A pyrotechnician cautiously mixes a composition through a sieve to achieve homogeneity. Eye and respiratory protection are worn, and great care is taken throughout this critical phase of the manufacturing process. Sensitive compositions, as well as large quantities of any pyrotechnic mixture, should be blended remotely. (Fireworks by Grucci)
4PYROTECHNIC PRINCIPLES
INTRODUCTION
The "secret" to maximizing the rate of reaction for a given pyrotechnic or explosive composition can be revealed in a single word -
homogeneity. Any operation that increases the degree of intimacy of a high-energy mixture should lead to an enhancement of reactivity. Reactivity, in general, refers to the rate - in grams or moles per second - at which starting materials are converted into products.
The importance of intimate mixing was recognized as early as 1831 by Samuel Guthrie, Jr. , a manufacturer of "fulminating powder" 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" [1]. 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 83
84
Chemistry of Pyrotechnics
Pyrotechnic Principles
85
composition, just as it had become thoroughly melted" [1]. An TABLE 4.1 Representative Heats of Reaction for enormous debt is owed to these pioneers in high-energy chemis-Pyrotechnic Systemsa
try who were willing to experiment in spite of the obvious hazards, and reported their results so others could build on their o Hreaction ,
knowledge.
Composition (% by weight)
kcal /gram
Application
Varying degrees of homogeneity can be achieved by altering either the extent of mixing or the particle size of the various Magnesium
50
2.0
Illuminating flare
components. Striking differences in reactivity can result from changes in either of these, as Mr. Guthrie observed with his Sodium nitrate, NaNO 3
44
"fulminating powder."
Laminac binder
6
A number of parameters related to burning behavior can be experimentally measured and used to report the "reactivity" or Potassium perchlorate, KCIO
performance of a particular high-energy mixture [2]: 4 60
1.8
Photoflash
Aluminum
40
1. Heat of reaction : This value is expressed in units of Boron
25
1.6
I gniter
calories (or kilocalories) per mole or calories per gram, and is determined using an instrument called a "calorime-Potassium nitrate, KNO 3
75
ter." One calorie of heat is required to raise the tempera-VAAR binder
1
ture of one gram of water by one degree (Celsius) , so the temperature rise of a measured quantity of water, brought Potassium nitrate, KNO
about by the release of heat from a measured amount of 3
71
1.0
Starter mixture
high-energy composition, can be converted into calories Charcoal
29
of heat. Depending upon the intended application, a mixture liberating a high, medium, or low value may be de-Black powder
91
0.85
Flash and report
sired. Some representative heats of reaction are given in Aluminum
9
Military simulator
Table 4. 1.
2. Burning rate: This is measured in units of inches, cen-Barium chromate, BaCr0
85
0.5
Delay mixture
timeters or grams per second for slow mixtures, such as 4
delay compositions, and in meters per second for "fast"
Boron
15
materials. Burning rates can be varied by altering the materials used, as well as the ratios of ingredients, as Silicon
25
0.28
First fire mixture
shown in Table 4.2. Note: Burning "rates" are also Red lead oxide, Pb
sometimes reported in units of seconds /cm or seconds/
30,,
50
gram - the inverse of the previously-stated units. Al-Titanium
25
ways carefully read the units when examining burning rate data!
Tungsten
50
0.23
Delay mixture
3. Light intensity: This is measured in candela or candle-Barium chromate, BaCrO,,
40
power. The intensity is determined to a large extent by the temperature reached by the burning composition. In-Potassium perchlorate, KC10 y 10
tensity will increase exponentially as the flame temperature rises, provided that no decomposition of the emitting spe-aSource: F. L. McIntyre, "A Compilation of Hazard and Test Data cies occurs.
for Pyrotechnic Compositions," Report AD-E400-496, U.S. Army 4. Color quality: This will be determined by the relative in-Armament Research and Development Command, Dover, New Jersey, tensities of the various wavelengths of light emitted by October 1980.
a
pyrotechnic Principles
87
86
Chemistry of Pyrotechnics
TABLE 4.2 Burning Rates of Binary Mixtures of Nitrate Oxidizers with Magnesium Metala
Burning rate (inches/minute)b
Barium nitrate
Potassium nitrate
% Oxidizer
oxidizer,
oxidizer,
(by weight)