- Program Areas
- Installation Energy and Water
- Environmental Restoration
- Munitions Response
- Resource Conservation and Resiliency
- Weapons Systems and Platforms
Evaluation of Metal-Bound Porphyrins and Porphyrazins as Lead Salt Replacements for Ballistic Modifiers in Minimum Signature XLDB Propellants
Dr. Scott Riley | ATK Missile Subsystems and Components
In an effort to reduce exposure to lead from minimum smoke propellants to both manufacturing operators and military personnel, alternate metal ions are being investigated in the role of ballistic modifiers in these propellants. Lead citrate has long been the burn rate modifier of choice due to its ability to reduce a propellant’s burn rate over a specific pressure range in many minimum smoke propellant formulations. Other metal ions have been investigated as burn rate modifiers, but have been found to either reduce service life or significantly reduce propellant cure times causing the propellant to solidify before it is cast.
The objective of this project was to investigate the usefulness of metal-bound porphyrins, metal-bound phthalocyanines (porphyrazins), and several bismuth salts for their effectiveness as replacements for lead salt burning rate modifiers in minimum smoke propellants. Specifically, their effectiveness in modifying the ballistics of an ammonium nitrate (AN) based propellant was determined.
Formulations using eighteen candidate compounds were generated for AN based propellants using a specific impulse range and amount of smoke produced as selection criteria. All materials were tested for thermal stability and compatibility with minimum smoke propellant ingredients. Ingredients that were found to thermally degrade below temperatures that the propellant is generally exposed to during processing or that were incompatible with energetic ingredients were removed from the list of investigated materials. The remaining ingredients were incorporated into AN based, minimum smoke, slurry cast propellant formulations by way of metal salt pastes. The slurry propellant was tested for pot life and sensitivity to impact, friction, and electrostatic discharge. Cured propellants were tested for shelf life stability via N-Methyl-p-Nitroanaline (MNA) depletion, sensitivity to impact, friction, and electrostatic discharge. Mechanical properties and burn rates were measured and pressure exponents calculated for propellants that showed promise.
The organic chelators selected for this project were from the porphyrin/porphyrazin family, phthalocyanines. Several of the commercially available metal centered phthalocyanines passed screening tests and were incorporated into a difficult to ballisticly tailor propellant formulation. It was found from these tests on stabilizer depletion, mechanical, and burn rate properties that the metalo-phthalocyanine compounds did not perform as expected. As compared to the same formulation containing a lead based ballistic modifier, the metalo-phthalocyanine modified propellants displayed shorter service life, lower burn rates, and in some instances, reduced tensile properties.
Several bismuth compounds were also investigated for their effects on propellant burn rate. Although all bismuth candidates passed the same screening tests that the phthalocyanine molecules were subjected to, and some bismuth compounds had useable pot lives as well as near equivalent physical properties, they did not have burn rates as high as that of propellant with lead as a ballistic modifier nor did they have the desired saddle at about 1000 psi.
This project investigated the possibility that by using an extensive organic chelating molecule, the negative characteristics of certain metals with regards to propellant pot life and service life might be mollified or blocked. Within this scope, the work demonstrates that the chelating organic compound does not influence burn rate or other properties relative to propellant processing or characteristics. Future work can be directed to finding a metal ion substitute for lead regardless of its processing or effects on propellant properties and then encasing it entirely in an organic structure such that it could not affect the propellant until the pressures and temperatures of combustion released it.