IGNITION PRINCIPLES
Successful performance of a high-energy mixture depends upon;
1. The ability to ignite the material using an external stimulus, as well as the stability of the composition in the absence of the stimulus.
2. The ability of the mixture, once ignited, to sustain burn- ing through the remainder of the composition.
Therefore, a composition is required that will readily ignite and burn, producing the desired effect upon demand, while remaining quite stable during manufacture and storage. This is not an easy requirement to meet, and is one of the main reasons why a relatively small number of materials are used in high-energy mixtures.
Application of the ignition stimulus (such as a spark or flame) initiates a complex sequence of events in the composition. The solid components may undergo crystalline phase transitions, melting, boiling, and decomposition. Liquid and vapor phases may be formed, and a chemical reaction will eventually occur at the surface where the energy input is applied, if the necessary activation energy has been provided.
The heat released by the occurrence of the high-energy reaction raises the temperature of the next layer of composition. If the heat evolution and thermal conductivity are sufficient to supply the required activation energy to this next layer, further reaction will occur, liberating additional heat and propagation of the reaction down the length of the column of mixture takes place. The rates, and quantity, of heat transfer to, heat production in, and heat loss from the high-energy composition are all critical factors in achieving propagation of burning and a self-sustaining chemical reaction.
The combustion process itself is quite complex, involving high temperatures and a variety of short-lived, high-energy chemical species. The solid, liquid, and vapor states may all be present in the actual flame, as well as in the region immediately adjacent to it. Products will be formed as the reaction proceeds, and they will either escape as gaseous species or accumulate as solids in the reaction zone (Figure 1).
Figure 1. Burning pyrotechnic composition
A moving, high-temperature reaction zone, progressing through the composition, is characteristic of a combustion (or "burning") reaction. This zone separates unreacted starting material from the reaction products. In "normal" chemical reactions, such as those carried out in a flask or beaker, the entire system is at the same temperature and molecules react randomly throughout the container. Combustion is distinguished from detonation by the absence of a pressure differential between the region undergoing reaction and the remainder of the unreacted composition.
A variety of factors affect the ignition temperature and the burning rate of a high-energy mixture, and the chemist has the ability to alter most of these factors to achieve a desired change in performance.
One requirement for ignition appears to be the need for either the oxidizer or fuel to be in the liquid (or vapor) state, and reactivity becomes even more certain when both are liquids. The presence of a low-melting fuel can substantially lower the ignition temperature of many compositions. Sulfur and organic compounds have been employed as "tinders" in high- energy mixtures to facilitate ignition. Sulfur melts at 119°C, while most sugars, gums, starches, and other organic polymers have melting points or decomposition temperatures of 300°C or less (Table1).
TABLE 1. Effect of Sulfur and Organic Fuels on Ignition Temperature
The oxidizers used in high-energy mixtures are generally ionic solids, and the "looseness" of the ionic lattice is quite important in determining their reactivity. A crystalline lattice has some vibrational motion at normal room temperature, and the amplitude of this vibration increases as the temperature of the solid is raised. At the melting point, the forces holding the crystalline solid to- gether collapse, producing the randomly-oriented liquid state. For reaction to occur in a high-energy system, the fuel and oxygen-rich oxidizer anion must become intimately mixed, on the ionic or molecular level. Liquid fuel can diffuse into the solid oxidizer lattice if the vibrational amplitude in the crystal is sufficient.
Once sufficient heat is generated to begin decomposing the oxidizer, the higher-temperature combustion reaction begins, involving free oxygen gas and very rapid rates. We are concerned here with the processes that initiate the ignition process. Professor G. Tammann, one of the pioneers of solid-state chemistry, considered the importance of lattice motion to reactivity, and used the ratio of the actual temperature of a solid divided by the melting point of the solid (on the Kelvin or "absolute" scale) to quantify this concept.
Tammann proposed that diffusion of a mobile species into a crystalline lattice should be "significant" at an a-value of 0.5 or halfway to the melting point, on the Kelvin scale). At this temperature, later termed the Tammann temperature, a solid has approximately 70% of the vibrational freedom present at the melting point, and diffusion into the lattice becomes probable. If this is the approximate temperature where diffusion becomes probable, it is therefore also the temperature where a chemical reaction between a good oxidizer and a mobile, reactive fuel becomes possible. This is a very important point from a safety standpoint - the potential for a reaction may exist at surprisingly low temperatures, especially with sulfur or organic fuels present. Table 2 lists the Tammann temperatures of some of the common oxidizers. The low temperatures shown for potassium chlorate and potassium nitrate may well account for the large number of mysterious, accidental ignitions that have occurred with compositions containing these materials.
TABLE 2. Tammann Temperatures of the Common Oxidizers
Ease of initiation also depends upon the particle size and surface area of the ingredients. This factor is especially important for the metallic fuels with melting point higher than or comparable to that of the oxidizer. Some metals - including aluminum, magnesium, titanium, and zirconium - can be quite hazardous when present in fine particle size (in the 1-5 micrometer range). Particles this fine may spontaneously ignite in air, and are quite sensitive to static discharge. For safety reasons, reactivity is sacrificed to some extent when metal powders are part of a mixture, and larger particle sizes are used to minimize accidental ignition.
