particles. A smooth sphere will possess the minimum sur-2. Particle size of ingredients: Homogeneity, and pyrotech-face area for a given mass of material. An uneven, porous nic performance, will increase as the particle size of the particle will exhibit much more free surface, and conse-various components is decreased. The finer the particle quently will be a much more reactive material. Particle
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 -
cussions of materials and factors that affect performance tion of the degree of confinement. Increased confinement leads to accelerated burning. Shimizu reports a burning 1. The composition produces the desired effect and is efficient rate in air of .03-.05 meters/second for black powder paste both in terms of effect /gram and effect /dollar.
impregnated in twine. The same material, enclosed in a
9 4
Chemistry of Pyrotechnics
Pyrotechnic Principles
95
2. The composition can easily and safely be manufactured, large quantities of bulk powder are present in one location, and handled, transported, stored, and used, assuming nor-if accidental ignition should occur, there is a good chance that mal treatment and the expected variations in temperature.
an explosive reaction rate may be reached.
3. Storage lifetime is acceptable, even in humid conditions, For this reason, mixing and drying operations should be iso-and there is reasonably low toxicity associated with both lated from all other plant processes, and remote control equip-the starting materials and reaction products.
ment should be used wherever and whenever possible. All high-energy manufacturing facilities should be designed with the idea in mind that an accident will occur at some time during the life These requirements seem rather simple, but they do restrict of the facility. The plant should be designed to minimize any or eliminate a number of potential starting materials. These com-damage to the facility, to the neighborhood, and most impor-pounds must either be deleted from our "acceptable" list or spe-tantly, to the operating personnel.
cial precautions must be taken in order to use them. Examples The manufacturing operation can be divided into several include
stages
1. Preparation of the individual components: Materials to be Potassium dichromate (K 2Cr2O ): This is a strong oxidizer, used in the manufacturing process may have to be dried, as well 7
but it only contains 16% oxygen by weight. It has a cor-as ground or crushed to achieve the proper particle size, or rosive effect on the mucous membranes, and its toxicity screened to separate out large particles or foreign objects. Ox-and suspected carcinogenicity suggest the use of alternate idizers should never be processed with the same equipment used oxidizers.
for fuels, nor should oxidizers and fuels be stored in the same Ammonium perchlorate (NH,,ClO,,): This is a good oxidizer, and area prior to use. All materials must be clearly labeled at all can be used to make excellent propellants and colored times.
flames. However, it is a self-contained oxidizer-fuel sys-2. Preparation o f compositions:
This step is the key to
tem (much like ammonium nitrate). The mixing of NH +
proper performance. The more homogeneous a mixture is, the 4
(fuel) and C1O -
greater its reactivity will be. The high-energy chemist is al-a
(oxidizer) occurs at the ionic level. The
potential for an explosion cannot be ignored. Conclusion: ways walking a narrow line in this area, however. By maxi-if this material is used, it must be treated with respect mizing reactivity - with small particle sizes and intimate mix-and minimum quantities of bulk powder should be pre-ing - you are also increasing the chance of accidental ignition pared.
during manufacturing and storage. A compromise is usually Magnesium metal ( Mg) : This is an excellent fuel and produces reached, obtaining a material that performs satisfactorily but is brilliant illuminating mixtures. The metal is water-reactive reasonably safe to work with. This compromise is reached by however, suggesting short shelf-life and possible sponta-careful specification of particle size, purity of starting materi-neous ignition if magnesium-containing mixtures become als, and safe operating procedures.
wet. Conclusion: replace magnesium with the more stable A variety of methods can be used for mixing. Materials can aluminum (or possibly titanium) metals. If magnesium gives be blended through wire screens, using brushes. Hand-screen-the best effect, coat the metal with an organic, water-re-ing is still used in the fireworks industry, but should never be pelling material.
used with explosive or unstable mixtures. Brushes provide a safer method of screening the oxidizer and fuel together. Materials can also be tumbled together to achieve homogeneity, and PREPARATION OF HIGH-ENERGY MIXTURES
this can (and should) be done remotely. Remote mixing is strongly recommended for sensitive explosive compositions such as the The most hazardous operations in the high-energy chemistry field
"flash and sound" powder used in firecrackers and salutes and involve the mixing of oxidizer and fuel in large quantities, and the the photoflash powders used by the military.
subsequent drying of the composition (if water or other liquid is 3. Granulation : Following mixing, the powders are often used in the mixing and granulating processes). In these operations, granulated, generally using a small percentage of binder to aid
9 6
Chemistry of Pyrotechnics
in the process. The composition is treated with water or an organic liquid (such as alcohol), and then worked through a large-mesh screen. Grains of well-mixed composition are produced which will retain the homogeneity of the composition better than loose powder.
Without the granulation step, light and dense materials might segregate during transportation and storage. The granulated material is dried in a remote, isolated area, and is then ready to be loaded into finished items. Remember: Sizable quantities of bulk powder are present at this stage, and the material must be protected from heat, friction, shock, and static spark.
4.
Loading:
An operator, working with the minimum quantity of bulk powder, loads the composition into tubes or other containers, or produces pellets for later use in finished items. The making of "stars" - small pieces of color-producing composition used in aerial fireworks - is an example of this pelleting operation.
5.
Testing:
An important final step in the manufacturing process is the continual testing of each lot of finished items to ensure proper performance. Significant differences in performance can be obtained by slight variation in the particle size or purity of any of the starting materials, and a regular testing program is the only way to be certain that proper performance is being achieved.
REFERENCES
1.
T. L. Davis, The Chemistry of Powder and Explosives, John Wiley & Sons, Inc. , New York, 1941.
2.
U.S. Army Material Command, Engineering Design Handbook, Military Pyrotechnic Series, Part One, "Theory and Application," Washington, D.C. , 1967 (AMC Pamphlet 706-185).
3.
A. A. Shidlovskiy, Principles of Pyrotechnics, 3rd Ed. , A magnesium-containing flare burns with a brilliant white flame in Moscow, 1964. (Translated by Foreign Technology Division, the test tunnel of the Applied Sciences Department, Naval Weapons Wright-Patterson Air Force Base, Ohio, 1974.) Support Center, Crane, Indiana. Special instrumentation can mea-4.
T. Shimizu, Fireworks from a Physical Standpoint, Part One, sure the intensity of the light output as a function of wavelength.
( trans. by A. Schuman), Pyrotechnica Publications, Austin,
"White light" compositions emit throughout the visible region of the Texas, 1982.
electromagnetic spectrum (380-780 nanometers) , with emission ex-5.
B. J. Thomson and A. M. Wild, "Factors Affecting the Rate tending into the infrared and ultraviolet regions.
Researchers at
of Burning of a Titantium - Strontium Nitrate Based Compo-the Crane facility have performed extensive research on the theory sition," Proceeding of Pyrochem International 1975, Pyro-and performance of illuminating flares, especially the sodium nitrate/
technics Branch, Royal Armament Research and Development magnesium system. (NWSC, Crane, Indiana)
Establishment, United Kingdom, July, 1975.
5IGNITION AND PROPAGATION
I GNITION PRINCIPLES
Successful performance of a high-energy mixture depends upon; 1. The ability to ignite the material using an external stimulus, as well as the stability of the composition in the absence of the stimulus.
2. The ability of the mixture, once ignited, to sustain burning through the remainder of the composition.
Therefore, a composition is required that will readily ignite and burn, producing the desired effect upon demand, while remaining quite stable during manufacture and storage. This is not an easy requirement to meet, and is one of the main reasons why a relatively small number of materials are used in high-energy mixtures.
For ignition to occur, a portion of the mixture must be heated to its ignition temperature, which is defined as the minimum temperature required for the initiation of a self-propagating reaction.
Upon ignition, the reaction then proceeds on its own, in the absence of any additional energy input.
Application of the ignition stimulus (such as a spark or flame) initiates a complex sequence of events in the composition. The solid components may undergo crystalline phase transitions, melting, boiling, and decomposition. Liquid and vapor phases may be formed, and a chemical reaction will eventually occur at the surface 97
98
Chemistry o f Pyrotechnics
Ignition and Propagation
99
where the energy input is applied, if the necessary activation energy has been provided.
The heat released by the occurrence of the high-energy reaction raises the temperature of the next layer of composition.
If the heat evolution and thermal conductivity are sufficient to supply the required activation energy to this next layer, further reaction will occur, liberating additional heat and propagation of the reaction down the length of the column of mixture takes place. The rates, and quantity, of heat transfer to, heat production in, and heat loss from the high-energy composition are all critical factors in achieving propagation of burning and a self-sustaining chemical reaction.
The combustion process itself is quite complex, involving high temperatures and a variety of short-lived, high-energy FIG. 5.1 Burning pyrotechnic composition. Several major regions chemical species. The solid, liquid, and vapor states may all are present in a reacting pyrotechnic composition. The actual be present in the actual flame, as well as in the region imme-self-propagating exothermic process is occurring in the reaction diately adjacent to it. Products will be formed as the reaction zone. High temperature, flame and smoke production, and the proceeds, and they will either escape as gaseous species or ac-likely presence of gaseous and liquid materials characterize this cumulate as solids in the reaction zone (Figure 5.1).
region. Behind the advancing reaction zone are solid products A moving, high-temperature reaction zone, progressing formed during the reaction (unless all products were gaseous).
through the composition, is characteristic of a combustion (or Immediately ahead of the reaction zone is the next layer of com-
"burning") reaction. This zone separates unreacted starting position that will undergo reaction. This layer is being heated by material from the reaction products. In "normal" chemical re-the approaching reaction, and melting, solid-solid phase transi-actions, such as those carried out in a flask or beaker, the entions, and low-velocity pre-ignition reactions may be occurring.
tire system is at the same temperature and molecules react ran-The thermal conductivity of the composition is quite important in domly throughout the container. Combustion is distinguished transferring heat from the reaction zone to the adjacent, unre-from detonation by the absence of a pressure differential be-acted material. Hot gases as well as hot solid and liquid par-tween the region undergoing reaction and the remainder of the ticles aid in the propagation of burning.
unreacted composition [1].
A variety of factors affect the ignition temperature and the burning rate of a high-energy mixture, and the chemist has the ability to alter most of these factors to achieve a desired change in performance.
The oxidizers used in high-energy mixtures are generally ionic One requirement for ignition appears to be the need for solids, and the "looseness" of the ionic lattice is quite important either the oxidizer or fuel to be in the liquid (or vapor) state, in determining their reactivity [3]. A crystalline lattice has some and reactivity becomes even more certain when both are liq-vibrational motion at normal room temperature, and the amplitude uids. The presence of a low-melting fuel can substantially lower of this vibration increases as the temperature of the solid is raised.
the ignition temperature of many compositions [2]. Sulfur and At the melting point, the forces holding the crystalline solid to-organic compounds have been employed as "tinders" in high-gether collapse, producing the randomly-oriented liquid state.
energy mixtures to facilitate ignition. Sulfur melts at 119°C, For reaction to occur in a high-energy system, the fuel and oxy-while most sugars, gums, starches, and other organic polymers gen-rich oxidizer anion must become intimately mixed, on the ionic have melting points or decomposition temperatures of 300°C or or molecular level. Liquid fuel can diffuse into the solid oxidizer less (Table 5.1).
lattice if the vibrational amplitude in the crystal is sufficient.
100
Chemistry o f Pyrotechnics
Ignition and Propagation
101
Organic Fuels on Ignition
I
TABLE 5. 1 Effect of Sulfur and
and used the ratio of the actual temperature of a solid divided by Temperature
the melting point of the solid (on the Kelvin or "absolute" scale) to quantify this concept.
Ignition tempera-
a = T(solid) /T (melting point) (in K)
(5.1)
Composition
(% by weight)
ture, °C
Tammann proposed that diffusion of a mobile species into a IA.
KC1O,,
66.7
446a
crystalline lattice should be "significant" at an a-value of 0.5
Al
33.3
(or halfway to the melting point, on the Kelvin scale). At this temperature, later termed the Tammann temperature, a solid has IB.
KC1O,,
64
360
approximately 70% of the vibrational freedom present at the melt-Al
22.5
ing point, and diffusion into the lattice becomes probable [3].
S
10
If this is the approximate temperature where diffusion becomes SbZS 3
3.5
probable, it is therefore also the temperature where a chemical reaction between a good oxidizer and a mobile, reactive fuel be-IIA.
BaCrO
comes possible. This is a very important point from a safety 4
90
615a (3.1 ml per
B
10
gram of evolved
standpoint - the potential for a reaction may exist at surprisingly gas)
low temperatures, especially with sulfur or organic fuels present.
Table 5.2 lists the Tammann temperatures of some of the common IIB.
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
102
Chemistry of Pyrotechnics
Ignition and Propagation
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.
104
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.
in observed values that can occur depending upon the experi-Data obtained in this type of study can be plotted to yield inmental conditions employed to measure the ignition points. Ratio teresting information, as shown in Figure 5.7.
106
Chemistry of Pyrotechnics
Ignition and Propagation
107
100
200
300
400
500
REFERENCE TEMPERATURE, °C
FIG. 5.5 Thermogram of pure potassium chlorate, KCIO
FIG. 5.6 The potassium chlorate/sulfur system. Sulfur endo-3 . No
thermal events are observed prior to the melting point (356°C).
therms are seen near 105° and 119°C, as expected. A violent Exothermic decomposition occurs above the melting point as oxy-exothermic reaction is observed below 150 1C. The ignition tem-gen gas is liberated.
perature is approximately 200 degrees below the melting point of the oxidizer (KC1O 3 m.p. = 356°C). Ignition occurs near the temperature at which S 3 molecules fragment into smaller units.
