% Magnesium
Ba(NO 3) 2
KNO 3
80
20
2.9
2.3
70
30
-
4.7
68
32
5.1
60
40
10.7
-
58
42
8.5
50
50
16.8
13.3
40
60
38.1
21.8
30
70
40.3
29.3
20
80
"Erratic"
26.4
aReference 2.
FIG. 4.1 Light output from a green flare. The radiant output bLoading pressure was 10,000 psi into 1.4 in' cases.
from a burning pyrotechnic composition can be analyzed using an instrument known as a spectrophotometer. Energy output can be monitored as a function of wavelength. A good "white light" mixture will emit reasonably intense light over the entire visible region. Color will be produced when the emission species present in the pyrotechnic flame. Only those wave-is concentrated in a narrow portion of the visible range. The lengths falling in the "visible" region of the electromag-output from this flare falls largely between 500-540 nm -- the netic spectrum will contribute to the color. An emission
"green" portion of the visible spectrum. Green light emission spectrum, showing the intensity of light emitted at each is usually associated with the presence of a barium compound wavelength, can be obtained if the proper instrumenta -
in the mixture, with molecular BaCI in the vapor state, typi-tion - an emission spectrometer - is available (Figure 4.1).
cally the primary emitter of green light. The mixture pro-5. Volume o f gas produced: Gaseous products are frequently ducing this emission pattern consisted of potassium perchlor-desirable when a high-energy mixture is ignited. Gas can ate (32.5%), barium nitrate (22.5%), magnesium (21%), copper be used to eject sparks, disperse smoke particles, and pro-powder (7%), polyvinyl chloride (12%), and 5% binder. Source vide propellant behavior; when confined, gas can be used H. A. Webster III, "Visible Spectra of Standard Navy Colored to create an explosion. Water, carbon monoxide and di-Flares," Proceedings, Pyrotechnics and Explosives Applications oxide, and nitrogen are the main gases evolved from high-Section, American Defense Preparedness Association, Fort energy mixtures. The presence of organic compounds can Worth, Texas, September, 1983.
88
Chemistry of Pyrotechnics
Pyrotechnic Principles
89
generally be counted upon to produce significant amounts TABLE 4. 3 Effect of Particle Size on Performance of a of gas. Organic binders and sulfur should be avoided if Flare Compositiona
a "gasless" composition is desired.
6. Efficiency: For a particular composition to be of practi-Average particle
cal interest, it must produce a significant amount of pyro-Composition :
Component
% by weight
size
technic effect per gram of mixture. Efficiency per unit volume is also an important consideration when available Magnesium metal
48
see table below
space is limited.
Sodium nitrate,
42
34 micrometers
7. Ignitibility : A pyrotechnic composition must be capable NaNO
of undergoing reliable ignition, and yet be stable in 3
(10 -6 meters)
transportation and storage. The ignition behavior of Laminae binder
8
every mixture must be studied, and the proper ignition Polyvinyl chloride
2
27 micrometers
system can then be specified for each. For easily-ignited materials, the "spit" from a burning black powder fuse is Magnesium average
Flare burning
often sufficient. Another common igniter is a "squib" or particle size,
Flare candlepower
rate, inches/
electric match, consisting of a metal wire coated with a micrometers
(1,000 candles)
minute
small dab of heat-sensitive composition. An electric current is passed through the wire, producing sufficient heat 437
130
2.62
to ignite the squib. The burst of flame then ignites the main charge. For pyrotechnic mixtures with high ignition 322
154
3.01
temperatures, a primer or first fire is often used. This 168
293
5.66
is an easily-ignited composition that can be activated by a fuse or squib. The flame and hot residue produced are 110
285
5.84
used to ignite the principal material. This topic will be treated in more detail in Chapter 5.
aReference 2.
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.
size, the more reactive a particular composition should be, Substantial deviations can result from variations in any of the with all other factors held constant. Table 4. 3 illustrates following [2]:
this principle for a sodium nitrate /magnesium flare composition. Note the similarity in performance for the two 1. Moisture: The best rule is to avoid the use of water in smallest particle sizes, suggesting that an upper per-processing pyrotechnic compositions, and to avoid the use formance limit may exist.
of all hygroscopic (water-attracting) ingredients. If wa-3. Surface area of the reactants: For a high-energy reaction ter is used to aid in binding and granulating, an efficient to rapidly proceed, the oxidizer must be in intimate contact drying procedure must be included in the manufacturing with the fuel. Decreasing particle size will increase this process. The final product should be analyzed for mois-contact, as will increasing the available surface area of the ture content, if reproducible burning behavior is critical.
particles. A smooth sphere will possess the minimum sur-2. Particle size of ingredients: Homogeneity, and pyrotech-face area for a given mass of material. An uneven, porous nic performance, will increase as the particle size of the particle will exhibit much more free surface, and conse-various components is decreased. The finer the particle quently will be a much more reactive material. Particle