The boiling points of some common combus-
with data reported by Shimizu [8].
tion products are given in Table 5.8.
This reduction of flame temperature can be minimized somewhat Mixtures using organic (carbon-containing) fuels usually at-by using binders with as high an oxygen content as possible. In tain lower flame temperatures than those compositions consisting such binders, the carbon atoms are already partially oxidized, of an oxidizer and a metallic fuel. This reduction in flame tem-and they will therefore consume less oxygen in going to carbon perature can be attributed to the lower heat output of the or-dioxide during the combustion process.
The balanced chemical
ganic fuels versus metals, as well as to some heat consumption equations for the combustion of hexane (C 6 H 1 ,,) and glucose going towards the decomposition and vaporization of the organic (C 6 H 12O 6 ) illustrate this (both are six-carbon molecules) fuel and its by-products. The presence of even small quantities of organic components can markedly lower the flame temperature, C 6 H 1 ,, + 9.5 0 2 -> 6 CO 2 + 7 H 2 O
as the available oxygen is consumed by the carbonaceous material C 6H 12 0 6 + 6 0,, 6 CO 2 + 6 H 2O
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Chemistry o f Pyrotechnics
Ignition and Propagation
121
TABLE 5.9 Effect of Organic Fuels on Flame Temperature TABLE 5.11
Flame Temperatures for Oxidizer/Shellac
of Magnesium /Oxidizer Mixturesa
Mixtures
Composition:
Oxidizer
55% by weight
Composition:
Oxidizer
75%
Magnesium
Shellac
15%
45% by weight
Shellac
either 0 or 10% additional
Sodium oxalate
10%
Approximate flame temperature, oCb
Approximate flame
Oxidizer
Oxidizer
temperature, oCa
KC1O,,
Ba(NO3)3
Potassium chlorate, KC1O 3
2160
Without shellac
3570
3510
Potassium perchlorate, KC1Oy
2200
With 10% shellac
2550
2550
Ammonium perchlorate, NH,,Cl0 4
2200
Potassium nitrate, KNO 3
1680
aReference 8.
bTemperature was measured 10 mm from the burning surface a Reference 8.
in the center of the flame.
Pyrotechnic flames typically fall in the 2000-3000°C range.
Binary mixtures of oxidizer with metallic fuel yield the highest Table 5. 10 lists approximate values for some common classes of flame temperatures, and the choice of oxidizer does not appear to high-energy reactions [1].
make a substantial difference in the temperature attained. For compositions without metal fuels, this does not seem to be the case.
Shimizu has collected data for a variety of compositions and has observed that potassium nitrate mixtures attain substantially lower flame temperatures than similar mixtures made with TABLE 5. 10 Maximum Flame Temperatures of Various Classes chlorate or perchlorate oxidizers.
This result can be attributed
of Pyrotechnic Mixturesa
to the substantially -endothermic decomposition of KNO 3 relative to the other oxidizers. Table 5.11 presents some of the Shimizu Maximum flame
data [ 8] .
Type of composition
temperature, °C
A final factor that can limit the temperature of pyrotechnic flames is unanticipated high-temperature chemistry. Certain re-Photoflash, illuminating
2500-3500
actions that do not occur to any measurable extent at room tem-Solid rocket fuel
perature become quite probable at higher temperatures. An ex-2000-2900
ample of this is the reaction between carbon (C) and magnesium Colored flame mixtures
1200-2000
oxide (MgO). Carbon can be produced from organic molecules in the flame.
Smoke mixtures
400-1200
C
+ MgO 3 CO + Mg
a Reference 1.
(solid)
(solid)
(gas)
(gas above 1100°C)
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Chemistry of Pyrotechnics
Ignition and Propagation
123
This is a strongly endothermic process, but it becomes possible at high temperature due to a favorable entropy change - formation of the random vapor state from solid reactants. Such reactions provide another reason for the lower flame temperatures achieved when organic binders are added to oxidizer/metal mixtures [3].
Propagation Index
A simple method for assessing the ability of a particular composition to burn is the "Propagation Index," originally proposed by McLain and later modified by Rose [3, 91. The original McLain expression was
PI = ~ Hreaction
T ignition
where PI - the Propagation Index -- is a measure of a mixture's tendency to sustain burning upon initial ignition by external stimulus. The equation contains the two main factors that determine burning ability - the amount of heat released by the chemical reaction (AH) and the ignition temperature of the mixture. If a substantial quantity of heat is released and the ignition temperature is low, then reignition from layer to layer should occur readily and propagation is likely. Conversely, mixtures with low heat output and high ignition temperature should propagate poorly, if at all. Propagation Index values for a variety of compositions are given in Table 5.12.
Rose recommended modifying the original McLain expression by the addition of terms for the pressed density of the composition and for the burning rate of the mixture. He reasoned, especially for delay compositions compressed in a tube, that ability to propagate should increase with increasing density, due to better heat transfer between grains of composition. Burning rate should also be a factor, he argued, because faster-burning mixtures should lose less heat to the surroundings than slower compositions [ 9] .
REFERENCES
1. A. A. Shidlovskiy, Principles of Pyrotechnics, 3rd Ed. , Moscow, 1964. (Translated by Foreign Technology Division, Wright-Patterson Air Force Base, Ohio, 1974.)
124
Chemistry of Pyrotechnics
2.
T. J. Barton, et al. , "Factors Affecting the Ignition Temperature of Pyrotechnics," Proceedings, Eighth International Pyrotechnics Seminar, IIT Research Institute, Steamboat Springs, Colorado, July, 1982, p. 99.
3.
J. H. McLain, Pyrotechnics from the Viewpoint of Solid State Chemistry, The Franklin Institute Press, Philadelphia, Penna., 1980.
4.
H. Ellern, Military and Civilian Pyrotechnics, Chemical Publ. Co., Inc. , New York, 1968.
5.
U.S. Army Material Command, Engineering Design Handbook, Military Pyrotechnic Series, Part One, "Theory and Application," Washington, D.C. , 1967 (AMC Pamphlet 706-185).
6.
H. Henkin and R. McGill, Ind. and Eng. Chem., 44, 1391