COLORED SMOKE MIXTURES
The generation of colored smoke by the volatilization of an organic dye is a fascinating pyrotechnic problem. The military and the fireworks and entertainment industries rely on this technique for the generation of copious quantities of brilliantly-colored smoke.
The requirements for an effective colored-smoke composition include
1. The mixture must produce sufficient heat to vaporize the dye, as well as produce a sufficient volume of gas to disperse the dye into the surrounding space.
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 colored smoke mixtures because of the high reaction temperatures they produce.
3. Although a low ignition temperature is required, the smoke mixture must be stable during manufacturing and storage, over the expected range of ambient temperatures.
4. The molecules creating the colored smoke must be of low toxicity (including low carcinogenicity). Further, they must readily sublime without decomposition at the temperature of the pyrotechnic reaction to yield a dense smoke of good color quality.
When requirements that include low ignition temperature and reliable propagation of burning at low reaction temperature are considered, the choice of oxidizer rapidly narrows to one candidate - potassium chlorate, KC103 . The ignition temperature of potassium chlorate combined with sulfur or many organic fuels is below 2500C. Good heat production is achieved with such mixtures, in part due to the exothermic decomposition of KC1O 3 at a temperature below 400°C, forming KCl and oxygen gas.
A mixture consisting of 70% KC1O 3 and 30% sugar ignites at 220°C and has a heat of reaction of approximately 0. 8 kcal /gram. Both chlorate-sulfur and chlorate-sugar mixtures are used in commercial colored smoke compositions. Sodium bicarbonate (NaHCO 3) is added to KC1O 3 /S mixtures to neutralize any acidic impurities that might stimulate premature ignition of the composition, and it also acts as a coolant by decomposing endothermically to evolve carbon dioxide gas (CO 2) . Magnesium carbonate (MgCO 3 ) is also used as a coolant, absorbing heat to decompose into magnesium oxide (MgO) and C0 2. The amount of coolant can be used to help obtain the desired rate of burning and the correct reaction temperature - if a mixture burns too rapidly, more coolant should be added.
The ratio of oxidizer to fuel will also affect the amount of heat and gas that are produced. A stoichiometric mixture of KC1O 3 and sulfur contains a 2.55:1 ratio of oxidizer to fuel, by weight. Colored smoke mixtures in use today contain ratios very close to this stoichiometric amount. The chlorate /sulfur reaction is not strongly exothermic, and a stoichiometric mixture is needed to generate the heat necessary to volatilize the dye.
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 as a hydrate - one water molecule crystallizes with each lactose molecule.)
The amount of heat can be controlled by adjusting the KC1O 3 : sugar ratio. Excess oxidizer should be avoided; it will encourage oxidation of the dye molecules. The quantity (and volatility) of the dye will also affect the burning rate. The greater the quan- tity of dye used, the slower will be the burning rate - the dye is a diluent in these mixtures. Typical colored smoke compositions contain 40-60% dye by weight. Table 1 shows a variety of colored smoke compositions.
TABLE 1. Colored Smoke Compositions
In colored smoke compositions, the volatile organic dye sublimes out of the reacting mixture and then condenses in air to form small solid particles. The dyes are strong absorbers of visible light. The light that is reflected off these particles is missing the absorbed wavelengths, and the complementary hue is perceived by observers. This color-producing process is different from that of colored flame production, where the emitted wavelengths are perceived as color by viewers.
A variety of dyes have been used in colored smoke mixtures; many of these dyes are presently under investigation for carcino- genicity and other potential health hazards because of their molecular similarity to known "problem" compounds. The materials 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 interionic 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 to release their oxygen, leading to oxidative decomposition of the dye molecules. Groups such as -NH2 and -NHR (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 2.
TABLE 2. Dyes for Colored Smoke Mixtures