Color Flame Whistle Formulations

The present article is being offered as an attempt to present some insights concerning colored flame whistles - more specifically red and green whistle formulations based on the efforts of Rembert Amons. Whistles made from the formulations mentioned here were primarily used in stand alone devices as opposed to being used in (as) rocket devices although one of us (Rembert) has used them in that application.

Blue flame whistles have been dealt with in previous articles 1a,1b,1c and will not be addressed here. In these AFN issues Rich had mentioned that Rembert had come up with a number of colored flame whistle formulations. In AFN 312 the following was mentioned:
"One pyro investigator, Rembert Amons, has conducted some extensive investigation into the use of these salts (and other carboxylic acid salts) in making colored flame pyrotechnic whistles. Rembert's red and green colored whistles are in particular worthy of mention. At the 2007 February WWB, J. Domanico presented some of Rembert's formulae for red and green whistles during his whistle seminar/ talk. Unfortunately Joe was pressed for time and he had to make only quick mention of them offering to provide those interested with the formulae after his talk. In those formulae, Rembert used strontium and barium salts of isophthalic acid to achieve some of his red and green colored flame whistles."

J. Domanico did provide a table of some of Rembert's red and green whistle formulae. However, for this article Rembert wanted to proceed in an informal fashion and felt that rather than present a list of the trial (they were not fully optimized) formulae given in that table, it would be better to consider more generic or general ways of examining formulae to effect red and green whistles.

In various works, Davies, Dilg, Domanico, and McLain, to name a few, have previously presented generalized stoichiometric equations showing the oxidizer potassium perchlorate reacting with organic (whistle) fuels. These equations can be used to calculate reactant percentages.

These percentages support Maxwell's3 70:30 binary mix proportions. Ellern4 referred to Maxwell's formulation in his works. As a starting point in considering coming up with a formula for a color flame whistle these proportions support the basis for one general formula for use as a color whistle that Rembert tried. That general formula is:
         Oxidizer about 70%
         Color fuel about 30%
         Chlorine donor 3-10% add.

As it turns out Rembert has shown that this formula will work well with isophthalic acid salts as the fuel, especially for effecting red whistles and using the strontium salt.

As Rembert's investigations went on another useful generalized formula that he found useful is :          Oxidizer about 60%
         Color fuel about 40%
         Chlorine donor 3-10% add.

Rembert's work has shown that the latter will work with nitrobenzoic acid salts.

Two of Rembert's trial formulations that make use of the preceding generalized ones are found in Table 1.
Table 1
RED 4

 

At first glance these formulae may seem a bit "exotic" but anyone familiar with pyrotechnic formulations shouldn't find them all that unusual.

Phthalate salts have been described previously in pyro whistle applications1e, 1f, 2a, 2b. The isophthalate salt indicated in the Table 1 would need to be prepared. However, it can be prepared from relatively inexpensive starting materials and using simple reactions (neutralization reactions) – ones very similar to those described by the Kosankes in making benzoate salts (see Kosankes'AFN 119 August 1991 article1d). The nitrobenzoate salt can also be similarly prepared.

Other green formulae given by Rembert in the table provided by Domanico utilized barium isophthalate but Rembert comments that this fuel tends to absorb water.

Speaking of phthalic acid isomers (Amons, Davies, and Dilg mention these) and their salts, Rembert wanted to mention that the para isomer salts tend not to yield the "right" stuff – it (formulations using them) will be colorful, but you will not hear a whistle. In this respect there is a difference with the potassium salt. Rembert tested this extensively. The ortho isomer will be very hygroscopic; Rembert has a reference from Arno Hahma (private communication) stating so.

Color flame enhancers and chlorine donors play an important role in formulating a color flame whistle. Their function, as in other pyrotechnic color flame compositions, is to promote/support the formation of color emitting species in the flame produced by the pyrotechnic reaction. In the red formula given in Table 1 Chlorez is an organic chlorine donor. In the green formula given in Table 1, as one of its functions, the barium chlorate can be said to function similarly, although most likely not in quite the same manner as an organic one, as a chlorine donor in that its presence helps in the generation of a green flame emitting specie. Although it can be a bit tricky, similarly, ammonium perchlorate can also be used in red and green formulations, but the particle size must not be too small/fine. Otherwise whistling is suppressed. In a January 2006 PML posting5 Myke Stanbridge very briefly noted the use of ammonium perchlorate in color whistle formulations where he wrote:
"…You need to use potassium benzoate as well (using potassium perchlorate) to get a reasonable sound. To get a reasonable colour you need to add about +5% ammonium perchlorate. Such compositions are fickle, with much work still to be done. A few years ago Rembert Amons was achieving good results with the modified phthalates, but understanding the vital nuance that gets both sound and colour is very time consuming. The oxidizer/fuel particle size and shape is crucial. …"

Typically, however, one would want to keep additions of organic chlorine donors to " a least as possible " amount to avoid unwanted degradation of the whistling sound effect.

It should be mentioned that the use of "catalysts" (such as those used in benzoate/salicylate formulations for whistling rockets) is not advisable for color whistle formulations since such use will adversely affect flame color.

Particle size of the whistle formulation materials is important to achieving the effects desired – whistle sound and visible color flame. For the whistle sound the potassium perchlorate should be at least 120 mesh (~0.125 mm)3,1f. The fuels (isophthalates and nitro benzoate) should be reduced to a very fine talc-like consistency. Particle sizing is also a concern when testing other added materials that may be used to enhance flame color and/or flame size as mentioned above.

Care should be taken that materials are very dry and unadulterated before beginning incorporation/blending of the whistle composition. Typically oven drying of the unmixed materials beforehand can be implemented.

Mixing and blending of the formulated mixes would need to assure intimate incorporation of fuels with the oxidizer. Keeping batch sizes small can add to the safety factor associated with the energetic mixes. "Wet" mixing such as the use of a solvent such as acetone can be useful but care is needed using this approach to safely handle/remove excess acetone vapors.

Half (½) inch i.d. convolute paper/cardboard tubes work well. J. Domanico had discussed compaction/ loading pressures6 for his whistles where he was working mainly with potassium benzoate mixes and he demonstrated that loading pressures in the 2500 to 7500 psi range worked well. Richard found that pressing color whistle comps at pressures in these psi ranges seems to work well. Rembert used pressures somewhat higher.

Acknowledgements
Rembert wanted to acknowledge Myke Stanbridge and would like to say thanks to him for having discussed colored whistles with Rembert a few years ago. It was from him that Rembert states he got the idea of incorporating ammonium perchlorate to colored whistle formulations.

Richard wanted to acknowledge Rembert for his willingness to share information that reawakened ideas Rich had long ago given some thought to but only recently pursued with renewed interest which led to Rich's blue whistle endeavors. Rich also wanted to thank Rembert for his cooperation in coauthoring this article.

(The author, Richard Dilg)