonly found in such mixtures due to its good particle sizes. Aluminum and magnesium are the most widely used heat output. The production of incandescent magnesium materials. Titanium, zirconium, and tungsten are also used, es-oxide particles in the flame aids in achieving good light in-pecially in military applications.
tensity. Atomic sodium, present in vapor form in a flame, The alkali and alkaline earth metals - such as sodium, potas-is a very strong light emitter, and sodium emission domi-sium, barium, and calcium -- would make excellent high-energy nates the light output from the widely used sodium nitrate/
fuels, but, except for magnesium, they are too reactive with magnesium compositions.
moisture and atmospheric oxygen. Sodium metal, for example, 3. Colored flame compositions : A high reaction temperature reacts violently with water and must be stored in an inert or-produces maximum light intensity, but color quality depends ganic liquid, such as xylene, to minimize decomposition.
upon having the proper emitters present in the flame, with A metal can initially be screened for pyrotechnic possibilities a minimum of solid and liquid particles present that are by an examination of its standard reduction potential (Table 2. 5).
emitting a broad spectrum of "white" light. Magnesium is A readily oxidizable material will have a large, negative value, sometimes added to colored flame mixtures to obtain higher meaning it possesses little tendency to gain electrons and a sig-intensity, but the color quality may suffer due to broad nificant tendency to lose them. Good metallic fuels will also be emission from MgO particles. Organic fuels (red gum, reasonably lightweight, producing high calories/gram values dextrine, etc.) are found in most color mixtures used in when oxidized. Table 3.4 lists some of the common metallic fuels the fireworks industry.
and their properties.
4. Colored smoke compositions: Gas evolution is needed to disperse the smoke particles. High temperatures are not desirable here because decomposition of the organic dye Aluminum (Al)
molecules will occur. Metals are not found in these mix-The most widely used metallic fuel is probably aluminum, with tures. Low heat fuels such as sulfur and sugars are com-magnesium running a close second. Aluminum is reasonable in monly employed.
cost, lightweight, stable in storage, available in a variety of 5. Ignition compositions: Hot solid or liquid particles are de-particle shapes and sizes, and can be used to achieve a variety sirable in igniter and first-fire compositions to insure the of effects.
transfer of sufficient heat to ignite the main composition.
Aluminum has a melting point of 660°C and a boiling point of Fuels producing mainly gaseous products are not com-approximately 2500°C. Its heat of combustion is 7.4 kcal/gram.
monly used.
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Chemistry
Components o f High-Energy Mixtures
67
of Pyrotechnics
Aluminum is available in either "flake" or "atomized" form.
The "atomized" variety consists of spheroidal particles. Spheres yield the minimum surface area (and hence minimum reactivity) for a given particle size, but this form will be the most reproducible in performance from batch to batch. Atomized aluminum, rather than the more reactive flake material, is used by the military for heat and light-producing compositions because the variation in performance from shipment to shipment is usually less.
Large flakes, called "flitter" aluminum, are widely used by the fireworks industry to produce bright white sparks. A special
"pyro" grade of aluminum is also available from some suppliers.
This is a dark gray powder consisting of small particle sizes and high surface area and it is extremely reactive. It is used to produce explosive mixtures for fireworks, and combinations of oxidizers with this "pyro" aluminum should only be prepared by skilled personnel, and only made in small batches. Their explosive power can be substantial, and they can be quite sensitive to ignition.
Aluminum surfaces are readily oxidized by the oxygen in air, and a tight surface coating of aluminum oxide (A120 3) is formed that protects the inner metal from further oxidation. Hence, aluminum powder can be stored for extended periods with little loss of reactivity due to air oxidation. Metals that form a loose oxide coating on exposure to air - iron, for example - are not provided this surface protection, and extensive decomposition can occur during storage unless appropriate precautions are taken.
Compositions made with aluminum tend to be quite stable.
However, moisture must be excluded if the mixture also contains a nitrate oxidizer. Otherwise, a reaction of the type 3KNO 3 +8Al+12H20-> 3KA1O2 +5A1(OH) 3 +3NH 3
can occur, evolving heat and ammonia gas. This reaction is accelerated by the alkaline medium generated as the reaction proceeds, and autoignition is possible in a confined situation. A small quantity of a weak acid such as boric acid (H 3B03) can effectively retard this decomposition by neutralizing the alkaline products and maintaining a weakly acidic environment. The hygroscopicity of the oxidizer is also important in this decomposition process. Sodium nitrate and aluminum can not be used together, due to the high moisture affinity of NaNO3 , unless the aluminum powder is coated with a protective layer of wax or similar material. Alternatively, the product can be sealed in a moisture-proof packaging to exclude any water [1]. Potassium nitrate/
aluminum compositions must be kept quite dry in storage to avoid