pyrotechnic compositions, especially with active metal fuels. Ex-Potassium nitrate has the additional property of not undergoing amples of this are the use of hexachloroethane (C
an explosion by itself, even when very strong initiating modes are 2 C1 6 ) with zinc
metal in white smoke compositions,
used [2].
3 Zn + C2C1 6 -> 3 ZnC12 + 2 C
Potassium Chlorate (KCIO 3)
and the use of Teflon with magnesium metal in heat-producing mixtures,
One of the very best, and certainly the most controversial, of the common oxidizers is potassium chlorate, KC1O 3 . It is a white, (C 2F,,) n + 2n Mg -} 2n C + 2n MgF 2 + heat crystalline material of low hygroscopicity, with 39.2% oxygen by In both of these examples, the metal has been "oxidized" - has weight. It is prepared by electrolysis from the chloride salt.
lost electrons and increased in oxidation number -- while the car-Potassium chlorate was used in the first successful colored-bon atoms have gained electrons and been "reduced."
flame compositions in the mid-1800's and it remains in wide use Table 3.2 lists some of the common oxidizers together with a today in colored smoke, firecrackers, toy pistol caps, matches, variety of their properties.
and color-producing fireworks.
Several oxidizers are so widely used that they merit special However, potassium chlorate has been involved in a large consideration. A few excellent books are available that provide percentage of the serious accidents at fireworks manufacturing
56
Chemistry of Pyrotechnics
Components of High-Energy Mixtures
57
plants, and it must be treated with great care if it is used at all.
TABLE 3.3 Ignition Temperatures of Potassium Other oxidizers are strongly recommended over this material, if Chlorate/Fuel Mixtures
one can be found that will produce the desired pyrotechnic ef-Ignition temperature of
fect.Potassium chlorate compositions are quite prone to accidental stoichiometric mixture,
ignition, especially if sulfur is also present. Chlorate /phosphor-Fuel
Ca
us mixtures are so reactive that they can only be worked with when quite wet. The high hazard of KC1O
Lactose, C1211 22011
195
3 mixtures was grad-
ually recognized in the late 19th century, and England banned Sulfur
220
all chlorate /sulfur compositions in 1894. United States factories have greatly reduced their use of potassium chlorate as well, Shellac
250
replacing it with the less-sensitive potassium perchlorate in Charcoal
335
many formulas. The Chinese, however, continue to use potassium chlorate in firecracker and color compositions. Details on Magnesium powder
540
their safety record are not available, although several accidents Aluminum powder
785
are known to have occurred at their plants in recent years.
Several factors contribute to the instability of potassium chlor-Graphite
890
ate-containing compositions. The first is the low (356°C) melting point and low decomposition temperature of the oxidizer. Soon aReference 1.
after melting, KC1O 3 decomposes according to equation 3.1.
2 KC10
} 2 KC1 + 3 02
(3.1)
3
This reaction is quite vigorous, and becomes violent at temperatures above 500°C [2]. The actual decomposition mechanism may be more complex than equation 3.1 suggests. Intermediate temperatures are observed for most such compositions. Higher formation of potassium perchlorate has been reported at tempera-ignition temperatures are found for KC1O 3 /metal mixtures, at-tures just above the melting point, with the perchlorate then de-tributable to the higher melting points and rigid crystalline lat-composing to yield potassium chloride and oxygen [5].
tices of these metallic fuels. However, these mixtures can be quite sensitive to ignition because of their substantial heat out-4 KC1O 3 -} 3 KC10 4 + KCl
put, and should be regarded as quite hazardous. Ignition tem-3 KC10,, - 3 KC1
+ 60,
peratures for some KC1O 3 mixtures are given in Table 3.3. Note: Ignition temperatures are quite dependent upon the experimental net: 4 KC1O 3 ; 4 KCl + 6 0 2
conditions; a range of +/-50 0 may be observed, depending on The decomposition reaction of potassium chlorate is rare among sample size, heating rate, degree of confinement, etc. [6].
the common oxidizers because it is exothermic, with a heat of re-Mixtures containing potassium chlorate can be quite suscep-action value of approximately -10.6 kcal/mole [ 2]. While most tible to the presence of a variety of chemical species. Acids other oxidizers require a net heat input for their decomposition, can have a dramatic effect - the addition of a drop of concen-potassium chlorate dissociates into KC1 and 0
trated sulfuric acid (H 2SO 4) to most KCIO 3 /fuel mixtures results 2 with the liberation
in immediate inflammation of the composition. This dramatic re-of heat. This heat output can lead to rate acceleration, and allows the ignition of potassium chlorate-containing compositions activity has been attributed to the formation of chlorine dioxide (C10
with a minimum of external energy input (ignition stimulus).
2 ) gas, a powerful oxidizer [5]. The presence of basic Potassium chlorate is particularly sensitive when mixed with
"neutralizers" such as magnesium carbonate and sodium bicarbonate in KC1O
sulfur, a low-melting (119°C) fuel. It is also sensitive when 3 mixtures can greatly lower the sensitivity of combined with low-melting organic compounds, and low ignition these compositions to trace amounts of acidic impurities.
Ah' AL_
5 8
Chemistry of Pyrotechnics
Components of High-Energy Mixtures
59
The ability of a variety of metal oxides -- most notably man-through an endothermic decomposition, in the flame, of the ganese dioxide, Mn0 2 - to catalyze the thermal decomposition of type
potassium chlorate into potassium chloride and oxygen has been known for years. Little use is made of this behavior in pyro-heat
MgCO3
MgO + CO2
technics, however, because KC10 3 is almost too reactive in its normal state and ways are not needed to enhance its reactivity.
Colored smoke mixtures also contain either sulfur or a carbohy-Materials and methods to retard its decomposition are desired drate as the fuel, and a volatile organic dye that sublimes from instead. However, knowledge of the ability of many materials to the reaction mixture to produce the colored smoke. These com-accelerate the decomposition of KC10