The Manufacture of Smokeless Powders and their
Forensic Analysis: A Brief Review
Robert M. Heramb
Graduate Student
Bruce R. McCord
Associate Professor of Analytical and Forensic Chemistry
Department of Chemistry
Ohio University
Athens, Ohio
Introduction
Smokeless powders are a class of propellants that were developed in the late 19th century to replace black powder. The term smokeless refers to the minimal residue left in the gun barrel following the use of smokeless powder. In forensic analysis, smokeless powders are often encountered as organic gunshot residue or as the explosive charge in improvised explosive devices.
All smokeless powders can be placed into one of three different classes according to the chemical composition of their primary energetic ingredients. A single-base powder contains nitrocellulose, whereas a double-base powder contains nitrocellulose and nitroglycerine. The energetic ingredients in triple-base powders are nitrocellulose, nitroglycerine, and nitroguanidine, but because triple-base powders are primarily used in large caliber munitions, they are difficult to obtain on the open market.
Composition and Manufacturing
The major classes of compounds in smokeless propellants include energetics, stabilizers, plasticizers, flash suppressants, deterrents, opacifiers, and dyes (Bender 1998; Radford Army Ammunition Plant 1987).
- Energetics facilitate the explosion. The base charge is nitrocellulose, a polymer that gives body to the powder and allows extrudability. The addition of nitroglycerine softens the propellant, raises the energy content, and reduces hygroscopicity. Adding nitroguanidine reduces flame temperature, embrittles the mixture at high concentration, and improves energy-flame temperature relationship.
- Stabilizers prevent the nitrocellulose and nitroglycerine from decomposing by neutralizing nitric and nitrous acids that are produced during decomposition. If the acids are not neutralized, they can catalyze further decomposition. Some of the more common stabilizers used to extend the safe life of the energetics are diphenylamine, methyl centralite, and ethyl centralite.
- Plasticizers reduce the need for volatile solvents necessary to colloid nitrocellulose, soften the propellant, and reduce hygroscopicity. Examples of plasticizers include nitroglycerine, dibutyl phthalate, dinitrotoluene, ethyl centralite, and triacetin.
- Flash suppressants interrupt free-radical chain reaction in muzzle gases and work against secondary flash. They are typically alkali or alkaline earth salts that either are contained in the formulation of the propellant or exist as separate granules.
- Deterrents coat the exterior of the propellant granules to reduce the initial burning rate on the surface as well as to reduce initial flame temperature and ignitability. The coating also broadens the pressure peak and increases efficiency. Deterrents may be a penetrating type such as Herkoteâ, dibutyl phthalate, dinitrotoluene, ethyl centralite, methyl centralite, or dioctyl phthalate; or an inhibitor type such as Vinsolâ resin.
- Opacifiers enhance reproducibility primarily in large grains and keep radiant heat from penetrating the surface. They may also enhance the burning rate. The most common opacifier is carbon black.
- Dyes are added mainly for identification purposes.
- Other ingredients may be one of the following:
- A graphite glaze used to coat the powder to improve flow and packing density as well as to reduce static sensitivity and increase conductivity
- Bore erosion coatings applied as a glaze to reduce heat transfer to the barrel, but uncommon in small-arms propellants
- Ignition aid coatings that are most commonly used in ball powders to improve surface oxygen balance
Figure 1- Common smokeless powder morphologies
Chemical composition is one important characteristic defining smokeless propellants; however, another important characteristic is its morphology. Shape and size have a profound effect on the burning rate and power generation of a powder (Meyer 1987). Common particle shapes of smokeless propellants include balls, discs, perforated discs, tubes, perforated tubes, and aggregates (Bureau of Alcohol, Tobacco and Firearms 1994; Selavka et al. 1989). A few common types of smokeless powder morphologies can be seen in Figure 1 (Bender 1998).
Morphology also lends clues to whether a powder is single- or double-base (Bender 1998). Most tube and cylindrical powders are single-base, with the exception of the Hercules Reloaderâseries. Disc powders, ball powders, and aggregates are double-base, with the exceptions being the PB and SR series powders manufactured by IMR Powder Company of Plattsburg, New York.