Data from time versus temperature studies can also be plotted as log time vs. 1/T, yielding straight lines as predicted by the Arrhenius Equation (eq. 2.4). Figure 5.8 illustrates this con-Ignition temperatures can also be determined by differential cept, using the same data plotted in Figure 5.7. Activation en-thermal analysis (DTA), and these values usually correspond well ergies can be obtained from such plots. Deviations from linear to those obtained by a Henkin-McGill study. Differences in heat-behavior and abrupt changes in slope are sometimes observed in ing rate can cause some variation in values obtained with this Arrhenius plots due to changes in reaction mechanism or other technique. For any direct comparison of ignition temperatures, complex factors.
it is best to run all of the mixtures of interest under identical
"Henkin-McGill" plots can be quite useful in the study of ig-experimental conditions, thereby minimizing the number of vari-nition, providing us with important data on temperatures at which ables.
spontaneous ignition will occur. These data can be especially use-One must also keep in mind that these experiments are mea-ful in estimating maximum storage temperatures for high-energy suring the temperature sensitivity of a particular composition, compositions - the temperature should be one corresponding to in which the entire sample is heated to the experimental tempera-infinite time to ignition (below the "spontaneous ignition temperature. Ignition sensitivity can also be discussed in terms of the ture," minimum - S.I.T (min) - shown in Figure 5.7). At any relative ease of ignition due to other types of potential stimuli, temperature above this point, ignition during storage is possible.
including static spark, impact, friction, and flame.
108
Chemistry of Pyrotechnics
Ignition and Propagation
109
TABLE 5.3 Ignition Temperatures of Pyrotechnic Mixtures TABLE 5.4 Ignition Temperatures of Magnesium-Containing Mixturesa
Ignition
temperature,
Ignition
Component a
Melting point, oC
oC
temperature,
Oxidizer
o C b
I.
KC1O 3
356
150
S
119
NaNO 3
635
II.
KC10 3
356
195b
Ba(N0 3 ) 2
615
Lactose
202
Sr(N0 3 )2
610
III.
KC1O 3
356
540b
Mg
649
KNO 3
650
IV.
KNO
KC10 4
715
3
334
390b
Lactose
202
Note:
All mixtures contain 50% magnesium by weight.
V.
KNO 3
334
340
aReference 5.
S
119
bLoading pressure was 10,000 psi.
VI.
KNO 3
334
565b
Mg
649
VII.
BaCr0 4 (90)
Decomposition at
685c
high temperatures
B (10)
liberate sufficient energy, in a sulfur mix, to generate a self-2300
propagating process. A greater quantity of material must react at once to produce ignition.
aMixtures were in stoichiometric proportions unless other-Another important factor is the thermal stability and heat of wise indicated.
decomposition of the oxidizer. Potassium chlorate mixtures tend bReference 1.
to be much more sensitive to ignition than potassium nitrate com-CReference 4.
positions, due to the exothermic nature of the decomposition of KC1O 3 . Mixtures containing very stable oxidizers - such as ferric oxide (Fe 2O 3 ) and lead chromate (PbCr0 4) - can be quite difficult to ignite, and a more-sensitive composition frequently has to be used in conjunction with these materials to effect ig-SENSITIVITY
nition.
A mixture of a good fuel (e.g., Mg) with an easily-decomposed Sensitivity of a high-energy mixture to an ignition stimulus is in-oxidizer (e.g., KC1O 3 ) should be quite sensitive to a variety of fluenced by a number of factors. The heat output of the fuel is ignition stimuli.
A composition with a poor fuel and a stable ox-quite important, with sensitivity generally increasing as the fuel's idizer should be much less sensitive, if it can be ignited at all!
heat of combustion increases. Mixes containing magnesium or alu-Ignition temperature, as determined by DTA or a Henkin-McGill minum metal, or charcoal, can be quite sensitive to static spark study, is but one measure of sensitivity, and there is not any or a fire flash, while mixes containing sulfur as the lone fuel are simple correlation between ignition temperature and static spark usually less sensitive, due to the low heat output of sulfur. Ig-or friction sensitivity.
Some mixtures with reasonably high ig-
nition of a small quantity of material by static energy does not nition temperatures (KC1O 4 and Al is a good example) can be
110
Chemistry o f Pyrotechnics
Ignition and Propagation
111
1 50
200
250
300
TEMPERATURE, °C
FIG. 5.7
Time to explosion versus temperature for nitrocellulose. As the temperature of the heating bath is raised, the time to explosion decreases exponentially, approaching an instantaneous value. The extrapolated temperature value corres-FIG. 5.8
"Henkin-McGill Plot" for nitrocellulose. The natural ponding to infinite time to explosion is called the spontaneous logarithm of the time to ignition is plotted versus the reciprocal ignition temperature, minimum (S.I.T. min). Source of the of the absolute temperature (°K). A straight line is produced, data: reference 6.
and activation energies can be calculated from the slope of the line. The break in the plot near 2.1 may result from a change in the reaction mechanism at that temperature. Source: reference 6.
quite spark sensitive, because the reaction is highly exothermic and becomes self-propagating once a small portion is ignited. Sensitivity and output are not necessarily related and PROPAGATION OF BURNING
are determined by different sets of factors. A given mixture can have high sensitivity and low output, low sensitivity and Factors
high output, etc. Those mixtures that have both high sensi-The ignition process initiates a self-propagating, high-tempera-tivity and substantial output are the ones that must be treated ture chemical reaction at the surface of the mixture. The rate with the greatest care. Potassium chlorate/sulfur/aluminum at which the reaction then proceeds through the remainder of
"flash and sound" mixture is an example of this type of danger-the composition will depend on the nature of the oxidizer ous composition.
and fuel, as well as on a variety of other factors. "Rate"
11 2
Chemistry o f Pyrotechnics
Ignition and Propagation
113
can be expressed in two ways - mass reacting per unit time or The fuel also plays an important role in determining the rate length burned per unit time. The loading pressure used, and of combustion. Metal fuels, with their highly exothermic heats the resulting density of the composition, will determine the re-of combustion, tend to increase the rate of burning. The pres-lationship between these two rate expressions.
ence of low-melting, volatile fuels (sulfur, for example) tends to Reaction velocity is primarily determined by the selection of retard the burning rate. Heat is used up in melting and vapor-the oxidizer and fuel. The rate-determining step in many high-izing these materials rather than going into raising the tempera-energy reactions appears to be an endothermic process, with de-ture of the adjacent layers of unreacted mixture and thereby ac-composition of the oxidizer frequently the key step. The higher celerating the reaction rate. The presence of moisture can greatly the decomposition temperature of the oxidizer, and the more en-retard the burning rate by absorbing substantial quantities of dothermic the decomposition, the slower the burning rate will be heat through vaporization. The heat of vaporization of water -
(with all other factors held constant).
540 calories/gram at 100°C - is one of the largest values found Shimizu reports the following reactivity sequence for the most-for liquids. Benzene, C 6H 6 , as an example, has a heat of vapor-common of the fireworks oxidizers [8]
ization of only 94 calories/gram at its boiling point, 80°C.
KC1O
The higher the ignition temperature of a fuel, the slower is 3 > NH,,C1O,, > KC1O q > KNO 3
the burning rate of compositions containing the material, again Shimizu notes that potassium nitrate is not slow when used in with all other factors equal. Shidlovskiy notes that aluminum black powder and metal-containing compositions in which a "hot"
compositions are slower burning than corresponding magnesium fuel is present. Sodium nitrate is quite similar to potassium ni-mixtures due to this phenomenon [1] .
trate in reactivity.
The transfer of heat from the burning zone to the adjacent Shidlovskiy has gathered data on burning rates for some of layers of unreacted composition is also critical to the combustion the common oxidizers [1]. Table 5.5 contains data for oxidizers process. Metal fuels aid greatly here, due to their high thermal with a variety of fuels. Again, note the high reactivity of potas-conductivity. For binary mixtures of oxidizer and fuel, combus-sium chlorate.
tion rate increases as the metal percentage increases, well past the stoichiometric point. For magnesium mixtures, this effect is observed up to 60-70% magnesium by weight. This behavior reTABLE 5.5 Burning Rates of Stoichiometric Binary sults from the increasing thermal conductivity of the composition Mixturesa
with increasing metal percentage, and from the reaction of excess magnesium, vaporized by the heat evolved from the pyrotechnic Linear burning rate, mm/secb
process, with oxygen from the atmosphere [1].
Stoichiometric mixtures or those with an excess of a metallic Oxidizer
fuel are typically the fastest burners. Sometimes it is difficult Fuel
KC1O 3
KNO 3
NaNO 3
Ba(NO3)2
to predict exactly what the stoichiometric reaction(s) will be at the high reaction temperatures encountered with these systems, Sulfur
2
Xc
X
so a trial-and-error approach is often advisable. A series of mixtures should be prepared - varying the fuel percentage Charcoal
6
2
1
0.3
while keeping everything else constant. The percentage yield-Sugar
2.5
1
0.5
0.1
ing the maximum burning rate is then experimentally determined.
Variation in loading density, achieved by varying the pressure Shellac
1
1
1
0.8
used to consolidate the composition in a tube, can also affect the burning rate. A "typical" high-energy reaction evolves a sub-aReference 1.
stantial quantity of gaseous products and a significant portion of bCompositions were pressed in cardboard tubes of the actual combustion reaction occurs in the vapor phase. For 16 mm diameter.
these reactions, the combustion rate (measured in grams con-cX indicates that the mixture did not burn.
sumed/second) will increase as the loading density decreases.
A loose powder should burn the fastest, perhaps reaching an
11 4
Chemistry of Pyrotechnics
Ignition and Propagation
115
explosive velocity, while a highly-consolidated mixture, loaded TABLE 5.6 Predicted Burning Rates for Black Powder under considerable pressure, will burn much more slowly. The at Various External Pressures
combustion front in such mixtures is carried along by hot gaseous products.
The more porous the composition is, the faster the re-External pres-
External pres-
Linear burning
action should be. The "ideal" fast composition is one that has sure, atm
sure, p.s.i.
rate, cm/sec
been granulated to achieve a high degree of homogeneity within each particle but yet consists of small grains of powder with high 1
14.7
1.21
surface area. Burning will accelerate rapidly through a loose collection of such particles.
2
29.4
1.43
The exception to this "loading pressure rule" is the category 5
73.5
of "gasless" compositions.
1.78
Here, burning is believed to propa-
gate through the mixture without the involvement of the vapor 10
147
2.10
phase, and an increase in loading pressure should lead to an in-15
221
2.32
crease in burning rate, due to more efficient heat transfer via tightly compacted solid and liquid particles.
Thermal conductiv-
20
294
2.48
ity is quite important in the burning rate of these compositions.
30
441
2.71
Table 4.6 illustrates the effect of loading pressure for the "gasless" barium chromate/boron system.
Note:
The Shidlovskiy equation is valid for the pressure range 2-30 atmospheres.
Effect of External Pressure
The gas pressure (if any) generated by the combustion products, combined with the prevailing atmospheric pressure, will also affect the burning rate. The general rule here predicts that an increase in burning rate will occur as the external pressure increases.
This factor can be especially important when oxygen For the ferric oxide/aluminum (Fe 20 3 /Al), manganese dioxide/
is a significant component of the gaseous phase. The magnitude aluminum (MnO 2 /Al), and chromic oxide/magnesium (Cr 2O 3 / Mg) of the external pressure effect indicates the extent to which the systems, slight gas phase involvement is indicated by the 3-4
vapor phase is involved in the combustion reaction.
fold rate increase observed as the external pressure is raised The effect of external pressure on the burning rate of black from 1 to 150 atm. The chromic oxide /aluminum system, how-powder has been quantitatively studied. Shidlovskiy reports ever, reportedly burns at exactly the same rate - 2.4 millime-the experimental empirical equation for the combustion of black ters /sec - at 1 and 100 atm ; suggesting that it is a true "gas-powder to be
less" system [1].
P(0.24)
Data for the burning rate of a delay system as a function of burning rate = 1.21
(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.
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
120
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)
122
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
(1952).
7.
J. E. Tanner, "Effect of Binder Oxygen Content on Adia-batic Flame Temperature of Pyrotechnic Flares," RDTR No.
181, Naval Ammunition Depot, Crane, Indiana, August, 1972.
8.
T. Shimizu, Fireworks - The Art, Science and Technique,
pub. by T. Shimizu, distrib. by Maruzen Co., Ltd., Tokyo, 1981.
9.
J. E. Rose, "Flame Propagation Parameters of Pyrotechnic Delay and Ignition Compositions," Report IHMR 71-168, Naval Ordnance Station, Indian Head, Maryland, 1971.
10.
F. L. McIntyre, "A Compilation of Hazard and Test Data for Pyrotechnic Compositions," Report ARLCD-CR-80047, U.S. Army Armament Research and Development Command, Dover, NJ, 1980.
A "set piece" outlines the seal of the United States. The pyrotechnician creates pictures and messages by attaching hundreds of cigar-sized tubes, loaded with color-producing composition, to a wooden lattice secured in the ground.
The pattern of the tubes and the
choice of colors determine the picture that is produced. Fast-burn-ning fuse-"quickmatch"-connects the tubes and permits rapid ignition of the entire pattern. Thread impregnated with fine black powder is covered by a loose-fitting paper wrapper to make quickmatch.
The hot gas and flame is confined inside the paper sheath, and burning is very rapid. (Zambelli Internationale)
6
HEAT AND DELAY COMPOSITIONS
HEAT PRODUCTION
All pyrotechnic compositions evolve heat upon ignition, and this release of energy can be used to produce color, motion, smoke, and noise.' There are applications as well for the chemically-produced heat itself, and these will be addressed in this chapter.The use of incendiary mixtures in warfare can be traced back to ancient times, when it provided an effective means of assault-ing well-fortified castles. Naval warfare was revolutionized by the use of flaming missiles to attack wooden ships, and much effort was put into improving the heat output, portability, and ac-curacy of these thermal weapons.
As both weaponry and the use of explosives for blasting developed, the need for a safe, reliable way to ignite these devices became obvious, and the concept of a pyrotechnic "delay"
emerged. A variety of terms are used for materials that either ignite or provide a delay period between ignition of a device and the production of the main explosive or pyrotechnic effect. These include
1.
Fuse:
A train of slow-burning powder (usually black powder), often covered with twine or twisted paper. Fuses are lit by a safety match or other hot object, and provide a time delay to permit the person igniting the device to retreat to a safe distance.
125
126
Chemistry of Pyrotechnics
Heat and Delay Compositions
127
TABLE 6.1 Electric Match (Squib) Compositionsa TABLE 6.2 Typical Primer Mixtures a
Component
Formula
% by weight
by
weight
1.
Potassium chlorate
KC10
Note
3
8.5
Component
Formula
Lead mononitroresorcinate
PbC 6H3NO,,
76.5
KC1O 3
45
Nitrocellulose
15
1.
