(5.2)
external pressure (a nitrogen atmosphere was used) are given t
in Table 5.7.
(cm /sec)
Another matter to consider is whether or not pyrotechnic com-where P = pressure, in atmospheres. Predicted burning rates positions will burn, and at what rate, at very low pressures. For for black powder, calculated using this equation, are given in reactions that use oxygen from the air as an important part of Table 5.6.
their functioning, a substantial drop in performance is expected For "gasless" heat and delay compositions, little external at low pressure. Mixtures high in fuel (such as the magnesium-pressure effect is expected.
This result, plus the increase in
rich illuminating compositions) will not burn well at low pres-burning rate observed with an increase in loading pressure, can sures. Stoichiometric mixtures - in which all the oxygen needed be considered good evidence for the absence of any significant to burn the fuel is provided by the oxidizer - should be the gas-phase involvement in a particular combustion mechanism.
least affected by pressure variations.
116
Chemistry of Pyrotechnics
Ignition and Propagation
117
TABLE 5.7 Burning Rate of a Delay Mixture as a a wide tube. The heat loss to the walls of the container is less Function of External Pressurea
significant for a wide-bore tube, relative to the heat retained by the composition. For each composition, and each loading pres-Composition: Potassium permanganate, KMnO
sure, there will be a minimum diameter capable of producing 4 64%
stable burning. This minimum diameter will decrease as the exo-Antimony, Sb
36%
thermicity of the composition increases.
A metal tube is particularly effective at removing heat from a External pressure,
Burning rateb,
burning composition, and propagation of burning down a narrow p.s.i.
cm /sec
column can be difficult for all but the hottest of mixtures if metal is used for the container material. On the other hand, the use 14.7
. 202
of a metal wire for the center of the popular wire "sparkler" re-30
. 242
tains the heat evolved by the barium nitrate /aluminum reaction and aids in propagating the burning down the length of thin 50
. 267
pyrotechnic coating.
80
. 296
A mixture that burns well in a narrow tube may possibly reach an explosive velocity in a thicker column, so careful ex-100
. 310
periments should be done any time a diameter change is made.
150
. 343
For narrow tubes, one must watch out for possible restriction of the tube by solid reaction products, thereby preventing the 200
. 372
escape of gaseous products. An explosion may result if this 300
. 430
occurs, especially for fast compositions.
500
. 501
External Temperature
800
. 529
Finally, with a knowledge of the Arrhenius rate-temperature re-1100
. 537
lationship, it can be anticipated that burning rate will also de-1400
. 543
pend on the initial temperature of the composition. Considerably more heat input is needed to provide the necessary activation a
energy at - 30°C than is needed when the mixture is initially at Source : Glasby, J.S. , "The Effect of Ambient Pressure on the Velocity of Propagation of Half-Second and
+40°C (or higher). Hence, both ignition and burning rate will Short Delay Compositions," Report No. D.4152, Imperial be affected by variations in external temperature; the effect Chemical Industries, Nobel Division, Ardeer, Scotland.
should be most pronounced for compositions of low exothermicity bCompositions were loaded into 10.5 mm brass tubes at and low flame temperature. For black powder, a 15% slower rate is reported at 0°C versus 100°C, at external pressure of one a loading pressure of 20,000 p.s.i.
atm [1]. Some high explosives show an even greater temperature sensitivity. Nitroglycerine, for example, is 2.9 times faster at 100°C than it is at 0°C [ 1] .
Burning Surface Area
Combustion Temperature
The burning rate - expressed either in grams/second or millimeters/second - will increase as the burning surface area in-A pyrotechnic reaction generates a substantial quantity of heat, creases. Small grains will burn faster than large grains due to and the actual flame temperature reached by these mixtures is their greater surface area per gram. Compositions loaded into an area of study that has been attacked from both the experimental and theoretical directions.
a narrow tube should burn more slowly than the same material in
11 8
Chemistry of Pyrotechnics
Ignition and Propagation
119
Flame temperatures can be measured directly, using special TABLE 5.8 Melting and Boiling Points of Common Non-Gaseous high-temperature optical methods. They can also be calculated Pyrotechnic Productsa
(estimated) using heat of reaction data and thermochemical values for heat of fusion and vaporization, heat capacity, and tran-Boiling point,
sition temperatures.
Calculated values tend to be higher than
Compound
Formula
Melting point, °C
°C
the actual experimental results, due to heat loss to the surroundings as well as the endothermic decomposition of some of the re-Aluminum oxide
A1 2 O 3
2072
2980
action products.
Details regarding these calculations, with several examples, have been published [5].
Barium oxide
BaO
1918
ca. 2000
Considerable heat will be used to melt and to vaporize the re-Boron oxide
B ,O,
450
ca. 1860
action products.
Vaporization of a reaction product is commonly the limiting factor in determining the maximum flame temperature.
Magnesium oxide
MgO
2852
3600
For example, consider a beaker of water at 25°C. As the water is Potassium chloride
KCl
770
1500 (sublimes)
heated, at one atmosphere pressure, the temperature of the liquid rises rather quickly to a value of 100 0 C.
To heat the water over
Potassium oxide
K 2 O
350 (decomposes)
this temperature range, a heat input of approximately 1 calorie Silicon dioxide
Si0 2
1610 (quartz)
2230
per gram per degree rise in temperature is required. To raise 500 grams of water from 25° to 100°C will require Sodium chloride
NaCl
801
1413
Heat required = (grams of water)(heat capacity)(°T change) Sodium oxide
Na 20
1275 (sublimes)
_ (500 grams)(1 cal /deg- gram) (75 deg)
Strontium oxide
SrO
2430
ca. 3000
= 37,500 calories
Titanium dioxide
Ti0 2
1830-1850
2500-3000
(r utile )
Once the water reaches 100°C, however, the temperature increase stops.
The water boils, as liquid is converted to the vapor state, Zirconium dioxide
Zr0 2
ca. 2700
ca. 5000
and 540 calories of heat is needed to convert 1 gram of water from liquid to vapor. To vaporize 500 grams of water, at 100°C, aSource: R. C. Weast (ed.), CRC Handbook of Chemistry and (500 grams)(540 cal/gram) = 270,000 calories Physics, 63rd ed. , CRC Press, Inc. , Boca Raton, Florida, 1982.
of heat is required. Until this quantity of heat is put into the system, and all of the water is vaporized, no further temperature increase will occur. Similar phenomena involving the vaporization of reaction products such as magnesium oxide (MgO) and aluminum oxide (A1 20 3 ) tend to limit the temperature attained in rather than metallic fuel [ 7] . Table 5. 9 illustrates this behavior, pyrotechnic flames.