Current medium caliber ammunition employs environmentally unfriendly materials that cause safe disposal of their propellants to be difficult. Although the main energetic component is nitrocellulose (NC), other harmful additives are used in the propellant mixture. Diphenylamine (DPA), a suspected carcinogen, is used as a stabilizer in the propellant to prevent the deterioration of the nitrocellulose. Barium nitrate, a heavy metal and also a hazardous material, is used in some medium caliber propellants as an oxidizing agent to make the propellant more readily ignitable. In the next 5 years, medium caliber munitions will employ 7,700 pounds per year of DPA and 2,000 pounds per year of barium nitrate.
The objective of this project was to identify a suitable replacement for medium caliber (25mm and 30mm) propellants that is more environmentally acceptable, offers good safety properties, provides an increased level of performance, and maintains a reasonably low cost. Thermoplastic elastomer (TPE)-based propellants offer a solution to this environmental problem because they do not require stabilizing additives or oxidizing agents.
In this effort, propellant performance parameters and a grain design were established, and a TPE toxicology study was performed. Promising candidate propellants were then selected for initial processing. Factors used to select propellants included previous use history, expected processing characteristics, density, impetus, flame temperature, material cost, and expected vulnerability. Two propellants were manufactured and processed. Twin screw extrusion enabled the propellants to be manufactured reproducibly and consistently in an environmentally friendly manner at a relatively low cost. After safety analysis, ammunition was manufactured from the propellants and medium caliber gun firings were conducted at various temperatures.
TPE propellants offer advantages over typical NC-based propellants in that they do not require hazardous stabilizing additives or oxidizing agents. In addition, they may be manufactured into advanced geometries, do not have plasticizer migration issues, are immune to moisture problems, and may be warmed and re-extruded into new geometries. TPE propellants also may be recycled, minimizing propellant waste. (Project Completed - 2004)