Potassium chlorate
Stab primer
Lead thiocyanate
Pb(SCN) 2
33
2.
Potassium chlorate
KC1O 3
55
Antimony sulfide
Sb 2 S 3
22
Lead thiocyanate
Pb(SCN) 2
45
2.
Potassium chlorate
KCI0 3
33
Stab primer
3.
Potassium perchlorate
KC1O,,
66.6
Antimony sulfide
Sb 2 S 3
33
Titanium
Ti
33.3
Lead azide
Pb(N 3) 2
29
Carborundum
5
a Reference 1.
3.
Potassium chlorate
KC10 3
50
Percussion primer
Lead peroxide
Pb0 2
25
Antimony sulfide
Sb 2 S 3
20
Trinitrotoluene
C 7 H S N 3 0 6
5
4.
Potassium perchlorate
KC10 4
50
Percussion primer
2.
Electric Match (Squib) : A metal wire is coated with a dab Zirconium
Zr
50
of heat-sensitive composition.
An electric current is
passed through the wire, and the heat that is produced ignites the match composition. A burst of flame occurs a Reference 1.
that ignites a section of fuse or a charge of pyrotechnic composition.
Squib compositions usually contain potas-
sium chlorate (low ignition temperatures! ). Lead mononitroresorcinate (LMNR) is also included in many squib mixtures.
Several squib formulas are listed in Table 6.1.
3.
First Fire:
An easily-ignited composition is placed in lim-5.
Primer:
A term for the device used to ignite smokeless ited quantity on top of the main pyrotechnic mixture. The powder in small arms ammunition. An impact-sensitive first fire is reliably ignited by a fuse or squib, and the composition is used.
When struck by a metal firing pin,
flame and hot residue that is produced then ignites the a primer emits a burst of flame capable of igniting the main charge. Black powder moistened with water contain-propellant charge. Several typical primer mixtures are ing a binder such as dextrine is used in the fireworks in-given in Table 6.2.
dustry as a first fire, and also secures the fuse to the 6.
Friction Igniter:
A truly "self-contained" device should item.
First fires are often referred to as "primes" - a be ignitible without the need for a safety match or other term similar to another with a distinct meaning (see #5, type of external ignition source. Highway flares (fusees), below).
other types of distress signals, and some military devices 4.
Delay Composition: A general term for a mixture that use a friction ignition system. The fusee uses a two-part burns at a selected, reproducible rate, providing a time igniter; when the two surfaces are rubbed together, a delay between activation and production of the main effect.
flame is produced and the main composition is ignited.
A fuse containing a core of black powder is an example of Typically, the scratcher portion of these devices contains a delay. Highly-reproducible delay mixtures are needed red phosphorus and the matchhead mixture contains po-for military applications, and much research effort has tassium chlorate (KC1O 3 ) and a good fuel. Several fric-been put into developing reliable compositions.
tion igniter systems are given in Table 6.3.
12 8
Chemistry of Pyrotechnics
Heat and Delay Compositions
129
a black powder core significantly improved the safety record of TABLE 6. 3
Friction Igniter Mixtures
the blasting industry.
However, the development of modern,
% by
Refer-
long-range, high-altitude projectiles created a requirement for a new generation of delay mixtures. Black powder, under speci-Component
Formula
weight
ence
fied conditions, gives reproducible burning rates at ground level.
However, it produces a considerable quantity of gas upon ignition 1.
Main composition
Potassium chlorate
KC1O
(approximately 50%of the reaction products are gaseous), and its 3
60
3
burning rate will therefore show a significant dependence on ex-Antimony sulfide
Sb 2S 3
30
ternal pressure (faster burning as external pressure increases).
Resin
10
To overcome this pressure dependence, researchers set out to Striker
develop "gasless" delays - mixtures that evolve heat and burn Red phosphorus
P
56
at reproducible rates with the formation of only solid and liquid Ground glass
Si0 2
24
products.
Such mixtures show little, if any, variation of burn-Phenol /formaldehyde
(C13H1202)7
20
ing rate with pressure.
resin
One could begin such a project by setting down the requirements for an "ideal" delay mixture [4] : 2.
Main composition
Shellac
-
40
2
Strontium nitrate
Sr(N0 3 ) 2
3
1.
The mixture should be stable during preparation and stor-Quartz
Si0 2
6
age.
Materials of low hygroscopicity must be used.
Charcoal
C
2
2.
The mixture should be readily ignitible from a modest ig-Potassium perchlorate
KC1O,,
14
nition stimulus.
Potassium chlorate
KCIO 3
28
3.
There must be minimum variation in the burning rate of Wood flour
5
the composition with changes in external temperature and Marble dust
CaCO 3
2
pressure.
The mixture must readily ignite and reliably burn at low temperature and pressure.
Striker
4.
There should be a minimum change in the burning rate with Lacquer
61
small percentage changes in the various ingredients.
Pumice
2.2
Red phosphorus
P
26
5.
There must be reproducible burning rates, both within a Butyl acetate
C
batch and between batches.
6 H 12 0 2
10.8
The newer "gasless" delays are usually a combination of a metal oxide or chromate with an elemental fuel. The fuels are metals or high-heat nonmetallic elements such as silicon or boron. If an organic binder (e.g. , nitrocellulose) is used, the resulting mixture DELAY COMPOSITIONS
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
chromate
cm /second
Mixture
BaCrO 4
KC1O 4
Mo
cm /second
I.
44
53
3
0.69
I.
10
10
80
25.4
II.
39
47
14
0.44
II.
40
5
55
1.3
III.
37
43
20
0.29
III.
55
10
35
0.42
IV.
33
36
31
0.19
IV.
65
5
30
0.14
a Reference 2. Data from H. Ellern, Military and Civilian Pyro-aReference 2. Data from H. Ellern, Military and Civilian Pyrotechnics, Chemical Publishing Co., Inc., New York, 1968.
technics, Chemical Publishing Co. , Inc. , New York, 1968.
be more stable at the high reaction temperature of the burning de-down the column of pyrotechnic material, and the thermal conduc-lay mixture [2].
tivity of the mixture plays a significant role. As the density of A small percentage of fuel in excess of the stoichiometric amount the mixture increases due to increased loading pressure, the com-increases the burning rate for most delay mixtures, presumably ponents are pressed closer together and better heat transfer oc-through increased thermal conductivity for the composition. The curs.
Table 4.6 presented data for the barium chromate/boron propagation of burning is enhanced by the additional metal, es-system, showing the modest increase that occurs as the loading pecially in the absence of substantial quantities of hot gas to aid pressure is raised.
in the propagation of burning. Air oxidation of the excess metal fuel can also contribute additional heat to increase the reaction rate
I
if the burning composition is exposed to the atmosphere.
GNITION COMPOSITIONS AND FIRST FIRES
The rate of burning of ternary mixtures can similarly be affected by varying the percentages of the components. Table 6.6
Compositions with high ignition temperatures (i.e., above 600°C) presents data for a three-component delay composition. In this can be difficult to ignite using solely the "spit" from a black pow-study, a decrease in the burning rate (in cm/second) is observed der fuse or similar mild ignition stimulus. In such situations, an as the metal percentage is lowered (giving poorer thermal conduc-initial charge of a more-readily-ignitible material, called a "first tivity) and the percentage of higher-melting oxidizer (BaCrO 4 ) fire," is frequently used. The requirements for such a mixture is increased at the expense of the lower-melting, more reactive include [ 3] :
lead chromate, PbCrO 4.
Table 6. 7 illustrates this same concept for the molybdenum /
1.
Reliable ignitibility from a small thermal impulse such as a barium chromate /potassium perchlorate system. Here, KC1O 4 is fuse.
The ignition temperature of a "first fire" should be the better oxidizer.
500°C or less.
Contrary to the behavior expected for "gassy" mixtures, the 2.
The mixture should attain a high reaction temperature, rate of burning for gasless compositions is expected to increase well above the ignition temperature of the main composi-
(in units of grams reacting per second) as the consolidation pres-tion.
Metal fuels are usually used when high reaction tem-sure is increased. "Gasless" delays propagate via heat transfer peratures are needed.
134
Chemistry o f Pyrotechnics
Heat and Delay Compositions
135
3. A mixture that forms a hot, liquid slag is preferred. Such slag will provide considerable surface contact with the main composition, facilitating ignition. The production of hot gas will usually produce good ignition behavior on the ground, but reliability will deteriorate at higher altitudes.
Liquid and solid products provide better heat retention to aid ignition under these conditions.
4. A slower-burning mixture is preferred over a more rapid one. The slower release of energy allows for better heat transfer to the main composition. Also, most "first fires"
are pressed into place or added as moist pastes (that harden on drying), rather than used as faster-burning loose powders.
Potassium nitrate is frequently used in igniters and first fires.
Compositions made with this oxidizer tend to have low ignition temperatures (typically below 500 1C), and yet the mixtures are reasonably safe to prepare, use in production, and store. Potassium chlorate formulations also tend to have low ignition temperatures, but they are considerably more sensitive (and hazardous).
Potassium nitrate mixed with charcoal can be used for ignition, as can black powder worked into a paste with water and a little dextrine. Shidlovskiy reports that the composition KNO 3 , 75
Mg, 15
Iditol, 10 (iditol is a phenol/formaldehyde resin) works well as an igniter mixture [3] ; the solid magnesium oxide (MgO) residue aids in igniting the main composition. Boron mixed with potassium nitrate is a frequently-used, effective igniter mixture, as is the combination of iron oxide with zirconium metal and diatomaceous earth (commonly known as A-lA ignition mixture).
Table 6.8 lists a variety of formulations that have been published.
THERMITE MIXTURES
Thermites are mixtures that produce a high heat concentration, usually in the form of molten products. Thermite compositions contain a metal oxide as the oxidizer and a metal -- usually aluminum - as the fuel, although other active metals may be used.
136
Chemistry o f Pyrotechnics
Heat and Delay Compositions
137
A minimum amount of gas is produced, enabling the heat of reacTABLE 6. 9 Calorific Data for Thermite Mixturesa tion to concentrate in the solid and liquid products. High reaction temperatures can be achieved in the absence of volatile ma-
% Al by
terials; typically, values of 2000-2800°C are reached [3]. A
% Active
weight in
metal product such as iron, with a wide liquid range (melting oxygen
thermite
~Hreaction,
point 1535°C, boiling point 2800°C) produces the best thermite Oxidizers
Formula by weight
mixture
kcal/gram
behavior.
Upon ignition, a thermite mixture will form aluminum oxide and the metal corresponding to the starting metal oxide: Silicon dioxide
SiO 2
53
37
. 56
Fe
Chromium(III) oxide
Cr
2 0 3 + 2 Al -} A1 2 0 3 + 2 Fe
2 0 3
32
26
. 60
Thermite mixtures have found application as incendiary compo-Manganese dioxide
MnO 2
37
29
1.12
sitions and spot-welding mixtures. They are also used for the Iron oxide
Fe
intentional demolition of machinery and for the destruction of 2O 3
30
25
. 93
documents.
Thermites are usually produced without a binder Iron oxide
Fe 30 4
28
24
. 85
(or with a minimum of binder), because the gaseous products Cupric oxide
CuO
20
19
. 94
resulting from the combustion of the organic binder will carry away heat and cool the reaction.
Lead oxide (red)
Pb 3 O 4
9
10
. 47
Iron oxide (Fe 2O 3 or Fe 3O 4 ) with aluminum metal is the classic thermite mixture.
The particle size of the aluminum should be
a
somewhat coarse to prevent the reaction from being too rapid.
Reference 3.
Thermites tend to be quite safe to manufacture, and they are rather insensitive to most ignition stimuli. In fact, the major problem with most thermites is getting them to ignite, and a strong first fire is usually needed.
Calorific data for a variety of aluminum thermite mixtures are starting materials, used one source of charcoal, and did given in Table 6.9.
not vary the extent of mixing or the amount of moisture contained in their product.
2.
Black powder has a relatively low gas output. Only about PROPELLANTS
50% of the products are gaseous; the remainder are solids.
3.
The solid residue from black powder is highly alkaline The production of hot gas to lift and move objects, using a pyro-
(strongly basic), and it is quite corrosive to many materi-technic system, began with the development of black powder.
als.
Rockets were in use in Italy in the 14th century [51, and cannons were developed at about the same time. The development of aerial
"Pyrodex" is a patented pyrotechnic composition designed to ful-fireworks was a logical extension of cannon technology.
fill many of the functions of black powder. It contains the three Black powder remained the sole propellant available for mili-ingredients found in black powder plus binders and burning rate tary and civilian applications until well into the 19th century. A modifiers that make the material somewhat less sensitive and slower number of problems associated with the use of black powder stim-burning.
A greater degree of confinement is required to obtain ulated efforts to locate replacements
performance comparable to "normal" black powder [6].
The advantages of black powder and Pyrodex include good ig-1.
Substantial variation in burning behavior from batch to nitibility, moderate cost, ready availability of the ingredients, batch.
The better black powder factories produced good and a wide range of uses (fuse powder, delay mixture, propellant, powder if they paid close attention to the purity of their and explosive) depending on the degree of confinement.
138
Chemistry of Pyrotechnics
Heat and Delay Compositions
139
As propellant technology developed, the ideal features for a GENERAL
better material became evident
1. A propellant that can safely be prepared from readily-available materials at moderate cost.
2. A material that readily ignites, but yet is stable during storage.
3. A mixture that forms the maximum quantity of low molecular weight gases upon burning, with minimum solid residue.
4. A mixture that reacts at the highest possible temperature, to provide maximum thrust.
The late 19th century saw the development of a new family of
"smokeless" powders, as modern organic chemistry blossomed and the nitration reaction became commercially feasible. Two "esters" - nitrocellulose and nitroglycerine - became the major components of these new propellants. An ester is a compound formed from the reaction between an acid and an alcohol. Figure 6.1 illustrates the formation of NC and NG from nitric acid and the pre-
( maximum of 3 -ON02 groups
cursor alcohols cellulose and glycerine.
per glucose unit)
"Single base" smokeless powder, developed mainly in the United States, uses only nitrocellulose. "Double base" smokeless powder, FIG. 6.1 The nitration reaction. Organic compounds containing developed in Europe, is a blend of nitrocellulose and nitroglycer-the -OH functional group are termed "alcohols." These com-ine. "Cordite," a British development, consists of 65% NC, 30%
pounds react with nitric acid to produce a class of compounds NG, and 5% mineral jelly. The mineral jelly (a hydrocarbon ma-known as "nitrate esters." Nitroglycerine and nitrocellulose are terial) functions as a coolant and produces substantial amounts among the numerous explosive materials produced using this re-of CO
action.