Except for ball powder, smokeless powder is manufactured by one of two general methods, differing in whether organic solvents are used in the process (Meyer 1987; Radford Army Ammunition Plant 1987). A single-base powder typically incorporates the use of organic solvents. Nitrocellulose of high- and low-nitrogen content are combined with volatile organic solvents, desired additives are blended with them, and the resulting mixture is shaped by extrusion and cut into specified lengths. The granules are screened to ensure consistency, and the solvents are removed. Various coatings, such as deterrents and graphite, are applied to the surface of the granules. The powder is dried and screened again, then blended to achieve homogeneity.
The manufacture of double-base powders requires the addition of nitroglycerine to the nitrocellulose. Two methods can be used. One method uses organic solvents, the other uses water. The organic solvent method mixes nitrocellulose and nitroglycerine with solvents and any desired additives to form a doughy mixture (Meyer 1987; National Research Council 1998; Radford Army Ammunition Plant 1987). The mixture is then pressed into blocks that can be fed into the extrusion press and cutting machine. The resulting granules are screened prior to solvent removal and the application of various coatings. The powder is dried, screened again, then blended to achieve homogeneity. The water method adds the nitroglycerine to a nitrocellulose water suspension to form a paste (Meyer 1987; Radford Army Ammunition Plant 1987). The water is removed by evaporation on hot rollers, then the dried powder is shaped by extrusion and cutting.
Triple-base powders use a solvent-based process similar to the double-base powder process (Meyer 1987; Radford Army Ammunition Plant 1987). Nitrocellulose and nitroglycerine are premixed with additives prior to the addition of a nitroguanidine solvent mixture. The nitroguanidine is incorporated into the overall mass without dissolving in the other materials. The final mixture is then extruded, cut, and dried.
The manufacture of smokeless ball powder requires a more specialized procedure (National Research Council 1998). Nitrocellulose, stabilizers, and solvents are blended into a dough, then extruded through a pelletizing plate and formed into spheres. The solvent is removed from the granules, and nitroglycerine is impregnated into the granules. The spheres are then coated with deterrents and flattened with rollers. Finally, an additional coating with graphite and flash suppressants is applied, and the batch is mixed to ensure homogeneity.
In the manufacturing process, smokeless powders are recycled and reworked (National Research Council 1998). When a powder within a batch is found to be unsatisfactory, it is removed and returned to the process for use in another lot. Manufacturers save money by recycling returns by distributors or the return of surplus or obsolete military powders. Hence, reworking and recycling the material assures good quality control of the final product, reduces costs by reusing materials, and reduces pollution by avoiding destruction by burning.
Distribution
The production of smokeless powders is big business in the United States, where approximately 10 million pounds of commercial smokeless powders are produced each year. Most of the powder is sold to the original-equipment manufacturers to be used for manufacturing ammunition. A large amount is sold to domestic and foreign militaries (National Research Council 1998). The rest is sold in individual canisters (ranging from ½-pound cans to 12- or 20-pound kegs) to gun stores or hunting and shooting clubs for hunters and target shooters who prefer to hand load their own ammunition.
There are several ways smokeless powders are distributed within the United States (National Research Council 1998). Some manufacturers, foreign or domestic, produce, package, and sell their own powders commercially. They may also sell in bulk to resellers and to original-equipment manufacturers that repackage and sell it under their own labels. The powder manufacturers and repackagers may disburse large quantities of canister powders to distributors who later sell to smaller distributors and wholesalers, who in turn, supply cans to dealers, gun shops, shooting clubs, and other retailers. At this point, consumers can purchase a 1-pound canister of powder for approximately $15 to $20 from a retailer, though the cost per pound can be cheaper if bought by the keg or acquired through a gun club (National Research Council 1998).
Manufacturers who produce smokeless powders for the U.S. military can distribute it either by selling the powder directly to the military or by selling them the preloaded ammunition. Powders can also be shipped to U.S. military subcontractors, foreign governments, or foreign loading companies for loading into military ammunition (National Research Council 1998).
http://firearmsid.com/Feature Articles/McCord_gunpowder/
Powder manufacturers supply safety data sheets (sds) showing the chemicals used.
