PRINCIPLES IN PYROTECHNIC
Reactivity, in general, refers to the rate - in grams or per second - at which starting materials are converted into products.
The importance of intimate mixing was recognized as early as1831 by Samuel Guthrie, Jr., a manufacturer of "fulminating used to prime firearms. Guthrie's mixture was a blend of potassium nitrate, potassium carbonate, and sulfur, and he discovered that the performance could be dramatically improved if he first melted together the nitrate and carbonate salts, and then blended in the sulfur. He wrote, "By the previously melting together of the nitro and carbonate of potash, a more intimate union of these substances was effected than could possibly be made by mechanical means". However, he also experienced the hazards associated with maximizing reactivity, reporting, "I doubt whether, in the whole circle of experimental philosophy, many cases can be found involving dangers more appalling, or more difficult to be overcome, than melting fulminating powder and saving the product, and reducing the process to a business operation. I have had with it some eight or ten tremendous explosions, and in one of them I received, full in my face and eyes, the flame of a quarter of a pound of the composition, just as it had become thoroughly melted". An enormous debt is owed to these pioneers in high-energy chemistry who were willing to experiment in spite of the obvious hazards, and reported their results so others could build on their knowledge.
Varying degrees of homogeneity can be achieved by altering either the extent of mixing or the particle size of the various components. Striking differences in reactivity can result from changes in either "fulminating powder."
A number of parameters related to burning behavior can be experimentally measured and used to report the "reactivity" or performance of a particular high-energy mixture:
1. Heat of reaction : This value is expressed in units of calories (or kilocalories) per mole or calories per gram, and is determined using an instrument called a "calorimeter." One calorie of heat is required to raise the temperature of one gram of water by one degree (Celsius) , so the temperature rise of a measured quantity of water, brought about by the release of heat from a measured amount of high-energy composition, can be converted into calories of heat. Depending upon the intended application, a mixture liberating a high, medium, or low value may be desired. Some representative heats of reaction are given in Table 1.
2. Burning rate: This is measured in units of inches, centimeters or grams per second for slow mixtures, such as delay compositions, and in meters per second for "fast" materials. Burning rates can be varied by altering the materials used, as well as the ratios of ingredients, as shown in Table 2. Note: Burning "rates" are also sometimes reported in units of seconds /cm or seconds/ gram - the inverse of the previouslystated units. Always carefully read the units when examining burning rate data!
3. Light intensity: This is measured in candela or candlepower. The intensity is determined to a large extent by the temperature reached by the burning composition. Intensity will increase exponentially as the flame temperature rises, provided that no decomposition of the emitting species occurs.
4. Color quality: This will be determined by the relative intensities of the various wavelengths of light emitted by species present in the pyrotechnic flame. Only those wavelengths falling in the "visible" region of the electromagnetic spectrum will contribute to the color. An emission spectrum, showing the intensity of light emitted at each wavelength, can be obtained if the proper instrumentation - an emission spectrometer - is available (Figure 1).
5. Volume o f gas produced: Gaseous products are frequently desirable when a high-energy mixture is ignited. Gas can be used to eject sparks, disperse smoke particles, and provide propellant behavior; when confined, gas can be used to create an explosion. Water, carbon monoxide and dioxide, and nitrogen are the main gases evolved from high- energy mixtures. The presence of organic compounds can generally be counted upon to produce significant amounts of gas. Organic binders and sulfur should be avoided if a "gasless" composition is desired.
6. Efficiency: For a particular composition to be of practical interest, it must produce a significant amount of pyrotechnic effect per gram of mixture. Efficiency per unit volume is also an important consideration when available space is limited.
7. Ignitibility : A pyrotechnic composition must be capable of undergoing reliable ignition, and yet be stable in transportation and storage. The ignition behavior of every mixture must be studied, and the proper ignition system can then be specified for each. For easily-ignited materials, the "spit" from a burning black powder fuse is often sufficient. Another common igniter is a "squib" or electric match, consisting of a metal wire coated with a small dab of heat-sensitive composition. An electric current is passed through the wire, producing sufficient heat to ignite the squib. The burst of flame then ignites the main charge. For pyrotechnic mixtures with high ignition temperatures, a primer or first fire is often used. This is an easily-ignited composition that can be activated by a fuse or squib. The flame and hot residue produced are used to ignite the principal material.
TABLE 1. Representative Heats of Reaction for Pyrotechnic Systems
TABLE 2. Burning Rates of Binary Mixtures of Nitrate Oxidizers with Magnesium
Metal
Figure 1.
To produce the desired pyrotechnic effect from a given mixture, the chemist must be aware of the large number of variables that can affect performance. These factors must be held constant from batch to batch and day to day to achieve reproducible behavior. Substantial deviations can result from variations in any of the following:
1. Moisture: The best rule is to avoid the use of water in processing pyrotechnic compositions, and to avoid the use of all hygroscopic (water-attracting) ingredients. If water is used to aid in binding and granulating, an efficient drying procedure must be included in the manufacturing process. The final product should be analyzed for moisture content, if reproducible burning behavior is critical.
