BaCrO 4
90
560 (29.5 ml per
oxidizers. The low temperatures shown for potassium chlorate B
10
gram of evolved
and potassium nitrate may well account for the large number of Vinyl alcohol/
1 (additional %)
gas)
mysterious, accidental ignitions that have occurred with compo-acetate resin
sitions containing these materials.
Ease of initiation also depends upon the particle size and sur-IIIA. NaNO 3
50
772b (50 mg sam-
face area of the ingredients. This factor is especially important Ti
50
ple, heated at
for the metallic fuels with melting point higher than or comparable 50°C/min.)
to that of the oxidizer. Some metals - including aluminum, magnesium, titanium, and zirconium - can be quite hazardous when IIIB. NaNO 3
50
357
present in fine particle size (in the 1-5 micrometer range). ParTi
50
ticles this fine may spontaneously ignite in air, and are quite Boiled linseed
6 (additional %)
sensitive to static discharge [4]. For safety reasons, reactiv-oil
ity is sacrificed to some extent when metal powders are part of a mixture, and larger particle sizes are used to minimize accidental ignition.
aReference 10.
Several examples will be given to illustrate these principles.
bReference 2.
In the potassium nitrate/sulfur system, the liquid state initially appears during heating with the melting of sulfur at 119°C. Sulfur occurs in nature as an 8-member ring - the S a molecule. This ring begins to fragment into species such as S 3 at temperatures above 140°C. However, even with these fragments present, re-Once sufficient heat is generated to begin decomposing the ox-action between sulfur and the solid KNO
idizer, the higher-temperature combustion reaction begins, in-3 does not occur at a
volving free oxygen gas and very rapid rates. We are concerned rate sufficient to produce ignition until the KNO 3 melts at 334°C.
Intimate mixing can occur when both species are in the liquid here with the processes that initiate the ignition process.
Professor G. Tammann, one of the pioneers of solid-state chem-state, and ignition is observed just above the KNO 3 melting point.
istry, considered the importance of lattice motion to reactivity, Although some reaction presumably occurs between sulfur and
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Chemistry of Pyrotechnics
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103
TABLE 5.2 Tammann Temperatures of the Common Oxidizers Melting
Melting
Tammann
point,
point,
temperature,
Oxidizer
Formula
°C
°K
°C
Sodium nitrate
NaN0 3
307
580
17
Potassium nitrate
KNO 3
334
607
31
Potassium chlorate
KC1O 3
356
629
42
Strontium nitrate
Sr(NO 3) 2
570
843
149
Barium nitrate
Ba(N0 3 ) 2
592
865
160
Potassium perchlorate
KC10 4
610
883
168
Lead chromate
PbCr0 4
844
1117
286
Iron oxide
Fe 2 0 3
1565
1838
646
FIG. 5.2 Thermogram of pure potassium nitrate. Endotherms are observed near 130° and 334°C. These peaks correspond to a solid KNO 3 below the melting point, the low heat output obtained rhombic-to-trigonal crystalline phase transition and melting. Note from the oxidation of sulfur combined with the endothermic de-the sharpness of the melting point endotherm near 334°C. Pure composition of KNO B prevent ignition from taking place until the compounds will normally melt over a very narrow range. Impure entire system is liquid.
Only then is the reaction rate great
compounds will have a broad melting point endotherm.
enough to produce a self-propagating reaction. Figures 5.2-5.4
show the thermograms of the components and the mixture. Note the strong exotherm corresponding to ignition for the KNO 3 /S
mixture.
In the potassium chlorate /sulfur system, a different result is generating oxygen to react with additional sulfur.
More heat is
observed. Sulfur again melts at 119°C and begins to fragment generated and an Arrhenius-type rate acceleration occurs, lead-above 140°C, but a strong exotherm corresponding to ignition of ing to ignition well below the melting point of the oxidizer. This the composition is found well below 200°C! Potassium chlorate combination of low Tammann temperature and exothermic decomposition helps account for the dangerous and unpredictable na-has a melting point of 356 11 C, so ignition is taking place well below the melting point of the oxidizer. We recall, though, that ture of potassium chlorate. Figures 5.5-5.6 show the thermal KC1O
behavior of the KC1O 3 /S system.
3 has a Tammann temperature of 42 1 C.
A mobile species --
such as liquid, fragmented sulfur - can penetrate the lattice As we proceed to higher-melting fuels and oxidizers, we see well below the melting point and be in position to react. We also a corresponding increase in the ignition temperatures of two-component mixtures containing these materials. The lowest ig-recall that the thermal decomposition of KC1O 3 is exothermic (10.6
nition temperatures are associated with combinations of low-melt-kcal of heat is evolved per mole of oxidizer that decomposes). A compounding of heat evolution is obtained -- heat is released by ing fuels and low-melting oxidizers, while high-melting combinations generally display higher ignition points. Table 5.3 gives the KC1O 3 /S reaction and by the decomposition of additional KC1O 3
some examples of this principle.
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Chemistry
Ignition
of Pyrotechnics
and Propagation
105
100
200
300
400
500
REFERENCE TEMPERATURE, "C
FIG. 5.4 The potassium nitrate /sulfur /aluminum system. Endo-FIG. 5.3 A sulfur thermogram. Endotherms for a rhombic-to-therms for sulfur can be seen near 105° and 119 1C, followed by monoclinic crystalline phase transition and melting are seen at the potassium nitrate phase transition near 130 1C. As the melt-105° and 119°C, respectively. An additional endotherm is ob-ing point of potassium nitrate is approached (334 1 C), an exo-served near 180°. This peak corresponds to the fragmentation therm is observed. A reaction has occurred between the oxidizer of liquid S
and fuel, and ignition of the mixture evolves a substantial amount 8 molecules into smaller units. Finally, vaporization is observed near 450°C.
of heat.
of components, degree of mixing, loading pressure (if any), heat -
ingrate, and quantity of sample can all influence the observed Table 5. 3 shows that several potassium nitrate mixtures with ignition temperature.
low-melting fuels have ignition temperatures near the 334°C melt-The traditional method for measuring ignition temperatures, ing point of the oxidizer. Mixtures of KNO 3 with higher-melting used extensively by Henkin and McGill in their classic studies metal fuels show substantially higher ignition temperatures.
of the ignition of explosives [6] , consists of placing small quan-Table 5. 4 shows that a variety of magnesium-containing compo-tities (3 or 25 milligrams, depending on whether the material sitions have ignition temperatures close to the 649°C melting detonates or deflagrates) of composition in a constant-tempera-point of the metal.
ture bath and measuring the time required for ignition to occur.
A problem with trying to develop logical theory using litera-Ignition temperature is defined, using this technique, as the ture values of ignition temperatures is the substantial variation temperature at which ignition will occur within five seconds.