Rheology and characterization of high-solids suspensions for direct ink writing of energetic materials

Abstract

Direct ink writing is a promising approach for preparing energetic materials with unique geometries that are of great interest in military and civil engineering fields due to their potential to control shock wave propagation and energy focus or dissipation. However, there are significant challenges to overcome in using additive manufacturing to produce energetics, particularly in using inks with high particle content (>60 vol% particles) while maintaining both extrusion capability and print quality. Voids and interfaces in energetics are areas of high risk for hot spot formation, and with the layer-by-layer additive manufacturing process, voids can manifest both between and within the extruded filaments as well as between printed layers. Concerns associated with the challenges of printing high-solids suspensions make understanding the flow and print capabilities of these materials of great importance. The binder used in suspensions for direct ink writing plays an important role in overall flow characteristics of the ink, and therefore has significant impact on final print quality. In this work, glass microspheres in polymer-solvent and photocurable monomer binders are examined as model systems to provide an in-depth study of polymer binder design. This work aims to understand how binder characteristics affect the viscosity and printability of such high-solids suspensions. We show that the suspension viscosity is primarily controlled by the particle volume fraction for the photocurable binder system, while both the particle volume fraction and polymer molecular weight influence the viscosity in the case of the polymer-solvent binder system. Both binder types can be tuned to make printable suspensions that result in lines of consistent width and 3D disc-shaped objects, indicating that both paths show promise for future direct ink writing formulations of energetic materials.