Chlorine Donor Pyrotechnic Chemicals
by Charley WilsonIn "Fireworks, the Art, Science, and Technique," Takeo Shimizu gives a very good explanation of color production. For blue, the color-producing firework chemical is thought to be copper monochloride, a molecular species that only exists in the colored flame. (This is NOT the same as cuprous chloride, CuCl). If there is no free chlorine in the flame, there can be no blue color from copper. Copper oxide, hydroxide, carbonate, and other copper compounds in a colored flame will emit green and red bands of light, and hardly any light in the blue spectrum.
To complicate matters, the cupric monochloride is destroyed at high temperatures, so to produce a good blue firework star the colored flame must be relatively cool by pyrotechnic standards. This is another reason why blue firework star compositions should not contain metal fuel.
The discovery of colors produced by chlorine along with other elements belongs to Chertier, who described the phenomena in 1836. As electricity began to become widely available, so did the chlorates to be used as oxidizers for color compositions. Potassium chlorate can be considered to be somewhat of a chlorine donor during combustion, which allows free chlorine to combine with other elements in the colored flame.
Where potassium perchlorate or a nitrate is used as the oxidizer, some supplemental chlorine must be included in the firework star composition. In years past, mercurous chloride, also known as calomel, was used. Calomel decomposes at ordinary flame temperatures into chlorine and mercury metal vapor. Even when potassium chlorate was used as the oxidizer in older blue firework star compositions, calomel was often included to insure that plenty of chlorine was available. Calomel was relatively safe to handle because of its very low solubility, and was even used medicinally.
Today we know that mercury is a very bad actor in the environment, witnessed by the birth defects in Japan resulting from mercury poisoning, not to mention the deaths! Unfortunately, the mechanism of calomel’s production of chlorine is unique.
Other metal chlorides have been tried in an attempt to mimic calomel. The alkaline earth metal chlorides typically have such a high decomposition temperature that they yield no free chlorine. Notably Tessier in good blue and blue related firework star compositions, with similar environmental problems, has used lead chloride.
Other means of supplying chlorine in the colored flame include organic compounds that contain chlorine. It is perhaps ironic that some of these chlorinated hydrocarbons are as nasty to the environment as mercury, perhaps even worse.
After all, the broad class of insecticides that includes DDT is chlorocarbons. One such insecticide is Mirex, also known as Dechlorane, with an empirical formula of C10Cl12. Although no longer manufactured in the United States, it is in use as a chlorine donor for pyrotechnics today. As "Dechlorane", the product was intended for use in plastics as a fire retardant. The now infamous Hooker Chemical Company, from 1957 to 1976, produced Dechlorane.
When an otherwise flammable plastic contains a chlorine donor along with antimony oxide, charring is promoted which in turn inhibits flame. In a flame, antimony oxide halides form an inert gas, which can preclude further oxygen from reaching the plastic. Again it is ironic that the same chlorine donor prevents fire in some cases and provides chlorine for colored flames in others. Bromine, the next halogen down from chlorine, has largely supplanted chlorine in the role of fire retardant. Metal bromides can also produce nice colored flames, but that is another topic!
It is the author’s untested assertion that the cinder formation in some color firework star compositions is due to the fire retardant action of metal oxide chlorides.
Other pyrotechnic chemicals that contain high percentages of chlorine include ammonium chloride (NH4Cl), hexachlorobenzene (C6Cl6), and hexachloroethane (C2Cl6). Hexachlorobenzene (HCB) was used extensively as a chlorine donor due to the aromatic carbon structure and high chlorine content. On paper, ammonium chloride and hexachloroethane (HCE) both look ideal. In reality, they both tend to vaporize out of the colored flame before contributing a significant amount of chlorine. They might find use as chlorine donors in the colored alcohol fireballs in vogue today.
There is an interesting use of the name benzene in the Shimizu text. There is an insecticide with the trade name of Lindane which is specifically 1,2,3,4,5,6 hexachloro gamma cyclohexane (C6H6Cl6). This compound is also referred to as benzene (gamma) hexachloride, even though it does not contain the benzene ring. Shimizu states that it is not as good for color production as PVC. Apparently, the good doctor had never tried true hexachlorobenzene, which is superior.
Where a firework star composition actually calls for HCB, Dechlorane can be used as a substitute. Dechlorane in amounts higher than about five percent seems to increase the whiteness of the colored flame.
Most of the above-chlorinated compounds are carcinogenic. Production of HCB was banned in this country (USA) several years ago. Much nicer to work with are the chlorinated plastics, beginning with "Parlon."
Parlon was made by Hercules Chemical many years ago, and is a chlorinated natural rubber. They no longer make it, but overseas sources of "Superchlon" and "pergut" are apparently the same material. These are probably what the chemical suppliers are selling. Parlon is relatively safe to handle, and it dissolves in ketones such as acetone or MEK.
The most confounded chlorine donor name is "Saran". Dow Chemical produces various types of Saran resin for different applications. Ideally for firework stars, Saran should be pure PVDC (polyvinylidene chloride) powder. This would be the best choice if it were available. However, there are seemingly hundreds of different versions of "Saran", which are copolymers of PVDC with some other plastic, including PVC and acrylonitrile. When these copolymers are mostly not PVDC, the chlorine content is lowered and the burning characteristics become worse.
A simple test for a given sample of Saran is to test for solubility in acetone or MEK. If the powder eventually dissolves at room temperature, it has a high copolymer component. If it does not dissolve, or requires high heat to dissolve, then it can be recommended for use in pyrotechnics. Saran with a high percentage of PVDC is highly recommended for any firework star composition which requires a chlorine donor, and typically gives the best color purity, burning speed, and ignition.
Similar to Saran in the aspect of identity crisis is "Chlorowax", produced by Occidental Chemical. There are at least ten different Chlorowax compounds, which range from white powder to oily liquid. The material, which has been reported as a good pyrotechnic chlorine donor, notably by Joel Baechle, is the waxy substance, which behaves like paraffin. Chlorowax was intended for use as a flame retardant.
PVC (polyvinyl chloride) is another plastic that is used as a chlorine donor. PVC produces hydrogen chloride gas when it burns, which can be environmentally bad but pyrotechnically good. It does not contain as much chlorine as PVDC or Parlon, and also acts as a fuel more so than they do. It works very well as an adjunct chlorine donor in chlorate firework star compositions, and it is typically the least expensive compound of those discussed here. It also works well in combination with magnesium, where the HCl gas reacts with the metal oxides in the colored flame to improve the color. PVC will dissolve in MEK.
There are many potential pyrotechnic chlorine donors, which have never been tested. Some of the newer flame-retardants may be superior in fact. One in particular that should be tested is TCPA, tetrachlorophthalic acid. This should be similar to HCB, and would supply the aromatic ring, which seems to make better colors.
In conclusion, mention must be made of ammonium perchlorate. NH4ClO4 is unique among oxidizers in that it combusts to produce HCl gas, and is superior as a chlorine donor in this respect. Most of today’s "award winning" firework star compositions are made with ammonium perchlorate.
In the following table, percentages of chlorine are given from either the calculated maximum by molecular weight or from the manufacturers own data when the molecular structure is irregular (such as Parlon).