2 , CO, and H 2O gas to improve the propellant characteristics. "Triple base" smokeless powder, containing nitroguanidine as a third component with nitroglycerine and nitrocellulose is also manufactured.
An advantage of the smokeless powders is their ability to be extruded during the manufacturing process. Perforated grains can be produced that simultaneously burn inwardly and outwardly such that a constant burning surface area and constant gas production are achieved.
Nitrocellulose does not contain sufficient internal oxygen for complete combustion to C0 2 , H2O, and N2 , while nitroglycerine contains excess oxygen [7]. The double base smokeless propellants therefore achieve a slightly more complete combustion and benefit from the substantial exothermicity of NG (1486 calories/
gram) [7].
140
Chemistry o f Pyrotechnics
Heat and Delay Compositions
141
Shuttle.
The pyrotechnic boosters used for these launches typi-4.
U.S. Army Material Command, Engineering Design Handbook, cally contain
Military Pyrotechnic Series, Part One, "Theory and Application, " Washington, D . C . , 1967 (AMC Pamphlet 706-185).
1.
A solid oxidizer:
Ammonium perchlorate (NH,,C1O,,) is the
5.
J. R. Partington, A History of Greek Fire and Gunpowder, current favorite due to the high percentage of gaseous W. Heffer & Sons, Ltd., Cambridge, England, 1960.
products it forms upon reaction with a fuel.
6.
G. D. Barrett, "Venting of Pyrotechnics Processing Equip-2.
A small percentage of light, high-energy metal: This
ment," Proceedings, Explosives and Pyrotechnics Applica-metal produces solid combustion products that do not aid tions Section, American Defense Preparedness Assn. , Los in achieving thrust, but the considerable heat evolved by Alamos, New Mexico, October, 1984.
the burning of the metal raises the temperature of the 7.
"Military Explosives, " U.S. Army and U.S. Air Force Tech-other gaseous products. Aluminum and magnesium are nical Manual TM 9-1300-214, Washington, D.C. , 1967.
the metals most commonly used.
8.
R. F. Gould (Ed.), Advanced Propellant Chemistry, American 3.
An organic fuel that also serves as binder and gas-former: Chemical Society Publications, Washington, D.C. , 1966.
Liquids that polymerize into solid masses are preferred, for simpler processing, and a binder with low oxygen content is desirable to maximize heat production.
A negative oxygen balance is frequently designed into these propellant mixtures to obtain CO gas in place of CO 2 .
CO is
lighter and will produce greater thrust, all other things being equal.
However, the full oxidation of carbon atoms to CO 2 evolves more heat, so some trial-and-error is needed to find the optimum ratio of oxidizer and fuel [8].
Propellant compositions are also used in numerous "gas generator" devices, where the production of gas pressure is used to drive pistons, trigger switches, eject pilots from aircraft, and perform an assortment of other critical functions. The military and the aerospace industry use many of these items, which can be designed to function rapidly and can be initiated remotely.
REFERENCES
1.
F. L. McIntyre, A Compilation of Hazard and Test Data for Pyrotechnic Compositions," Report ARLCD-CR-80047, U.S.
Army Armament Research and Development Command, Dover, NJ, 1980.
2.
J. H. McLain, Pyrotechnics from the Viewpoint of Solid State Chemistry, The Franklin Institute Press, Philadelphia, Penna., 1980
3.
A. A. Shidlovskiy, Principles of Pyrotechnics, 3rd Ed., Moscow, 1964. (Translated by Foreign Technology Division, Wright-Patterson Air Force Base, Ohio, 1974.)
A "weeping willow" aerial shell bursts high in the sky and leaves its characteristic pattern as the large, slow-burning stars descend to the ground. Charcoal is frequently used to produce the attractive gold color, with potassium nitrate selected as the oxidizer to achieve a slow-burning mixture. (Zambelli Internationale)
7
COLOR AND LIGHT PRODUCTION
The production of bright light and vivid color is the primary purpose of many pyrotechnic compositions. Light emission has a variety of applications, ranging from military signals and highway distress flares to spectacular aerial fireworks. The basic theory of light emission was discussed in Chapter 2, and several good articles have been published dealing with the chemistry and physics of colored flames [1, 21.
The quantitative measurement of light intensity (candle power) at any instant and the light integral (total energy emitted, with units of candle-seconds/gram) can be affected by a variety of test parameters such as container diameter, burning rate, and the measuring equipment. Therefore, comparisons between data obtained from different reports should be viewed with caution.
WHITE LIGHT COMPOSITIONS
For white-light emission, a mixture is required that burns at high temperature, creating a substantial quantity of excited atoms or molecules in the vapor state together with incandescent solid or liquid particles. Incandescent particles emit a broad range of wavelengths in the visible region of the electromagnetic spectrum, and white light is perceived by the viewer. Intense emission from sodium atoms in the vapor state, excited to higher-energy electronic states by high flame temperature, is the principal light source in the sodium nitrate /magnesium /organic binder flare compositions widely used by the military [3, 41.
143
144
Chemistry of Pyrotechnics
Color and Light Production
145
Magnesium or aluminum fuels are found in most white-light com-fireworks mixtures. Several published formulas for white light positions. These metals evolve substantial heat upon oxidation, compositions are given in Table 7.1.
and the high-melting magnesium oxide (MgO) and aluminum oxide The ratio of ingredients, as expected, will affect the perform-
(A1203 ) reaction products are good light emitters at the high re-ance of the composition. Optimum performance is anticipated near action temperatures that can be achieved using these fuels. Ti-the stoichiometric point, but an excess of metallic fuel usually in-tanium and zirconium metals are also good fuels for white-light creases the burning rate and light emission intensity. The addi-compositions.
tional metal increases the thermal conductivity of the mixture, In selecting an oxidizer and fuel for a white-light mixture, a thereby aiding burning, and the excess fuel - especially a vola-main consideration is maximizing the heat output. A value of 1.5
tile metal such as magnesium (boiling point 1107°C) - can vapor-kcal/gram has been given by Shidlovskiy as the minimum for a ize and burn with oxygen in the surrounding air to produce extra usable illuminating composition [5]. A flame temperature of less heat and light. The sodium nitrate/magnesium system is exten-than 2000°C will produce a minimum amount of white light by emis-sively used for military illuminating compositions. Data for this sion from incandescent particles or from excited gaseous sodium system are given in Table 7.2.
atoms.
The anticipated reaction between sodium nitrate and magnesium Therefore, the initial choice for an oxidizer is one with an is
exothermic heat of decomposition such as potassium chlorate (KC1O
5 Mg + 2 NaNO 3 -> 5 MgO + Na2O + N 2
3). However, mixtures of both chlorate and perchlorate salts with active metal fuels are too ignition-sensitive for commer-grams 121.5
170
cial use, and the less-reactive - but safer - nitrate compounds
% by weight 41.6
58.4 (for a stoichiometric mixture)
are usually selected. Potassium perchlorate is used with aluminum Formula A in Table 7.2 therefore contains an excess of oxidizer.
and magnesium in some "photoflash" mixtures ; these are extremely It is the slowest burning mixture and produces the least heat.
reactive compositions, with velocities in the explosive range.
Formula B is very close to the stoichiometric point. Formula C
The nitrates are considerably endothermic in their decomposi-contains excess magnesium and is the most reactive of the three; tion and therefore deliver less heat than chlorates or perchlor-the burning of the excess magnesium in air must contribute subates, but they can be used with less fear of accidental ignition.
stantially to the performance of this composition.
Barium nitrate is often selected for white-light mixtures. The A significant altitude effect will be shown by these illuminating barium oxide (BaO) product formed upon reaction is a good, compositions, especially those containing excess metal. The de-broad-range molecular emitter in the vapor phase (the boiling creased atmospheric pressure - and therefore less oxygen - at point of BaO is ca. 2000°C), and condensed particles of BaO
higher altitudes will slow the burning rate as the excess fuel will found in the cooler parts of the flame are also good emitters of not be consumed as efficiently.
incandescent light.
Sodium nitrate is another frequent choice. It is quite hygroscopic however, so precautions must be taken during production
"Photoflash" Mixtures
and storage to exclude moisture. Sodium nitrate produces good To produce a burst of light of short duration, a composition is heat output per gram due to the low atomic weight (i.e. , 23) of required that will react very rapidly. Fine particle sizes are sodium, and the intense flame emission from atomic sodium in the used for the oxidizer and fuel to increase reactivity, but sensi-vapor state contributes substantially to the total light intensity.
tivity is also enhanced at the same time. Therefore, these mix-Potassium nitrate, on the other hand, is not a good source of tures are quite hazardous to prepare, and mixing operations atomic or molecular emission, and it is rarely - if ever - used should always be carried out remotely. Several representative as the sole oxidizer in white-light compositions.
photoflash mixtures are given in Table 7.3.
Magnesium metal is the fuel found in most military illuminating An innovation in military photoflash technology was the decompositions, as well as in many fireworks devices. Aluminum and velopment of devices containing fine metal powders without any titanium metals, the magnesium /aluminum alloy "magnasium," and oxidizer. A high-explosive bursting charge is used instead.
antimony sulfide (Sb2S 3 ) are used for white light effects in many This charge, upon ignition, scatters the metal particles at high
14 6
Chemistry of Pyrotechnics
Color and Light Production
147
TABLE 7.2 The Sodium Nitrate /Magnesium Systema
% Sodium
Linear burning Heat of reaction,
nitrate
% Magnesium
rate, mm/sec
kcal/gram
A.
70
30
4.7
1.3
B.
60
40
11.0
2.0
C .
50
50
14. 3
2.6
aReference 5.
temperature and they are then air-oxidized to produce light emission. No hazardous mixing of oxidizer and fuel is required to prepare these illuminating devices.
SPARKS
The production of brilliant sparks is one of the principal effects available to the fireworks manufacturer and to the "special effects" industry. Sparks occur during the burning of many pyrotechnic compositions, and they may or may not be a desired feature.Sparks are produced when liquid or solid particles - either original components of a mixture or particles created at the burn-ning surface - are ejected from the composition by gas pressure produced during the high-energy reaction. These particles --
heated to incandescent temperatures - leave the flame area and proceed to radiate light as they cool off or continue to react with atmospheric oxygen. The particle size of the fuel will largely determine the quantity and size of sparks; the larger the particle size, the larger the sparks are likely to be. A combination of fine fuel particles for heat production with larger particles for the spark effect is often used by manufacturers.
Metal particles - especially aluminum, titanium, and "magnalium" alloy - produce good sparks that are white in appearance. Charcoal of sufficiently large particle size also works well, producing sparks with a characteristic orange color. Sparks from iron particles vary from gold to white, depending on the
148
Chemistry of Pyrotechnics
Color and Light Production
149
TABLE 7.3 Photoflash Mixtures
TABLE 7.4 Spark-Producing Compositions
Refer-
% by
Oxidizer (% by weight)
Fuel (% by weight)
ence
Composition
weight
Effect
Reference
I.
Potassium per-
40
Magnesium
34
7
I.
Potassium nitrate,
58
Gold sparks
6
chlorate, KC10,,
Aluminum
26
KNO 3
Sulfur
7
II.
Potassium per-
40
Magnesium aluminum
60
7
Pure charcoal
35
chlorate, KC1O,,
alloy, "Magnalium"
(50/50)
II.
Barium nitrate,
50
Gold sparks (gold
6
Ba(N0 3 ) 2
sparkler)
III.
Potassium per-
30
Aluminum
40
7
Steel filings
30
chlorate, KC10,,
Dextrine
10
Barium nitrate,
30
Aluminum powder
8
Ba(N03)2
Fine charcoal
0.5
IV.
Barium nitrate,
54.5
Magnalium
45.5
8
Boric acid
1.5
Ba(NO 3 ) 2
Aluminum
4
III.
Potassium perchlorate,
42.1
White sparks
9
KC1O,,
Titanium
42.1
De xt rine
15.8
(Make a paste from dextrine
reaction temperature; they are the brilliant sparks seen in the and water, then mix in ox-popular "gold sparkler" ignited by millions of people on the 4th idizer and fuel)
of July.
IV.
Potassium perchlorate,
50
White sparks "water-
6
Magnesium metal does not produce a good spark effect. The KClO,,
falls" effect
metal has a low boiling point (1107°C), and therefore tends to
"Bright" aluminum
25
vaporize and completely react in the pyrotechnic flame [6]. "Mag-powder
nalium" can produce good sparks that burn in air with a novel,
"Flitter" aluminum ,
12.5
crackling sound. Several spark-producing formulas are given in 30-80 mesh
Table 7.4.
Remember, the particle size of the fuel is very impor-
"Flitter" aluminum,
12.5
tant in producing sparks - experimentation is needed to find the 5-30 mesh
ideal size.
For a good spark effect, the fuel must contain particles large enough to escape from the flame prior to complete combustion.
Note:
Particle size of the fuel is very important in determining Also, the oxidizer must not be too effective, or complete reac-the size of the sparks.
tion will occur in the flame. Charcoal sparks are difficult to achieve with the hotter oxidizers; potassium nitrate (KNO 3 ) -
with its low flame temperatures - works best. Some gas production is required to achieve a good spark effect by assisting in FLITTER AND GLITTER
the ejection of particles from the flame. Charcoal, other organic fuels and binders, and the nitrate ion can provide gas for this Several interesting visual effects can be achieved by careful se-purpose.
lection of the fuel and oxidizer for a spark-producing composition.
15 0
Chemistry of Pyrotechnics
Color and Light Production
151
A thorough review article discussing this topic in detail -- with TABLE 7.5 Glitter Formulasa
numerous formulas - has been published [101.
"Flitter" refers to the large white sparks obtained from the burning of large aluminum flakes. These flakes burn continuously upon ejection from the flame, creating a beautiful white effect, and they are used in a variety of fireworks items.
I.
