will be "low gas" rather than "gasless," due to the carbon dioxide (C0 2), carbon monoxide (CO), and nitrogen (N 2) that will form The purpose of a delay composition is obvious - to provide a time upon combustion of the binder. If a truly "gasless" mixture is re-delay between ignition and the delivery of the main effect. Crude quired, leave out all organic materials!
delays can be made from loose powder, but a compressed column If a fast burning rate is desired, a metallic fuel with high heat is capable of much more reproducible performance. The burning output per gram should be selected, together with an oxidizer of rates of delay mixtures range from very fast (millimeters/millisec low decomposition temperature.
The oxidizer should also have a
ond) to slow (millimeters /second).
small endothermic - or even better, exothermic - heat of de-Black powder was the sole delay mixture available for several composition.
For slower delay mixtures, metals with less heat centuries.
The development and use of "safety fuse" containing output per gram should be selected, and oxidizers with higher
130
Chemistry of Pyrotechnics
Heat and Delay Compositions
131
TABLE 6.4 Typical Delay Compositions a
TABLE 6.5 The Barium Chromate/Boron System -
Effect of % Boron on Burning Timea
by
Burning rate,
Component
Formula weight
cm /second
Average burning time
Heat of reaction
% B
seconds /gram
cal /gram
1.
Red lead oxide
Pb 30,,
85
1.7 (10.6 ml/g of gas)
Silicon
Si
15
3
3.55
278
Nitrocellulose /
1.8
5
. 51
420
acetone
7
. 33
453
10
. 24
515
2.
Barium chromate
BaCr0 4
90
5.1 (3.1 ml/g of gas)
13
.
Boron
B
10
21
556
15
. 20
551
3.
Barium chromate
BaCr0 4
40
- (4.3 ml/g of gas)
17
. 21
543
Potassium
KC1O,,
10
21
. 22
526
perchlorate
25
. 27
497
Tungsten
W
50
30
. 36
473
4.
Lead chromate
PbCr0
35
. 64
446
4
37
0.30 (18.3 ml /g of gas)
40
Barium chromate
BaCrO.
30
1.53
399
45
Manganese
Mn
33
3.86
364
5.
Barium chromate
BaCrO,,
80
0.16 (0.7 ml /g of gas)
Zirconium-nickel
Zr-Ni
17
a Reference 4.
alloy (50/50)
Potassium
KC1O,,
3
perchlorate
the relative burning rates of various delay candidates. For high aReference 1.
reactivity, look for low melting point, exothermic or small endothermic heat of decomposition (in the oxidizer), and high heat of combustion (in the fuel).
The ratio of oxidizer to fuel can be altered for a given binary mixture to achieve substantial changes in the rate of burning.
decomposition temperatures and more endothermic heats of decom-The fastest burning rate should correspond to an oxidizer/fuel position should be chosen. By varying the oxidizer and fuel, it ratio near the stoichiometric point, with neither component pres-is possible to create delay compositions with a wide range of burn-ent in substantial excess.
Data have been published for the
ing rates.
Table 6.4 lists some representative delay mixtures.
barium chromate /boron system. Table 6. 5 gives the burn time Using this approach, lead chromate (melting point 844°C) and heat output per gram for this system [4].
would be expected to produce faster burning mixtures than barium McLain has proposed that the maximum in performance cen-chromate (higher melting point), and barium peroxide (melting tered at approximately 15% boron by weight indicates that the point 450°C) should react more quickly than iron oxide (Fe principal pyrotechnic reaction for the BaCrO„/B system is 20 3 ,
melting point 1565°C). Similarly, boron (heat of combustion =
4B +BaCrO,,- 4BO+Ba+Cr
14.0 kcal/gram) and aluminum (7.4 kcal/gram) should form quicker delay compositions than tungsten (1.1 kcal/gram) or iron (1.8
Although B 20 3 is the expected oxidation product from boron in kcal/gram).
Tables 3.2, 3.4, and 3.5 can be used to estimate a room temperature situation, the lower oxide, BO, appears to
132
Chemistry of Pyrotechnics
Heat and Delay Compositions 133
TABLE 6.6 A Ternary Delay Mixture - The PbCrO 4 /BaCrO 4 /
TABLE 6.7 The BaCrO4/KCIO 4 /Mo System a
Mn Systema
% Barium
% Potassium
% Molyb-
% Manganese,
% Lead
°% Barium
Burning rate,
chromate,
perchlorate,
denum,
Burning rate,
Mixture
Mn
chromate