2. Particle size of ingredients: Homogeneity, and pyrotechnic performance, will increase as the particle size of the various components is decreased. The finer the particle size, the more reactive a particular composition should be, with all other factors held constant. Table 3 illustrates this principle for a sodium nitrate /magnesium flare composition. Note the similarity in performance for the two smallest particle sizes, suggesting that an upper performance limit may exist.
TABLE 3. Effect of Particle Size on Performance of a Flare Compositions
3. Surface area of the reactants: For a high-energy reaction to rapidly proceed, the oxidizer must be in intimate contact with the fuel. Decreasing particle size will increase this contact, as will increasing the available surface area of the particles. A smooth sphere will possess the minimum surface area for a given mass of material. An uneven, porous particle will exhibit much more free surface, and consequently will be a much more reactive material. Particle size is important, but surface area can be even more critical in determining reactivity. Several examples of this phenomenon are presented in Tables 4 and 5.
TABLE 4. Effect of Particle Size on the Burning Rate of Tungsten Delay Mixtures
TABLE 5 Effect of Particle Size on Burning Rate
4. Conductivity: For a column of pyrotechnic composition to burn smoothly, the reaction zone must readily travel down the length of the composition. Heat is transferred from layer to layer, raising the adjacent material to the ignition temperature of the particular composition. Good thermal conductivity can be essential for smooth propagation of burning, and this is an important role played by metals in many mixtures. Metals are the best thermal conductors, with organic compounds ranking among the worst. Table 10 lists the thermal conductivity values of some common materials.
5. Outside container material : Performance of a pyrotechnic mixture can be affected to a substantial extent by the type of material used to contain the mixed composition. If a good thermal conductor, such as a metal, is used, heat may be carried away from the composition through the wall of the container to the surroundings. The thickness of such a metal wall will also be an important consideration. If sufficient heat does not pass down the length of the pyrotechnic mixture, burning may not propagate and the device will not burn completely. Organic materials, such as cardboard, are widely used to contain low-energy pyrotechnic compositions - such as highway fuses and fireworks - to minimize this problem (cardboard is a poor thermal conductor).
6. Loading pressure: There are two general rules to describe the effect of loading pressure on the burning behavior of a pyrotechnic composition. If the pyrotechnic reaction, in the postignition phase, is propagated via hot gases, then too high of a loading pressure will retard the passage of these hot gases down the column of composition. A lower rate, in units of grams of composition reacting per second, will be observed at high loading pressures. (Note: One must be cautious in interpreting burn rate data, because an increase in loading pressure usually leads to an increase in the density of the composition. What may appear to be a slower rate, expressed in units of millimeters/second, may actually be a faster rate in terms of grams/second. ) If the propagation of the pyrotechnic reaction is a solid- solid or solid - liquid phenomenon, without the significant involvement of gas-phase components, then an increase in loading pressure should lead to an increase in burn rate (in grams per second). An example of this possibility is given in Table 6.
TABLE 6. Effect of Loading Pressure on the Burning Rate of a Delay Mixture
7. Degree o f confinement : The variation in the burning behavior of black powder was discussed as a function of the degree of confinement. Increased confinement leads to accelerated burning. Shimizu reports a burning rate in air of .03-.05 meters/second for black powder paste impregnated in twine. The same material, enclosed in a paper tube of one cm inside diameter, had a burning rate of 4.6-16.7 meters /second - over 100 times faster! This behavior is typical of loose powders, and points out the potential danger of confining mixtures that burn quite sluggishly in the open air. This effect is particularly important when consideration is given to the storage of pyrotechnic compositions. Containers and storage facilities should be designed to instantly vent in the case of pressure buildup. Such venting can quite effectively prevent many fires from progressing to explosions.
Two factors contribute to the effect of confinement on burning rate. An increase in temperature produces an exponential increase in rate of a chemical reaction. In a confined high-energy system, the temperature of the reactants can rise dramatically upon ignition, as heat is not effectively lost to the surroundings. A sharp rise in reaction rate occurs, liberating more heat, raising the temperature further, accelerating the reaction until an explosion occurs or the reactants are consumed. The minimum quantity of material needed to produce an explosion, under a specified set of conditions, is referred to as the critical mass. Also, in a confined system, the hot gases that are produced can build up substantial pressure, driving the gases into the high-energy mixture and causing a rate acceleration.
Burning behavior can therefore be summarized in two words: homogeneity and confinement. An increase in either should lead to an increase in burning rate for most high-energy mixtures. Note, however, that "gasless" compositions do not show the dramatic confinement effects found for "gassy" compositions.