Potassium nitrate, KNO 3
55
Good white Used in aerial
"Glitter" is the term given to the effect produced by molten
"Bright" aluminum powder 5
glitter
stars
droplets which, upon ejection from the flame, ignite in air to Dextrine
4
produce a brilliant flash of light. A nitrate salt (KNO
Antimony sulfide, Sb
3 is best)
2S3
16
and sulfur or a sulfide compound appear to be essential for the Sulfur
10
glitter phenomenon to be achieved. It is likely that the low melt-Charcoal
10
ing point (334°C) of potassium nitrate produces a liquid phase II. Potassium nitrate, KNO
that is responsible, at least in part, for this effect. Several B
55
Gold glitter Used in aerial
"Bright" aluminum powder 5
stars
"glitter" formulas are given in Table 7.5. The ability of certain Dextrine
4
compositions containing magnesium or magnalium alloy to burn in Antimony sulfide, Sb2S3
14
a pulsing, "strobe light" manner is a novel phenomenon believed Charcoal
8
to involve two distinct reactions. A slow, "dark" process occurs Sulfur
8
until sufficient heat is generated to initiate a fast, light-emitting reaction. Dark and light reactions continue in an alternate man-III. Potassium nitrate, KNO 3
55 Good white Used in foun-
ner, generating the strobe effect [11, 12].
Sulfur
10
glitter
tains
Charcoal
10
Atomized aluminum
10
Iron oxide, Fe
COLOR
203
5
Barium carbonate, BaCO 3 5
I ntroduction
Barium nitrate, Ba(N0 3) 2 5
Certain elements and compounds, when heated to high temperature, have the unique property of emitting lines or narrow bands of light in the visible region (380-780 nanometers) of the electro-a Reference 10.
magnetic spectrum. This emission is perceived as color by an observer, and the production of colored light is one of the most important goals sought by the pyrotechnic chemist. Table 7.6 lists the colors associated with the various regions of the visible spec-blue and red light in the proper proportions will produce a purple trum. The complementary colors - perceived if white light minus effect. Color theory is a complex topic, but it is one that should a particular portion of the visible spectrum is viewed -- are also be studied by anyone desiring to produce colored flames [2].
given in Table 7.6.
The production of a vividly-colored flame is a much more chal-To produce color, heat (from the reaction between an oxidizer lenging problem than creating white light. A delicate balance of and a fuel) and a color-emitting species are required. Sodium factors is required to obtain a satisfactory effect compounds added to a heat mixture will impart a yellow color to the flame. Strontium salts will yield red, barium and copper compounds can give green, and certain copper-containing mixtures 1. An atomic or molecular species that will emit the desired will produce blue. Color can be produced by emission of a narrow wavelength, or blend of wavelengths, must be present in band of light (e.g. , light in the range 435-480 nanometers is per-the pyrotechnic flame.
ceived as blue), or by the emission of several ranges of light that 2. The emitting species must be sufficiently volatile to exist combine to yield a particular color. For example, the mixing of in the vapor state at the temperature of the pyrotechnic
15 2
Chemistry of Pyrotechnics
Color and Light Production
153
TABLE 7.6 The Visible Spectruma
A temperature range is therefore required, high enough to achieve the excited electronic state of the vaporized species Observed color - if
but low enough to minimize dissociation.
this wavelength is
5. The presence of incandescent solid or liquid particles in Wavelength
removed from
the flame will adversely affect color quality. The result-
(nanometers)
Emission color
white light
ing "black body" emission of white light will enhance overall emission intensity, but the color quality will be lessened.
<380
None (ultraviolet region)
A "washed out" color will be perceived by viewers. The 380-435
Violet
Yellowish-green
use of magnesium or aluminum metal in color compositions will yield high flame temperatures and high overall inten-435-480
Blue
Yellow
sity, but broad emission from incandescent magnesium ox-480-490
Greenish-blue
Orange
ide or aluminum oxide products may lower color purity.
6. Every effort must be made to minimize the presence of un-490-500
Bluish-green
Red
wanted atomic and molecular emitters in the flame. Sodium 500-560
Green
Purple
compounds can not be used in any color mixtures except yellow. The strong yellow atomic emission from sodium 560-580
Yellowish-green
Violet
(589 nanometers) will overwhelm other colors. Potassium 580-595
Yellow
Blue
emits weak violet light (near 450 nanometers), but good red and green flames can be produced with potassium com-595-650
Orange
Greenish-blue
pounds present in the mixture. Ammonium perchlorate is 650-780
Red
Bluish-green
advantageous for color compositions because it contains no metal ion to interfere with color quality. The best oxidizer
>780
None (infrared region)
to choose, therefore, should contain the metal ion whose emission, in atomic or molecular form, is to be used for a
color production, if such an oxidizer is commercially avail-Source : H. H. B auer , G. D. Christian, and J. E. O'Reilly, Inable, works well, and is safe to use. Using this logic, the strumental Analysis, Allyn & Bacon, Inc., Boston, 1979.
chemist would select barium nitrate or barium chlorate for green flame mixtures. Strontium nitrate, although hygroscopic, is frequently selected for red compositions. The use of a salt other than one with an oxidizing anion (e.g. , strontium carbonate for red) may be required by hygro-reaction. The flame temperature will range from 1000-scopicity and safety considerations. However, these inert 2000°C (or more), depending on the particular composition ingredients will tend to lower the flame temperature and used.
therefore lower the emission intensity. A low percentage 3. Sufficient heat must be generated by the oxidizer/fuel re-of color ingredient must be used in such cases to produce action to produce the excited electronic state of the emitter.
a satisfactory color.
A minimum heat requirement of 0.8 kcal/gram has been men-7. If a binder is required in a colored flame mixture, the mini-tioned by Shidlovskiy [5].
mum possible percentage should be used. Carbon-contain-4. Heat is necessary to volatilize and excite the emitter, but ing compounds may be oxidized to the atomic carbon level you must not exceed the dissociation temperature of mo-in the flame and produce an orange color. The use of a lecular species (or the ionization temperature of atomic binder that is already substantially oxidized (one with a species) or color quality will suffer. For example, the high oxygen content, such as dextrine) can minimize this green emitter BaC1 is unstable above 2000°C and the best problem. Binders such as paraffin that contain little or blue emitter, CuCl, should not be heated above 1200°C [5].
no oxygen should be avoided unless a hot, oxygen-rich composition is being prepared.
Color and
15 4
Chemistry of Pyrotechnics
Light Production
155
TABLE 7.7 Flame Temperatures for Oxidizer/Shellac Mixtures Oxidizer Selection
The numerous requirements for a good oxidizer were discussed in Flame temperatures for various oxidizers (°C)a detail in Chapter 3. An oxidizer for a colored flame composition Potassium
Ammonium
must meet all of those requirements, and in addition must either perchlor-perchlor-
emit the proper wavelength light to yield the desired color or not Potassium
Potassium
ate
ate
chlorate
emit any light that interferes with the color produced by other nitrate
Composition
KClO,,
NH,,C10,,
KCIO
components.
3
KNO 3
In addition, the oxidizer must react with the selected fuel to I.
75% Oxidizer
2250
2200
2180
produce a flame temperature that yields the maximum emission of 1675
15% Shellac
light in the proper wavelength range. If the temperature is too 10% Sodium
low, not enough "excited" molecules are produced and weak color oxalateb
intensity is observed.
A flame temperature that is too hot may
decompose the molecular emitter, destroying color quality.
II.
70% Oxidizer
2125
2075
2000
1700
Table 7.7 gives some data on flame temperatures obtained by 20% Shellac
Shimizu for oxidizer/shellac mixtures. Sodium oxalate was added 10% Sodium
to yield a yellow flame color and permit temperature measurement oxalate
by the "line reversal" method [11].
III.
65% Oxidizer
1850
1875
1825
The data in Table 7. 7 show that potassium nitrate, with its 1725
25% Shellac
highly endothermic heat of decomposition, produces significantly 10% Sodium
lower flame temperatures with shellac than the other three oxi-oxalate
dizers.
The yellow light intensity will be substantially less for the nitrate compositions.
To use potassium nitrate in colored flame mixtures, it is nec-a Reference 11.
essary to include magnesium as a fuel to raise the flame tempera-bThe sodium oxalate (Na
ture.
A source of chlorine is also needed for formation of volatile 2 C 20,,) produces a yellow flame. The intensity of the yellow light emission can be used to determine the BaCl (green), or SrCl (red) emitters. The presence of chlorine flame temperature.
in the flame also aids by hindering the formation of magnesium oxide and strontium or barium oxide, all of which will hurt the color quality.
Shidlovskiy suggests a minimum of 15% chlorine donor in a color composition when magnesium metal is used as a often contain sawdust as a coarse, slow-burning retardant to help fuel [5].
achieve lengthy burning times.
To achieve rapid burning - such as in the brightly-colored Fuels and Burning Rates
"stars" used in aerial fireworks and Very pistol cartridges -
compositions will contain charcoal or a metallic fuel (usually mag-Applications involving colored flame compositions will require nesium). Fine particle sizes will be used, and all ingredients will either a long-burning composition or a mixture that burns rap-be well-mixed to achieve a very homogeneous - and fast burning -
idly to give a burst of color.
mixture.
Highway flares ("fusees") and the "lances" used to create fireworks set pieces require long burning times ranging from 1-30 minutes. "Fast" fuels such as metal powders and charcoal are Color Intensifiers
usually not included in these slow mixtures. Partially-oxidized Chlorine is the key to the production of good red, green, and blue organic fuels such as dextrine can be used. Coarse oxidizer and flames, and its presence is required in a pyrotechnic mixture to fuel particles can also retard the burning rate. Highway flares
156
Chemistry of Pyrotechnics
Color and Light Production
157
TABLE 7.8 Chlorine Donors for Pyrotechnic Mixtures MgO particles is thereby reduced, and color quality improves significantly.
Melting point,
% Chlorine
MgO + HCl + MgCl + OH
Material
Formula
°C
by weight
Polyvinyl chloride
(-CH2CHC1-)n Softens ca. 80
56
Red Flame Compositions
decomposes
The best flame emission in the red region of the visible spectrum ca. 160
is produced by molecular strontium monochloride, SrCl. This
"Parlon" (chlorinated
Softens 140
ca. 66
species - unstable at room temperature - is generated in the polyisopropylene )
pyrotechnic flame by a reaction between strontium and chlorine atoms. Strontium dichloride, SrC1 2 , would appear to be a logi-Hexachlorobenzene C 6C16
229
74.7
cal precursor to SrCl, and it is readily available commercially,
"Dechlorane"
C10C112
160
78.3
but it is much too hygroscopic to use in pyrotechnic mixtures.
(hexachloropenta-
The SrCl molecule emits a series of bands in the 620-640 mano-diene dimer)
meter region - the "deep red" portion of the visible spectrum.
Other peaks are observed. Strontium monohydroxide, SrOH, is Hexachloroethane
C 2C16
185
89.9
another substantial emitter in the red and orange-red regions
[1, 11]. The emission spectrum of a red flare is shown in Figure 7.1.
Strontium nitrate - Sr(NO 3) 2 - is often used as a combination achieve a good output of these colors. Chlorine serves two impor-oxidizer/color source in red flame mixtures. A "hotter" oxidizer, tant functions in a pyrotechnic flame. It forms volatile chlorine-such as potassium perchlorate, is frequently used to help achieve containing molecular species with the color-forming metals, en-higher temperatures and faster burning rates. Strontium nitrate suring a sufficient concentration of emitters in the vapor phase.
is rather hygroscopic, and water can not be used to moisten a Also, these chlorine-containing species are good emitters of nar-binder for mixtures using this oxidizer. Strontium carbonate is row bands of visible light, producing the observed flame color.
much less hygroscopic and can give a beautiful red flame under Without both of these properties - volatility and light emission -
the proper conditions. However, it contains an inert anion - the good colors would be difficult to achieve.
carbonate ion, C032 - and low percentages must be used to avoid The use of chlorate or perchlorate oxidizers (KC1O 3 , KC1O,,, burning difficulties.
etc.) is one way to introduce chlorine atoms into the pyrotechnic To keep the SrCl from oxidizing in the flame, Shidlovskiy rec-flame. Another method is to incorporate a chlorine-rich organic ommends using a composition containing a negative oxygen balance compound into the mixture. Table 7.8 lists some of the chlorine (excess fuel). Such a mixture will minimize the reaction donors commonly used in pyrotechnic mixtures. A dramatic in-2 SrCl + 0
crease in color quality can be achieved by the addition of a small 2 -> 2 SrO + C1 2
percentage of one of these materials into a mixture. Shimizu rec-and enhance color quality [ 51. Several red formulas are presented ommends the addition of 2-3% organic chlorine donor into compo-in Table 7.9
sitions that don't contain a metallic fuel, and the addition of 10-15% chlorine donor into the high temperature mixtures containing Green Flame Compositions
metallic fuels [11].
Shimizu attributes much of the value of these chlorine donors Pyrotechnic compositions containing a barium compound and a good in magnesium-containing compositions to the production in the chlorine source can generate barium monochloride, BaCl, in the flame of hydrogen chloride, which reacts with magnesium oxide flame and the emission of green light will be observed. BaCl - an to form volatile MgCl molecules. The incandescent emission from unstable species at room temperature - is an excellent emitter in
158
Chemistry of Pyrotechnics
Color and Light Production
159
TABLE 7.9 Red Flame Compositions
% by
Composition
weight
Use
Reference
I.
Ammonium perchlorate,
70
Red torch
6
NH,,ClO,,
Strontium carbonate,
10
SrC O 3
Wood meal (slow fuel)
20
II.
Potassium perchlorate,
67
Red fireworks
6
K C 10,,
star
Strontium carbonate,
13.5
SrCO 3
Pine root pitch
13.5
Rice starch
6
III.
Potassium perchlorate,
32.7
Red fireworks
9
KCIO,,
star
Ammonium perchlorate,
28.0
NH„ CIO,,
Strontium carbonate,
16.9
SrCO 3
FIG. 7.1 Emission spectrum of a red flare. Emission is concen-Red gum
14.0
trated in the 600-700 nm region. The primary emitting species Hexamethylenetetra-2.8
are SrCI and SrOH molecules in the vapor state. The composi-mine, C 6 H 12 N,,
tion of the flare was potassium perchlorate (20.5%) , strontium ni-Charcoal
1.9
trate (34.7%), magnesium (24.4%), polyvinylchloride (11.4%), and Dextrine (dampen with
3.7
asphaltum (9.0%). Source : H. A. Webster III, "Visible Spectra 3:1 water/alcohol)
of Standard Navy Colored Flares," Proceedings, Explosives and IV.
