Many of the existing fabrication technologies for gun propellants utilize solvents [volatile organic compounds (VOCs)] and generate a large amount of hazardous waste. In addition, solventless formulations that were designed to circumvent these issues have significant manufacturing hazards, as well as performance, aging, and safety concerns. This project aims to reduce the adverse environmental impact of gun propellant fabrication through the use of a novel direct-write additive manufacturing (AM) technology that enables the printing of extremely viscous (µ > 10000 Pa·s) mixtures, which requires less solvent for formulations while benefiting from the ability to fabricate unique designs through AM. The work will also characterize the mechanical properties and performance of printed parts as a function of process and material parameters.

The project will explore the following basic research questions:

• How does varying the amount of solvent affect the viscosity, flow rates and self-adhesion of the gun propellant formulations (double-base, triple-base, modified double-base composites and composite propellants) and similar heterogeneous materials?
• How do pertinent processing parameters, such as vibration amplitudes and vibration mode shapes, influence the flow rates of low-solvent mixtures and particle-TPE composites close to their melting temperatures, and how do these parameters affect print quality?
• How are the mechanical properties and burning characteristics of the gun propellants affected by the printed grain structures (functional gradation, pore geometry, etc.) and the printing process?
• How much solvent waste reduction can be realized using this manufacturing process and how does this reduction, along with remote operation, influence human exposure levels during manufacturing?

The vibration assisted direct write system enables the rapid deposition of extremely viscous materials through sub-mm sized nozzles and yields fully dense 3D printed structures. The high resolution (~0.1 mm) of the system can enable the in situ deposition of low solvent formulations, or different components of existing double-base or triple-base formulations with enough solvents and additives to ensure safety, completely eliminating mixing cycles that generate most of the solvent waste. Furthermore, thermoplastic elastomer (TPE)-energetic (RDX, CL-20) composites are compatible with the system and can be printed at lower than typical temperatures, while using pellets instead of pre-formed filaments since flow is controlled at the nozzle. This can alleviate the brittleness issues associated with filaments which contain solids loadings in excess of 50 wt.%. The mild printing conditions can also enable the use of novel material formulations that cannot be processed using existing methods. The AM system enables the fabrication of graded compositions and the inclusion of designed porosity, which allow for significant improvements in propellant performance and tailorability.

If successful, this project will enable the AM of gun propellants with both lower environmental impact and cost, due to reduced solvent usage. Furthermore, the AM of gun propellants can enable tunable burning characteristics and reduced sensitivity through thoughtful structural design. These advancements will be realized through the development of a fundamental understanding of the effects of material-process-structure-function relationships.