M
90
Chemistry of Pyrotechnics
pyrotechnic Principles
91
TABLE 4.4 Effect of Particle Size on the Burning Rate of TABLE 4.5 Effect of Particle Size on Burning Ratea Tungsten Delay Mixturesa
Composition Titanium metal
48 % by weight
Mix A
Mix B
("M 10")
Strontium nitrate
45
("ND 3499")
Linseed oil
4
% Tungsten, W
40
38
Chlorinated rubber
3
% Barium chromate, BaCrO4
51.8
52
Titanium size range,
Relative
Potassium perchlorate, KC1O 4
4.8
4.8
micrometers
burning rate
% Diatomaceous earth
3.4
5.2
less than 6
1.00 (fastest)
Tungsten surface area, cm 2 /gram
1377
709
6-10
0.68
Tungsten average diameter, 10 -6 m
2.3
4.9
10-14
0.63
14-18
0.50
Burning rate of mixture, in/sec
0.24
0.046
greater than 18
0.37 (slowest)
a
Note: Curiously, the system showed the opposite ef-Reference 2.
fect for strontium nitrate. Decreasing the particle size of the oxidizer from 10.5 to 5.6 micrometers produced a 25% decrease in burning rate.
aReference 5.
size is important, but surface area can be even more critical in determining reactivity. Several examples of this phenomenon are presented in Tables 4.4 and 4.5.
4. Conductivity: For a column of pyrotechnic composition of such a metal wall will also be an important consideration.
to burn smoothly, the reaction zone must readily travel If sufficient heat does not pass down the length of the py-down the length of the composition. Heat is transferred rotechnic mixture, burning may not propagate and the de-from layer to layer, raising the adjacent material to the vice will not burn completely. Organic materials, such as ignition temperature of the particular composition. Good cardboard, are widely used to contain low-energy pyrotech-thermal conductivity can be essential for smooth propaga-nic compositions - such as highway fuses and fireworks -
tion of burning, and this is an important role played by to minimize this problem (cardboard is a poor thermal con-metals in many mixtures. Metals are the best thermal ductor).
conductors, with organic compounds ranking among the 6. Loading pressure: There are two general rules to describe worst. Table 2.10 lists the thermal conductivity values the effect of loading pressure on the burning behavior of of some common materials.
a pyrotechnic composition. If the pyrotechnic reaction, in 5. Outside container material : Performance of a pyrotechnic the post-ignition phase, is propagated via hot gases, then I
mixture can be affected to a substantial extent by the type too high of a loading pressure will retard the passage of of material used to contain the mixed composition. If a these hot gases down the column of composition. A lower good thermal conductor, such as a metal, is used, heat rate, in units of grams of composition reacting per second, may be carried away from the composition through the will be observed at high loading pressures. (Note: One wall of the container to the surroundings. The thickness must be cautious in interpreting burn rate data, because
92
Chemistry
Pyrotechnic
93
o f Pyrotechnics
Principles
TABLE 4.6 Effect of Loading Pressure on the
paper tube of one cm inside diameter, had a burning rate Burning Rate of a Delay Mixture
of 4. 6-16.7 meters /second - over 100 times faster [ 4] !
This behavior is typical of loose powders, and points out Composition: Barium chromate, BaCrO
the potential danger of confining mixtures that burn quite 4
90
sluggishly in the open air.
Boron
10
This effect is particularly important when consideration is given to the storage of pyrotechnic compositions. Con-Loading pressure
Burning rate
tainers and storage facilities should be designed to instantly (1000 psi)
(seconds/gram) a
vent in the case of pressure buildup. Such venting can quite effectively prevent many fires from progressing to 36
. 272 (fastest)
explosions.
18
. 276
Two factors contribute to the effect of confinement on burning rate. First, as was discussed in Chapter 2, an 9
. 280
increase in temperature produces an exponential increase 3.6
. 287
in rate of a chemical reaction. In a confined high-energy system, the temperature of the reactants can rise dramat-1.3
. 297
ically upon ignition, as heat is not effectively lost to the 0.5
. 309 (slowest)
surroundings. A sharp rise in reaction rate occurs, liberating more heat, raising the temperature further, accelerating the reaction until an explosion occurs or the Note: This is a "gasless" delay mixture - the reactants are consumed. The minimum quantity of ma-burning rate increases as loading pressure in-terial needed to produce an explosion, under a specified creases. "Gassy" mixtures will show the oppo-set of conditions, is referred to as the critical mass. Also, site behavior.
in a confined system, the hot gases that are produced can aReference 2.
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: an increase in loading pressure usually leads to an increase homogeneity and confinement. An increase in either should lead in the density of the composition. What may appear to be to an increase in burning rate for most high-energy mixtures.
a slower rate, expressed in units of millimeters/second, may Note, however, that "gasless" compositions do not show the dra-actually be a faster rate in terms of grams/second. ) matic confinement effects found for "gassy" compositions.
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 REQUIREMENTS FOR A GOOD HIGH-ENERGY
loading pressure should lead to an increase in burn rate MIXTURE
(in grams per second). An example of this possibility is given in Table 4.6.
The requirements for a commercially-feasible high-energy mixture can be summarized as follows, keeping in mind the preceding dis-7. Degree o f confinement : In Chapter 1, the variation in the burning behavior of black powder was discussed as a func -