Potassium perchlorate,
44
Red signal
Unpublished
Pyrotechnics Applications Section, American Defense Preparedness KClO,,
flare (very
Association, Fort Worth, Texas, September, 1983.
Strontium nitrate,
31
little residue)
Sr(NO3)2
Epoxy fuel/binder
25
the 505-535 nanometer region of the visible spectrum - the "deep green" portion [1, 11]. The emission spectrum of a green flare was shown in Figure 4. 1.
Barium nitrate - Ba(NO 3) 2 - and barium chlorate - Ba(C 1 03)2 -
are used most often to produce green flames, serving both as the high decomposition temperature and endothermic heat of decomposition.
oxidizer and color source. Barium chlorate can produce a deep Barium carbonate (BaCO 3 ) is another possibility, but it green, but it is somewhat unstable and can form explosive mix-must be used in low percentage due to its inert anion, CO 3 .
tures with good fuels. Barium nitrate produces an acceptable An oxygen-deficient flame is required for a good-quality green green color, and it is considerably safer to work with due to its flame. Otherwise, barium oxide (BaO) will form and emit a series
160
Chemistry of Pyrotechnics
Color and Light Production
161
of bands in the 480-600 nanometer range, yielding a dull, yellow-TABLE 7.10 Green Flame Compositions
ish-green color. The reaction
2 BaCl + 0
% by
2 ~ 2 BaO + C1 2
Composition
weight
will shift to the left-hand side when chlorine is present in abun-Use
Reference
dance and oxygen is scarce, and a good green color will be I.
Ammonium perchlorate,
50
Green torch
6
achieved. A flame temperature that is too high will decompose N H,,C1O,,
BaCl, however, so metal fuels must be held to a minimum, if they Barium nitrate,
34
are used at all. A "cool" flame is best.
Ba(N0
This temperature dependence and need for chlorine source are 3)2
Wood meal
8
important to remember. A binary mixture of barium nitrate and Shellac
8
magnesium metal will produce a brilliant white light upon ignition, from a combination of MgO and BaO emission at the high tempera-II. Barium chlorate,
65
Green torch
Unpublished
ture achieved by the mixture. Addition of a chlorine-containing Ba(Cl0 3) 2 - H 20
organic fuel to lower the temperature and provide chlorine atoms Barium nitrate,
25
to form BaCI can produce a green flame. Several green flame Ba(NO3)2
compositions are given in Table 7. 10.
Red gum
10
III. Potassium perchlorate,
46
Green fireworks
6
Blue Flame Compositions
KC10y
star
Barium nitrate,
32
The generation of an intense, deep-blue flame represents the ulti-Ba(N0
mate challenge to the pyrotechnic chemist. A delicate balance of 3 ) 2
Pine root pitch
16
temperature and molecular behavior is required to obtain a good Rice starch
6
blue, but it can be done if the conditions are right.
The best flame emission in the blue region of the visible spec-IV. Barium nitrate,
59
Russian green
5
trum (435-480 nanometers) is obtained from copper monochloride, Ba(N 0 3)2
fire
CuCl. Flame emission from this molecular species yields a series Polyvinyl chloride
22
of bands in the region from 428-452 nanometers, with additional Magnesium
19
peaks between 476-488 nanometers [1, 11].
In an oxygen-rich flame, and at temperatures above 12000C, CuCl is unstable and will react to form CuO and CuOH. CuOH
emits in the 525-555 nanometer region (green!) and substantial emission may overpower any blue effect that is also present. Copper oxide, CuO, emits a series of bands in the red region, and among the materials used in blue flame mixtures. Potassium per-this reddish emission is often seen at the top of blue flames, where chlorate and ammonium perchlorate are the oxidizers found in most sufficient oxygen from the atmosphere is present to convert CuCI blue compositions. Potassium chlorate would be an ideal choice to Cu0 [111.
because of its ability to sustain reaction at low temperatures (reParis green - copper acetoarsenite, (CuO) 3 As2 O3 Cu(C2H302) -
member, CuCl is unstable above 1200°C), but copper chlorate is was widely used in blue flame mixtures until a few years ago. It an extremely reactive material. The chance of it forming should produces a good blue flame, but it has all but vanished from com-a blue mixture get wet precludes the commercial use of KC1O
mercial formulas because of the health hazards associated with its 3 .
Several formulas for blue flame compositions are given in Table use. (It contains arsenic! )
7.11. An extensive review of blue and purple flames, concentra-Copper oxide (CuO), basic copper carbonate - CuCO 3 • C u(OH) 2 , ting on potassium perchlorate mixtures, has been published by and copper sulfate - available commercially as CuS0,, • 5H
Shimizu [131.
20 - are
162
Chemistry o f Pyrotechnics
'
Color and Light Production
163
TABLE 7.12 Purple Flame Compositions
Composition
% by weight
Commenta
I. Potassium perchlorate, KC10,, 70
"Excellent"
Polyvinyl chloride
10
Red gum
5
Copper oxide, CuO
6
Strontium carbonate, SrCO 3
9
Rice starch
5 (additional %)
II. Potassium perchlorate, KC10,,
70
"Excellent"
Polyvinyl chloride
10
Red gum
5
Copper powder, Cu
6
Strontium carbonate, SrCO 3
9
Rice starch
5 (additional %)
a Reference 13.
Purple Flame Compositions
A purple flame, a relative newcomer to pyrotechnics, can be achieved by the correct balance of red and blue emitters. The additive blending of these two colors produces a perception of purple by an observer. Several comprehensive review articles on purple flames have recently been published [131.
The compositions given in Table 7.12 received an "excellent"
rating in the review article written by Shimizu [131.
Yellow Flame Compositions
Yellow flame color is achieved by atomic emission from sodium.
The emission intensity at 589 nanometers increases as the reaction temperature is raised; there is no molecular emitting species here to decompose. Ionization of sodium atoms to sodium ions will occur at very high temperatures, however, so even here there is an upper limit of temperature that must be avoided for maximum color quality. The emission spectrum of a yellow flare is shown in Figure 7.2.
164
Chemistry of Pyrotechnics
Color and Light Production
165
TABLE 7.13 Yellow Flame Compositions
% by
Refer-
Composition
weight
Use
ence
I.
Potassium perchlorate, KC104
70
Yellow fire-
6
Sodium oxalate, Na 2C 204
14
works star
Red gum
6
Shellac
6
Dextrine
4
II.
Potassium perchlorate, KC10 4
75
Yellow fire
6
Cryolite, Na 3A1F6
10
Red gum
15
III. Sodium nitrate, NaN03
56
Yellow fire
5
Magnesium
17
(Russian)
Polyvinyl chloride
27
IV. Potassium nitrate, KNO 3
37
Yellow fire
5
Sodium oxalate, Na 2C 20y
30
(Russian)
Magnesium
30
Resin
3
FIG. 7.2 Emission spectrum of a yellow flare. The primary emit-V.
Barium nitrate, Ba(N0 3) 2
17
Yellow flare
8
ting species is atomic sodium, with intensity centered near 589 nm.
Strontium nitrate, Sr(N0 3) 2
16
A background continuum of "blackbody" emission and bands from Potassium perchlorate, KC104
17
vaporized BaO, BaOH, and BaCl are also observed. The compo-Sodium oxalate, Na 2C 20,,
17
sition of the flare was potassium perchlorate (21.0%), barium ni-Hexachlorobenzene, C 6 C16
12
trate (20.0%), magnesium (30.3%), sodium oxalate (19.8%), as-Magnesium
18
phaltum (3.9%), and binder (5.0%). This is apparently a former Linseed oil
3
green flare formula to which sodium oxalate was added to obtain a yellow flame. The intense atomic sodium emission at 589 nm overwhelms the green bands from barium-containing species!
Source: H. A. Webster III, "Visible Spectra of Standard Navy Colored Flares," Proceedings, Explosives and Pyrotechnics Appli-before, during, and after the manufacturing process. Sodium cations Section, American Defense Preparedness Association, Fort oxalate (Na
Worth, Texas, September, 1983.
2C 20 4 ) and cryolite (Na 3 AlF G ) are low in hygroscopicity and they are therefore the color agents used in most commercial yellow flame mixtures. Some representative yellow compositions are given in Table 7.13.
Most sodium compounds tend to be quite hygroscopic, and REFERENCES
therefore simple compounds such as sodium nitrate (NaNO 3 ), sodium chlorate (NaG10 3 ), and sodium perchlorate (NaC10,,) - com-1.
B. E. Douda, "Theory of Colored Flame Production," RDTN
bining the oxidizing anion with the metallic emitter - can not be No. 71, U.S. Naval Ammunition Depot, Crane, Indiana, 1964.
used unless precautions are taken to protect against moisture
166
Chemistry of Pyrotechnics
2.
K. L. Kosanke, "The Physics, Chemistry and Perception of Colored Flames," Pyrotechnica VII, Pyrotechnica Publications, Austin, Texas, 1981.
3.
B. E. Douda, "Spectral Observations in Illuminating Flames,"
Proceedings, First International Pyrotechnics Seminar, Denver Research Institute, Estes Park, Colorado, August, 1968, p. 113 (available from NTIS as AD 679 911).
4.
D. R. Dillehay, "Pyrotechnic Flame Modeling for Sodium D-Line Emissions," Proceedings, Fifth International Pyrotechnics Seminar, Denver Research Institute, Vail, Colorado, July, 1976, p. 123 (available from NTIS as AD A087 513).
5.
A. A. Shidlovskiy, Principles of Pyrotechnics, 3rd Ed. , Moscow, 1964. (Translated by Foreign Technology Division, Wright-Patterson Air Force Base, Ohio, 1974.) 6.
T. Shimizu in R. Lancaster's Fireworks Principles and Practice, Chemical Publishing Co., Inc., New York, 1972.
7.
U.S. Army Material Command, Engineering Design Handbook, Military Pyrotechnic Series, Part One, "Theory and Application," Washington, D.C., 1967 (AMC Pamphlet 706-185).
8.
F. L. McIntyre, "A Compilation of Hazard and Test Data for Pyrotechnic Compositions," Report ARLCD-CR-80047, U.S. Army Armament Research and Development Command, Dover, NJ, 1980.
9.
Pyrotechnica IV, Pyrotechnica Publications, Austin, Texas, 1978.
10.
R. M. Winokur, "The Pyrotechnic Phenomenon of Glitter,"
Pyrotechnica II, Pyrotechnica Publications, Austin, Texas, 1978.
11.
T. Shimizu, Fireworks - The Art, Science and Technique,
pub. by T. Shimizu, distrib. by Maruzen Co., Ltd., Tokyo, 1981.
12.
T. Shimizu, "Studies on Strobe Light Pyrotechnic Composi-A portion of the "finale" of a fireworks display. Several hundred tions," Pyrotechnica VIII, Pyrotechnica Publications, aerial shells are usually launched in a brief period of time to over-Austin, Texas, 1982.
whelm the senses of the audience. A Japanese "chrysanthemum"
13.
T. Shimizu, "Studies on Blue and Purple Flame Composi-shell with its characteristic large, symmetrical burst of color can tions Made With Potassium Perchlorate," Pyrotechnica VI, be seen near the center of the photograph. Several American aerial Pyrotechnica Publications, Austin, Texas, 1980.
shells, with their more-random bursting pattern, can also be seen.
14.
Pyrotechnica I, Pyrotechnica Publications, Austin, Texas, The bright "dots" of light seen in the picture are the bursts of 1977.
"salutes"; these are tubes containing "flash and sound" composition that explode to create a booming noise and a flash of light.
(Zambelli Internationale)
8SMOKE AND SOUND
SMOKE PRODUCTION
Most explosive and pyrotechnic reactions produce significant quantities of smoke, and this visible phenomenon may or may not be desirable. Smoke can obscure colored flames, and therefore attempts are made to keep the production of smoke to a minimum in such mixtures. However, a variety of smoke-producing compositions are purposefully manufactured for use in daytime signalling and troop and equipment obscuration, as well as for amuse-ment and entertainment purposes.
Two basic processes are used to create smoke clouds: the condensation of vaporized material and the dispersion of solid or liquid particles. Materials can either be released slowly via a pyrotechnic reaction or they can instantaneously be scattered using an explosive material. Technically, a dispersion of fine solid particles in air is termed a smoke, while liquid particles in air create a fog. A smoke is created by particles in the 10 -5-10-9 meter range, while larger suspended particles create a dust (1) .
A variety of events that will lead to smoke production can occur in the pyrotechnic flame. Incomplete burning of an organic fuel will produce a black, sooty flame (mainly atomic carbon). A highly-oxidized fuel such as a sugar is not likely to produce carbon. Materials such as naphthalene (C 10H 8) and anthracene ( C 1,,H 10 ) - volatile solids with high carbon content - are good candidates for soot production. Several mixtures that will produce black smokes are listed in Table 8. 1.
The heat from the reaction between an oxidizer and fuel can vaporize a volatile ingredient, with no chemical change occurring 167
168
Chemistry of Pyrotechnics
Smoke and Sound
169
phosphorus oxides, creates dense white smoke as the oxides atTABLE 8.1
Black Smoke Compositions
tract moisture to form acids such as phosphoric acid, H 3POa .
% by
Composition
weight
Reference
COLORED SMOKE MIXTURES
I.
Potassium chlorate, KC10 3
55
1
The generation of colored smoke by the volatilization of an or-Anthracene, C 1,,H10
45
ganic dye is a fascinating pyrotechnic problem. The military II. Potassium chlorate, KC1O 3
45
1
and the fireworks and entertainment industries rely on this tech-Naphthalene, C 10 H8
40
nique for the generation of copious quantities of brilliantly-col-Charcoal
15
ored smoke.
The requirements for an effective colored-smoke composition III. Potassium perchlorate, KC10 4
56
2
include
Sulfur
11
Anthracene, C1,,H 10
33
1. The mixture must produce sufficient heat to vaporize the IV. Hexachloroethane, C 2C16
62
2
dye, as well as produce a sufficient volume of gas to dis-Magnesium
15
perse the dye into the surrounding space.