Several examples will be given to illustrate these principles. In the potassium nitrate/sulfur system, the liquid state initially appears during heating with the melting of sulfur at 119°C. Sulfur occurs in nature as an 8-member ring - the S a molecule. This ring begins to fragment into species such as S 3 at temperatures above 140°C. However, even with these fragments present, reaction between sulfur and the solid KNO 3 does not occur at a rate sufficient to produce ignition until the KNO 3 melts at 334°C. Intimate mixing can occur when both species are in the liquid state, and ignition is observed just above the KNO3 melting point. Although some reaction presumably occurs between sulfur and solid KNO 3 below the melting point, the low heat output obtained from the oxidation of sulfur combined with the endothermic decomposition of KNOB prevent ignition from taking place until the entire system is liquid. Only then is the reaction rate great enough to produce a self-propagating reaction. Figures 2 - 4 show the thermograms of the components and the mixture. Note the strong exotherm corresponding to ignition for the KNO 3 /S mixture.
FIG. 2. Thermogram of pure potassium nitrate
FIG. 3. A sulfur thermogram
FIG. 4. The potassium nitrate /sulfur /aluminum system
In the potassium chlorate /sulfur system, a different result is observed. Sulfur again melts at 119°C and begins to fragment above 140°C, but a strong exotherm corresponding to ignition of the composition is found well below 200°C! Potassium chlorate has a melting point of 356 11C, so ignition is taking place well below the melting point of the oxidizer. We recall, though, that KC1O 3 has a Tammann temperature of 42 1C. A mobile species -- such as liquid, fragmented sulfur - can penetrate the lattice well below the melting point and be in position to react. We also recall that the thermal decomposition of KC1O 3 is exothermic (10.6 kcal of heat is evolved per mole of oxidizer that decomposes). A compounding of heat evolution is obtained -- heat is released by the KC1O 3 /S reaction and by the decomposition of additional KC1O 3 generating oxygen to react with additional sulfur. More heat is generated and an Arrhenius-type rate acceleration occurs, leading to ignition well below the melting point of the oxidizer. This combination of low Tammann temperature and exothermic decomposition helps account for the dangerous and unpredictable nature of potassium chlorate. Figures 5 - 6 show the thermal behavior of the KC1O 3 /S system.
FIG. 5. Thermogram of pure potassium chlorate, KCIO 3
FIG. 6. The potassium chlorate/sulfur system
As we proceed to higher-melting fuels and oxidizers, we see a corresponding increase in the ignition temperatures of twocomponent mixtures containing these materials. The lowest ignition temperatures are associated with combinations of low-melting fuels and low-melting oxidizers, while high-melting combinations generally display higher ignition points. Table 3 gives some examples of this principle.
TABLE 3. Ignition Temperatures of Pyrotechnic Mixtures
Table 3 shows that several potassium nitrate mixtures with low-melting fuels have ignition temperatures near the 334°C melt- ing point of the oxidizer. Mixtures of KNO 3 with higher-melting metal fuels show substantially higher ignition temperatures. Table 4 shows that a variety of magnesium-containing compositions have ignition temperatures close to the 649°C melting point of the metal.
A problem with trying to develop logical theory using literature values of ignition temperatures is the substantial variation in observed values that can occur depending upon the experimental conditions employed to measure the ignition points. Ratio of components, degree of mixing, loading pressure (if any), heat - ingrate, and quantity of sample can all influence the observed ignition temperature.
The traditional method for measuring ignition temperatures, used extensively by Henkin and McGill in their classic studies of the ignition of explosives, consists of placing small quantities (3 or 25 milligrams, depending on whether the material detonates or deflagrates) of composition in a constant-temperature bath and measuring the time required for ignition to occur. Ignition temperature is defined, using this technique, as the temperature at which ignition will occur within five seconds. Data obtained in this type of study can be plotted to yield interesting information, as shown in Figure 7.
FIG. 7. Time to explosion versus temperature for nitrocellulose
Data from time versus temperature studies can also be plotted as log time vs. 1/T, yielding straight lines as predicted by the Arrhenius Equation. Figure 8 illustrates this concept, using the same data plotted in Figure 7. Activation energies can be obtained from such plots. Deviations from linear behavior and abrupt changes in slope are sometimes observed in Arrhenius plots due to changes in reaction mechanism or other complex factors.
FIG. 8. "Henkin-McGill Plot" for nitrocellulose
"Henkin-McGill" plots can be quite useful in the study of ignition, providing us with important data on temperatures at which spontaneous ignition will occur. These data can be especially useful in estimating maximum storage temperatures for high-energy compositions - the temperature should be one corresponding to infinite time to ignition (below the "spontaneous ignition temperature," minimum - S.I.T (min) - shown in Figure 7). At any temperature above this point, ignition during storage is possible.
Ignition temperatures can also be determined by differential thermal analysis (DTA), and these values usually correspond well to those obtained by a Henkin-McGill study. Differences in heating rate can cause some variation in values obtained with this technique. For any direct comparison of ignition temperatures, it is best to run all of the mixtures of interest under identical experimental conditions, thereby minimizing the number of variables.
One must also keep in mind that these experiments are measuring the temperature sensitivity of a particular composition, in which the entire sample is heated to the experimental temperature. Ignition sensitivity can also be discussed in terms of the relative ease of ignition due to other types of potential stimuli, including static spark, impact, friction, and flame.