ORGANIC SUBSTITUTES FOR CHARCOAL IN "BLACK POWDER" TYPE PYROTECHNIC FORMULATIONS Sean Wise Ronald A. Sassé Hughes E. Holmes July 1984 Technical Report ARBRL-TR-02569 Aberdeen Proving Ground, Maryland ***** scanned and you know what that means! ***** ABSTRACTED /djh/ INTRODUCTION AND BACKGROUND Black powder is a mixture of 75 percent potassium nitrate, 10 percent sulfur, and 15 percent charcoal. It is probably the oldest known energetic material and has been used throughout the world for centuries. Even though black powder has been in use for years, the factors that control its combustion properties are not known and certainly not well understood. The reasons for this ambiguity are related to the nature of the composition as it is a heterogeneous mixture of three solids, pressed to about 95 percent theoretical maximum density. Also, charcoal in black powder is a naturally derived substance which contains up to 35 percent tar-like constituents and varies from one source to another. Such variance has been found to have a great impact on the combustion properties of black powder.[1] Recently poor combustion properties of one lot of black powder has been cited as a cause for weapon malfunctions. [2] One problem area of prime concern is that various lots of black powder made by a particular manufacturer and black powder made by various manufacturers, using apparently equivalent processes, produce a pyrotechnic with different combustion characteristics. In fact, it has been possible to identify "good" and "bad" lots, in relation to device performance without a clear understanding as to the particular differences involved. Such variances are believed to be due to the varying chemical and physical properties of charcoal and to the physical properties of black powder. The reasons for the difficulties in characterizing charcoal used in black powder are many. It is an amorphous substance; it reacts and changes on heating; it is a mixture of many components; and only small portions of it dissolve in solvents. Since the material cannot easily be characterized, it has been impossible to learn what reactions might be important in combustion. One hope embraced in this work was to determine if a pure organic compound could be identified that would be an adequate substitute for charcoal and render the same performance, in a reproducible manner, as does "good" black powder. Such a substitute should lead to a more easily studied system to model the combustion processes of black powder. An added benefit may be a new type of pyrotechnic material in which a non-varying chemistry of combustion could exist and uniform physical properties could be maintained. In choosing organic compounds as substitutes for charcoal in black powder' it is necessary to make an assumption about the important functionality that may contribute to combustion. Since the oxidation of charcoal in combustion is an electron transfer process, it follows that charcoal's combustion should be made more rapid by functional groups which make electron transfer easier or by easily oxidizable groups present in the material. This hypothesis suggests two classes of model compounds that should be evaluated to study the reactivity of charcoal in black powder: polynuclear aromatics and organic reducing agents. In the first class of model compounds, we will determine if electrons delocalized over large aromatic pi systems facilitate electron transfer and therefore oxidation. If this is important, then polynuclear aromatic compounds, when substituted for charcoal in black powder, should support combustion. The second class of compounds studied probes a hypothetical role for the "volatiles" in charcoal during combustion. Rose observed that the "volatiles" in the charcoal play a crucial role in combustion, [1], [4] and fine papers relating volatiles to burning rate were also offered by Hintze [5] and by Kirshenbaum. Gray, March, and Robertson [7] related volatile content to roasting temperature and Sasse' [8] presented complete analysis of charcoal used to make black powder. Although the subject of volatiles appears to be well presented, the mechanisms of combustion are not understood. It is well known that charcoal is not just carbon; it contains 5 to 20 percent (by weight) oxygen, up to 5 percent hydrogen, and smaller amounts of other elements. We suspected that a significant amount of oxygen might be present in the charcoal as catechol or hydroquinone moieties. These compounds are very good organic reducing agents and can easily undergo two-electron oxidations to quinones. In addition, the catechol structure is known to occur in lignin which accounts for approximately 20 to 30 percent of the weight of wood before pyrolysis. [9] The conditions for the pyrolysis of wood required to make a good black-powder charcoal are stringent but not severe. Thermal analysis has shown that significant amounts of lignin remain in charcoal used for black powder [7], [8] and extreme pyrolysis, to 900C in an inert atmosphere, destroys these organics resulting in a weight loss. Therefore, it is proposed that the lignins originally present act as reducing agents in black powder making charcoal's combustion more facile. To probe the importance of this type of reaction in combustion then, a large number of polyphenolic compounds were evaluated as substitute compounds for charcoal. Some of these were capable of a facile two-electron oxidation and others were not. Another factor considered in choosing the organic substitutes for charcoal was melting point. Effort was made to choose high-melting materials of the types described above. It was felt that a low-melting-point material would liquefy and agglomerate prior to reacting. This would prevent good mixing of the three components which is required for combustion. The compounds selected are listed in Table 1 showing chemical structure, chemical composition, and melting point. [snip] ------------ Characterization of Maple Charcoal Used to Make Black Powder Ronald A Sasse' Memorandum Report ARBRL-MR-03322 November 1983 The total carbon content of the [charcoal] samples ranged from 71 to 80%, but the variance does not relate to volatile conent. Although the values vary, the empirical formula dor charcoal is near C8H4O which could represent an extensive fused ring system.