Naphthalene (or anthracene)
23
2. The mixture must ignite at a low temperature and continue to burn smoothly at low temperature (well below 1000°C).
If the temperature is too high, the dye molecules will decompose and the color quality as well as volume of the smoke will deteriorate. Metal fuels are not used in col-in the vaporized material. The vaporized component, which was ored smoke mixtures because of the high reaction tempera-part of the original mixture, then condenses as fine, solid parti-tures they produce.
cles upon leaving the reaction zone and a smoke is created. Or-3. Although a low ignition temperature is required, the smoke ganic dyes, ammonium chloride, and sulfur can be used to create mixture must be stable during manufacturing and storage, smokes using this method.
over the expected range of ambient temperatures.
Alternately, the pyrotechnic reaction can occur in a separate 4. The molecules creating the colored smoke must be of low container, and the heat that is produced volatilizes a smoke-form-toxicity (including low carcinogenicity). Further, they ing component contained in an adjacent compartment. The vapor-must readily sublime without decomposition at the tem-ization and dispersion of heavy oils to create white smoke uses perature of the pyrotechnic reaction to yield a dense this technique.
smoke of good color quality [3].
Finally, a product of a pyrotechnic reaction may vaporize from the reaction zone and subsequently condense as fine particles in air, creating a smoke. Potassium chloride (boiling point 1407°C) When requirements that include low ignition temperature and produces smoke in many potassium chlorate and potassium per-reliable propagation of burning at low reaction temperature are chlorate compositions, although smoke is usually not a goal sought considered, the choice of oxidizer rapidly narrows to one candi-from these mixtures.
date - potassium chlorate, KC10 3 . The ignition temperature of A good white smoke can be obtained by the formation of zinc potassium chlorate combined with sulfur or many organic fuels chloride, ZnC1
is below 2500C. Good heat production is achieved with such mix-2, from a reaction between zinc metal and a chlorinated organic compound (the chlorine-containing species serves tures, in part due to the exothermic decomposition of KC1O 3 at a as the oxidizer). Reaction products that strongly attract mois-temperature below 400°C, forming KCl and oxygen gas.
ture (such as ZnCl
A mixture consisting of 70% KC1O
2 ) will have an enhanced smoke effect in humid 3 and 30% sugar ignites at
atmospheres. The burning of elemental phosphorus, producing 220°C and has a heat of reaction of approximately 0. 8 kcal /gram
170
Chemistry o f Pyrotechnics
Smoke and Sound
171
[5]. Both chlorate-sulfur and chlorate-sugar mixtures are used TABLE 8.2
Colored Smoke Compositions
in commercial colored smoke compositions. Sodium bicarbonate (NaHCO 3) is added to KC1O 3 /S mixtures to neutralize any acidic
% by
impurities that might stimulate premature ignition of the compo-Composition
weight
Reference
sition, and it also acts as a coolant by decomposing endothermi-cally to evolve carbon dioxide gas (CO 2) . Magnesium carbonate Green smoke
(MgCO 3 ) is also used as a coolant, absorbing heat to decompose Potassium chlorate, KC1O 3
25.4
8
into magnesium oxide (MgO) and C0 2. The amount of coolant Sulfur
10.0
can be used to help obtain the desired rate of burning and the Green dye
40.0
correct reaction temperature - if a mixture burns too rapidly, Sodium bicarbonate, NaHCO
more coolant should be added.
3
24.6
The ratio of oxidizer to fuel will also affect the amount of Red smoke
heat and gas that are produced. A stoichiometric mixture of Potassium chlorate, KC1O 3
29.5
8
KC1O
Lactose
18.0
3 and sulfur (equation 8.1) contains a 2.55:1 ratio of oxidizer to fuel, by weight. Colored smoke mixtures in use today Red dye
47.5
contain ratios very close to this stoichiometric amount.
The
Magnesium carbonate, MgCO 3
5.0
chlorate /sulfur reaction is not strongly exothermic, and a New yellow smoke
stoichiometric mixture is needed to generate the heat necessary Potassium chlorate, KC1O
to volatilize the dye.
3
22.0
4
Sucrose
15.0
2 KC1O
Chinoline yellow dye
42.0
3 + 3 S -> 3 SO 2 + 2 KC1
(8.1)
Magnesium carbonate, MgCO 3
21.0
grams
245
96
%
71.9
28.1
(a 2.55 to 1.00 ratio)
The reaction of potassium chlorate with a carbohydrate (e.g. , lactose) will produce carbon monoxide (CO), carbon dioxide (CO2 ) or a mixture depending on the oxidizer:fuel ratio.
The balanced
equations are given as equations 8.2 and 8. 3. (Lactose occurs The amount of heat can be controlled by adjusting the KC1O 3 : as a hydrate - one water molecule crystallizes with each lactose sugar ratio. Excess oxidizer should be avoided; it will encourage molecule.)
oxidation of the dye molecules. The quantity (and volatility) of CO
the dye will also affect the burning rate. The greater the quan-2 Product
tity of dye used, the slower will be the burning rate - the dye 8 KC10 3 + C12H22011'H20 - 8 KCI + 12 C0 2 + 12 H2O (8.2) is a diluent in these mixtures. Typical colored smoke compositions grams
980
360.3
contain 40-60% dye by weight. Table 8. 2 shows a variety of colored smoke compositions.
%
73.1
26.9
(2.72 to 1.00 ratio)
In colored smoke compositions, the volatile organic dye sub-Heat of reaction = 1.06 kcal /gram .[ 1 ]
limes out of the reacting mixture and then condenses in air to form small solid particles. The dyes are strong absorbers of CO Product:
visible light. The light that is reflected off these particles is 4 KC1O
missing the absorbed wavelengths, and the complementary hue 2 + C12H22O11 • H ZO -
4 KCl + 12 CO + 12 H 2O (8. 3)
is perceived by observers. This color-producing process is dif-grams
490
360.3
ferent from that of colored flame production, where the emitted
%
57.6
42.4
(1.36 to 1.00 ratio)
wavelengths are perceived as color by viewers. Table 7.6 lists the complementary colors for the various regions of the visible Heat of reaction = 0.63 kcal/gram [1]
spectrum.
172
Chemistry of Pyrotechnics
Smoke and Sound
173
A variety of dyes have been used in colored smoke mixtures; TABLE 8.3 Dyes for Colored Smoke Mixtures
many of these dyes are presently under investigation for carcinogenicity and other potential health hazards because of their mo-Orange 7
Solvent green 3
lecular similarity to known "problem" compounds [4]. The ma-a-xylene-a zo- S-naphthol
1, 4-di-p -toluidino-anthraquinone
terials that work best in colored smokes have several properties in common, including
1. Volatility: The dye must convert to the vapor state on heating, without substantial decomposition. Only low molecular weight species (less than 400 grams/mole) are usually used - volatility typically decreases as molecular weight increases. Salts do not work well; ionic species generally have low volatility due to the strong inter-ionic attractions present in the crystalline lattice. Therefore, functional groups such as -COO - (carboxylate ion) and
- NR +
3
(a substituted ammonium salt) can not be present.
2.
Chemical stability: Oxygen-rich functional groups (-NO 21
-SO3H) can't be present.
At the typical reaction tem-
peratures of smoke compositions, these groups are likely Disperse red 9
Violet
to release their oxygen, leading to oxidative decomposi-1-methylamino-anthraquinone
1,4-diamino-2,3-dihydroanthraquinone
tion of the dye molecules. Groups such as -NH and -NHR
2
(amines) are used, but one must be cautious of possible oxidative coupling reactions that can occur in an oxygen-rich environment.
Structures for some of the dyes used in colored smoke mixtures are given in Table 8.3.
WHITE SMOKE PRODUCTION
Chinoline yellow
Vat yellow 4
The processes used to generate a white smoke by means of a pyro-2-( 2-quinolyl)-1 , 3-indandione
dibenzo(a,h)pyrene-7,14-dione
technic reaction include:
0
1. Sublimation of sulfur, using potassium nitrate as the oxidizer: A 1:1 ratio of sulfur to KNO 3 is used in such mixtures. Caution: some toxic sulfur dioxide gas will be formed. Ignition of these mixtures must be done in a well-ventilated area.
2. Combustion of phosphorus: White or red phosphorus burns to produce various oxides of phosphorus, which then attract moisture to form dense white smoke. Research and
174
Chemistry of Pyrotechnics
Smoke and Sound
175
TABLE 8.4 White Smoke Compositions
4.
Formation of zinc chloride ("HC Smokes"): A reaction of
the type
by
Refer-
C x Cly + y/2 Zn } x C + y/2 ZnC1 2 + heat
Composition
weight
Note
ence
produces the zinc chloride vapor, which condenses in air I.
Hexachloroethane, C
and attracts moisture to create an effective white smoke.
2C1 6
45.5
HC type C
6
Zinc oxide, ZnO
47.5
These mixtures have been widely used for over forty years Aluminum
7.0
with an excellent safety record during the manufacturing process.
However, ZnC1 2 can cause headaches upon con-II.
Hexachlorobenzene, C 6 C1 6
34.4
Modified HC
6
tinued exposure and replacements for the HC smokes are Zinc oxide, ZnO
27.6
actively being sought due to health concerns relating to Ammonium perchlorate, NH,,C10,,
24.0
the various reaction products.
Zinc dust
6.2
The original HC smoke mixtures (Type A) contained Laminac
7.8
zinc metal and hexachloroethane, but this composition is III.
Red phosphorus
63
Under de-
extremely moisture- sensitive and can ignite spontaneously 8
if moistened.
An alternative approach involves adding a
Butyl rubber, methylene
37
velopment
small amount of aluminum metal to the composition, and chloride
zinc oxide (ZnO) is used in place of the moisture-sensi-IV.
Red phosphorus
51.0
4
tive metal.
Upon ignition, a sequence of reactions en-
Magnesium
10.5
sues of the type [6]
Manganese dioxide, MnO,
32.0
Magnesium oxide, MgO
1.5
2 Al + C 2 C1 6 } 2 AIC1 3 + 2 C
(8.4)
Microcrystalline wax
5.0
2 A1C1 3 + 3 ZnO -> 3 ZnC1 2 + A1 2 0 3
(8.5)
V.
Potassium nitrate, KNO 3
48.5
Contains
9
ZnO + C -* Zn + Co
(8.6)
Sulfur
48.5
arsenic
Arsenic disulfide, As
3 Zn + C
2 S 2
3.0
2 C1 6 - 3 ZnCl 2 + 2 C
(8.7)
Alternatively, the original reaction has been proposed to be [ 7]
2 Al + 3 ZnO - 3 Zn + A1 2O 3
(8.8)
In either event, the products are ZnCl
development work relating to red phosphorus-based smoke 2 , CO, and A1 20 3 .
The zinc oxide cools and whitens the smoke by consuming mixtures is actively being pursued to find substitutes for atomic carbon in an endothermic reaction that occurs spon-the zinc chloride smokes. A typical red phosphorus mix-taneously above 1000°C (equation 8.6). The reaction with ture is given in Table 8.4. An explosive bursting charge aluminum (equation 8.4 or 8.8) is quite exothermic, and is often used with the very-hazardous white phosphorus.
this heat evolution controls the burning rate of the smoke Caution :
Phosphorus-based smokes generate acidic com-
mixture.
A minimum amount of aluminum metal will yield pounds which may be irritating to the eyes, skin, and the best white smoke. Several "HC" smoke compositions respiratory tract.
are listed in Table 8.4.
3.
Volatilization of oil:
A pyrotechnic reaction produces the
5.
"Cold Smoke":
White smoke can also be achieved by non-
heat needed to vaporize high molecular weight hydrocar-thermal means. A beaker containing concentrated hydro-bons. The subsequent condensation of this oil in air cre-chloric acid placed near a beaker of concentrated ammonia ates a white smoke cloud. The toxicity of this smoke is will generate white smoke by the vapor-phase reaction probably the least of all the materials discussed here.
176
Chemistry of Pyrotechnics
Smoke and Sound
177
HC1 (gas) + NH 3 (gas) -> NH,,Cl (solid)
TABLE 8.5 "Flash and Sound" Compositionsa Similarly, titanium tetrachloride (TiC1 4) rapidly reacts with moist air to produce a heavy cloud of titanium hy-
% by
Refer-
droxide - Ti(OH)
Composition
weight
Use
ence
4 - and HC1.
I.
Potassium perchlorate,
50
Military simulator
8
KC1O,,
NOISE
Antimony sulfide,
33
Two basic audible effects are produced by explosive and pyro-Sb 2S 3
technic devices: a loud explosive noise (called a "report" or Magnesium
17
"salute" in the fireworks industry) and a whistling sound.
II. Potassium perchlorate,
64
M-80 firecracker for
8
A report is produced by igniting an explosive mixture, usually KC1O,,
military training
under confinement in a heavy-walled cardboard tube. Potassium Aluminum
22.5
chlorate and potassium perchlorate are the most commonly used Sulfur
10
oxidizers for report composi
s , which are also referred to as
Antimony sulfide,
3.5
"flash and sound" mixtures.
hese mixtures produce a flash of
Sb 2S 3
light and a loud "bang" upon ignition. Black powder under substantial confinement also produces a report.
III. Potassium chlorate,
43
Japanese "flash thun- 5
"Flash and sound" compositions are true explosives, and they KC1O 3
der" for aerial fire-
will detonate if a sufficient quantity of powder (perhaps 100
Sulfur
26
works
Aluminum
grams or more) is present in bulk form, even if unconfined!
31
Chlorate-based mixtures are considerably more hazardous than IV. Potassium perchlorate,
50
Japanese "flash thun- 5
perchlorate compositions because of their substantially lower ig-KCIO,,
der" for aerial fire-
nition temperatures. However, flash and sound compositions Sulfur
27
works
made with either oxidizer must be considered very dangerous.
Aluminum
23
They have killed many people at fireworks manufacturing plants in the United States and abroad. Mixing should only be done using remote means, and the smallest feasible amount of com-aNote: These mixtures are explosive and very dangerous. They position should be prepared at one time. Bulk flash and sound must only be prepared by trained personnel using adequate pro-powder must never be stored anywhere near operating person-tection, and should be mixed by remote means.
nel.The famous Chinese firecracker uses a mixture of potassium chlorate, sulfur, and aluminum. The chlorate combined with sulfur makes this mixture doubly dangerous for the manufacturer.
