In this work, ambient reactive extrusion (ARE) will be used to additively manufacture gun propellant formulations with improved performance while addressing several of the adverse environmental consequences of current gun propellant manufacturing. Along with removing solvent, ARE enables a wide variety of chemistries that will eliminate nitrocellulose based formulations, reduce nitramine composition, and deviate from isocyanate curing agents. Compared to other additive manufacturing (AM) techniques, ARE produces minimal volatile organic compounds (from the monomers) and requires fewer processing steps at lower operating temperatures, thereby reducing heating costs and personnel risk in propellant manufacturing and handling. The focus of this effort will be to develop novel propellant formulations based on new energetic binders for a customizable ARE and demonstrate manufacturing/performance on medium caliber gun charges.

This effort is a three-pronged approach that will culminate in printing at the US Army Combat Capabilities Development Command - Armament Center (CCDC) pilot plant and demonstrating performance with sub-scale tests, such as friability and closed bomb, and a gun firing on an optimized geometry and propellant formulation. (i) One objective is to develop new, energetic polymeric binders that provide a balance between mechanical and combustion properties. The research team will develop an understanding of how the structure, placement, and composition of energetic moieties in the polymer backbone influence mechanical properties and combustion behavior. They will start with polyurea chemistry that is tailored for ARE and replace inert monomers with energetic monomers. They will then move to Michael Addition (MA) chemistry as a more environmentally friendly alternative by removing isocyanates. (ii) Another objective is to adapt the ambient reactive extrusion (ARE) AM technique for energetic formulations by pushing the limits of solids loading and expanding the ARE polymers to include energetic, greener formulations. The research team will adapt these new formulations while simultaneously improving print resolution. (iii) The final objective is to implement a novel interior ballistic (IB) modeling concept of using topology optimization for charge geometry specifically geared for ARE manufacturing. Throughout the course of the project, life cycle analysis and toxicity testing will be performed new materials of interest.

This work is rooted in fundamental science that can have significant impact beyond gun propulsion or any specific application. If successful, the research team will have developed new, energetic polymer binders that can improve performance, exhibit better mechanical properties, are derived from environmentally friendlier materials, and is integrated with a scale-able, commercial AM technique. These materials and processes can be used for explosives, rocket propulsion, or any other energetic applications. Furthermore, the novel approach for merging topology optimization and interior ballistics can provide benefit to gun propulsion researchers. The US Army Research Laboratory’s (ARL) partnership with CCDC paves a clear transition path to supplant traditional gun propellant manufacturing with an inherently greener, safer, and better performing AM solution.