The standard American flash and sound composition is a blend The ignition temperature of the potassium chlorate/sulfur system of potassium perchlorate, sulfur or antimony sulfide, and alu-is less than 200°C! The presence of aluminum - an excellent minum. The ignition temperature of this formulation is several fuel - guarantees that the pyrotechnic reaction will rapidly prop-hundred degrees higher than chlorate-based mixtures, but these agate once it begins. Safety data from China is unavailable, but are still very dangerous compositions because of their extreme one has to wonder how many accidents occur annually from the sensitivity to spark and flame. Ignition of a small portion of a preparation of this firecracker composition. The preparation of
"flash and sound" mixture will rapidly propagate through the en-potassium chlorate/sulfur compositions was banned in Great Britain tire sample. These mixtures should only be prepared remotely, in 1894 because of the numerous accidents associated with this mix-by experienced personnel. Table 8.5 lists several "flash and ture!
sound" formulas.
178
Chemistry of Pyrotechnics
Smoke and Sound
179
TABLE 8.6 Whistle Compositionsa
be stored near operating personnel. Several formulas for whistle compositions are given in Table 8.6.
% by
Refer-
Composition
weight
Note
ence
REFERENCES
I.
Potassium chlorate
73
Military simulator
8
KC1O
1.
3
A. A. Shidlovskiy, Principles of Pyrotechnics, 3rd Ed. , Gallic Acid, C 7 H 6 0 5 .
24
Moscow, 1964. (Translated by Foreign Technology Divi-H ,O
sion, Wright-Patterson Air Force Base, Ohio, 1974.) Red gum
3
2.
T. Shimizu in R. Lancaster's Fireworks Principles and Practice, Chemical Publishing Co. , Inc. , New York, 1972.
II.
Potassium perchlorate,
70
Perhaps the safest
5
3.
A. Chin and L. Borer, "Investigations of the Effluents KC10,,
to prepare and use
Produced During the Functioning of Navy Colored Smoke Potassium benzoate,
30
Devices," Proceedings, Eighth International Pyrotechnics KC 7 H 5 0 2
Seminar, IIT Research Institute, Steamboat Springs, III.
Potassium perchlorate,
75
Hygroscopic-does
5
Colorado, July, 1982, p. 129.
KC1O,,
not store well
4.
M. D. Smith and F. M. Stewart, "Environmentally Accep-Sodium salicylate,
25
table Smoke Munitions," Proceedings, Eighth International NaC
Pyrotechnics Seminar,
7 H 5 O 3
IIT Research Institute, Steamboat
Springs, Colorado, July, 1982, p. 623.
IV.
Potassium perchlorate,
75
Chinese whistle
Unpub-
5.
T. Shimizu, Fireworks - The Art, Science and Technique, KC10,
composition
lished
pub. by T. Shimizu, distrib. by Maruzen Co., Ltd., Tokyo, Potassium hydrogen
25
1981.
phthalate, KC B H 5 O 4
6.
U.S. Army Material Command, Engineering Design Handbook, Military Pyrotechnic Series, Part One, "Theory and a
Application," Washington, D.C. , 1967 (AMC Pamphlet 706-Note:
These mixtures are very sensitive to ignition and can be 185).
quite dangerous to prepare. They should only be mixed by trained 7.
J. H. McLain, Pyrotechnics from the Viewpoint personnel using adequate protection.
of Solid
State Chemistry, The Franklin Institute Press, Philadelphia, Penna., 1980.
8.
F. L. McIntyre, "A Compilation of Hazard and Test Data for Pyrotechnic Compositions," Report ARLCD-CR-80047, Whistles
U.S. Army Armament Research and Development Command, Dover, NJ, 1980.
A unique, whistling phenomenon can be produced by firmly press-9.
R. Lancaster, Fireworks Principles and Practice, Chemical ing certain oxidizer/fuel mixtures into cardboard tubes and ig-Publishing Co., Inc., New York, 1972.
niting the compositions.
A detailed analysis of this phenomenon,
10.
W. R. Maxwell, "Pyrotechnic Whistles," 4th Symposium on both from a chemical and physical view, has been published by Combustion, Williams and Wilkins, Baltimore, Md., 1953, Maxwell [10].
p. 906.
A reaction that produces a whistling effect is burning intermit-tently from layer to layer in the pressed composition. A whistling reaction is on the verge of an explosion, so these mixtures must be cautiously prepared and carefully loaded into tubes. Large quantities of bulk powder should be avoided, and they should never
APPENDIXES
APPENDIX A: OBTAINING PYROTECHNIC
LITERATURE
Many of the technical reports and publications referenced in this book are available through the U.S. Department of Commerce's National Technical Information Service (NTIS) located in Springfield, Virginia.
Publications can be ordered from NTIS if the "accession numbers" are known; these are the numbers assigned by NTIS to technical documents in their files.
NTIS can supply you with an
"accession number" if you provide them with the title and author of a document. Current prices, order forms, accession numbers, and other needed information can be obtained from National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
NTIS numbers for several of the major references used in this book are:
A. A. Shidlovskiy, Principles of pyrotechnics, 3rd Edition.
NTIS # AD-A001859
Military Pyrotechnic Series, Part I, "Theory and Application."
NTIS # AD-817071
181
182
Chemistry of Pyrotechnics
Appendix
183
Military Pyrotechnic Series, Part III, "Properties of Materials may result. Weigh out the proper amount of each component and Used in Pyrotechnic Compositions." NTIS # AD-830394
combine the materials in the mortar. Carefully mix them together F. L. McIntyre, "A Compilation of Hazard and Test Data for with the soft brush to obtain a homogeneous blend. Caution: Do Pyrotechnic Compositions." NTIS # AD-A096248
not prepare more than 2 grams of any composition for evaluation purposes using this procedure.
In addition, copies of the various Proceedings of the Interna-Place a small pile of the mixed composition on the fireproof tional Pyrotechnics Symposia are available for purchase from the board, insert a section of safety fuse into the base of the pile, host organization, IIT Research Institute.
and carefully light the end of the fuse with a match. Step back For prices and ordering information, contact and observe the effect. Because of the generation of smoke by most pyrotechnic compositions, these tests are best conducted outdoors or in a well-ventilated area such as a laboratory fume Dr. Allen J. Tulis
hood. Be certain no flammable materials are near the test area, IIT Research Institute
for sparks may be produced.
10 West 35th Street
All testing of pyrotechnic compositions must be carried out Chicago, IL 60616
under the direct supervision of a responsible adult well trained in standard laboratory safety procedures. Serious injury can Information regarding availability, prices, and ordering of the result from working with larger amounts of composition or from Pyrotechnica publications can be obtained from the misuse of pyrotechnic mixtures, so caution and adequate supervision are mandatory. Warning: Do not attempt to prepare any of the explosive mixtures listed in Tables 8.5 or 8.6.
Mr. Robert G. Cardwell
These must be mixed only by remote means, or serious injuries Editor and Publisher
might result. The color-producing compositions listed in Tables 2302 Tower Drive
7.9-7.13 are recommended as a good starting point for persons Austin, TX 78703
preparing their first pyrotechnic compositions. The effects caused by variations from the specified percentages can easily be seen upon burning.
APPENDIX B: MIXING TEST QUANTITIES OF
PYROTECHNIC COMPOSITIONS
The pyrotechnic chemist always begins with a very small quantity of composition when carrying out initial experiments on a new formula. The preparation of one or two grams of a new mixture enables one to evaluate performance (color quality and intensity, smoke volume, etc.) without exposure to an unduly hazardous amount of material.
Eye protection -- safety glasses or goggles - is mandatory whenever any pyrotechnic composition is being prepared or tested. Necessary equipment includes a mortar and pestle, a laboratory balance, a soft bristle brush, several 2-3 inch lengths of fireworks-type safety fuse (available from many hobby stores), and a fireproof stone or composite slab on which to conduct burning tests.
Pre-grind the components individually to fine particle size.
Do not grind any oxidizer and fuel together - fire or explosion
I NDEX
A-lA composition, 134
Arsenic disulfide, 73
Acids, 38-39
Atomic weight, 7
catalytic ability of, 39
table of, 9-11
reactions of, 38-39
Atoms, theory of, 7-11
Aluminum
for "flitter" effect, 150
in delay mixtures, 130
Ballistite, 42
in white light mixtures, 144
Barium carbonate, 159
manufacture of, 4
Barium chlorate, 62
oxidation of, 23
for green flames, 153, 158
properties of, 65, 101, 108, 147
Barium chromate, 130, 132
reaction with nitrates, 67
with boron, 131
varieties of, 67
Barium nitrate
with metal oxides, 115
for green flames, 153, 158
Aluminum oxide, 118
in sparklers, 117
Ammonium chlorate, 58, 61
in white-light compositions, 144
Ammonium chloride
properties of, 62
for white smoke, 58, 168
thermal decomposition of, 62
with potassium chlorate, 58
Barium peroxide, 130
Ammonium perchlorate
Bases, 38-39
explosive behavior of, 61, 94
Binders
in color compositions, 153, 161
effect on flame temperature, 119
in propellants, 60
materials used as, 79
properties of, 60
selection of, 79, 153
thermal decomposition of, 60
Black body radiation, 47, 153
Antimony trisulfide, 73, 144, 177
Black powder
Arrhenius equation, 28, 106
as a delay, 128-129
185
186
Index
Index
187
[Black powder]
Confinement
"Flash and sound" mixtures,
Ignition
as a propellant, 136-137
effect on burning, 50, 92
110, 176, 177
compositions for, 133, 135
as an igniter, 126
storage implications of, 93
Flitter, 149-150
events during, 97-98
burning rate of, 114, 115, 117
Consumer Product Safety Com-
Formula weight, 16
factors affecting, 101
composition of, 1
mission, 73
Free energy, 21, 23
melting effect on, 98
factories, 3
Copper compounds, blue flames
Friction igniter, 127-128
systems for, 88
gas production by, 33
with, 160-161
Fuels
Ignition temperature, 97
history of, 3-5
Cordite, 138
metals used as, 65
determination of, 105-107
properties of, 1-2
Critical mass, 93
properties of, 66, 71, 75
tables of, 57, 100, 108, 109
thermogram of, 43
Cryolite, 165
requirements for, 64
Ions, 8, 11-12
Blue flame compositions, 160-162
Crystals, 35-39
selection of, 65
Iron, 69, 130, 147
Bond
diffusion in, 99
Fuse, 125
Iron oxide, 63, 109, 115, 130,
covalent, 12
"looseness" of, 99
Fusee, 127, 154
134
energy-rich, 30
Boric acid, 39, 67
Boron
Deflagration, 2
Lactose, 78
as a fuel, 72, 129, 130
Delay mixtures, 126, 128-133
"Gasless" compositions, 114,
Lances, 154
properties of, 72
Detonation, 2
129, 132
Lead chromate, 109, 130, 132
with potassium nitrate, 134
Dextrine, 78
Gases
Lead mononitroresorcinate, 126
Burning, rate of, 84, 113-117
Differential thermal analysis, 40-
equation for, 32-33
Light
stages of, 99
41
generators of, 140
energy of, 46
table of, 112
Dipoles, 14
ideal, 32
frequency of, 45
DTA (see Differential thermal
Glitter, 149-151
speed of , 4 5
analysis)
Glucose, combustion of, 74
theory of, 42-48
Carbohydrates (see also
Dyes, for colored smokes, 171-
Granulation, 95
wavelength of, 45
Sugars), 77
173
Greek fire, 3
Green flame compositions,
Liquids, 34
Charcoal, 77
157-160
as a fuel, 155
emission spectrum of, 87
effect on sensitivity, 108
Electrochemistry, 20
Magnalium, 69, 144, 147, 148
standard potentials for, 22
sparks from, 147
Magnesium
Electromagnetic spectrum, 45
Chlorate ion, 39
as a fuel, 68, 113
Chromic oxide, 115
Electronegativity, 13, 30
HC smoke, 175
in color compositions, 154
Electrons, transfer of, 18
Chlorine
Heat, production of, 125-128
manufacture of, 4
Endothermic process, 23
donors, 156
Heat of formation, 23
oxidation of, 47
Energy of activation, 28
for color intensity, 155
table of, 26-27
properties of, 68-69, 101, 108
Enthalpy, 21
Chromic oxide, 115
Heat of reaction, 23-25, 84
sparks from, 148
Entropy, 21, 23, 31
Color
table of, 85
with acids, 68
Exothermic process, 23
complementary, 150, 171
Henkin and McGill method,
with potassium perchlorate, 19
intensifiers for, 155
105-107, 110-111
with sodium nitrate, 143-144
production of, 150-165
Firecracker, 176
Hexachloroethane, 175
Magnesium carbonate, 39, 57, 80
temperature effect on, 154
Fireworks, history of, 4-6
Highway flare (see Fusee)
Magnesium oxide, 118, 121
Colored smoke compositions,
First fire, 126, 133, 135
Hydrocarbon, 74
Manganese dioxide, 58, 115
58-59
Flame, temperature of, 117-118,
Hydrogen chloride, 156
Match, electric, 126
theory of, 169-172
120-121, 154-155
Hygroscopicity, 51
Melting point, 35
188
Index
Index
189
Mixing pyrotechnic composi-
"Photoflash" mixtures, 145-148
Propagation Index, 122-123
Sodium oxalate, 154, 155, 165
tions, 182
Photon, energy of, 46
Propellants, 136-140
Solids, nature of, 35-36
Mixture, stoichiometric, 17
Potassium chlorate
for large rockets, 139
Solubility, factors in, 14
Moisture 88
acid with, 57
requirements for, 64
Space Shuttle, 139
Mole concept, 15-17
discovery of, 4
Purple flame compositions,
Sparkler, 62, 117, 148
Molecular weight, 16
hazards of, 59, 109, 134
163
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Отборные классические книги
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ВКОНТАКТЕБиблиотека с любовными романами, которая наверняка придётся по вкусу женской части аудитории
Любовные романы
Библиотека с фантастикой и фэнтези, а также смежных жанров
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