Centrifugal and acoustic mixing technologies for the Homogenization of propellant powder formulations

Mixing is an essential process step in manufacturing propellant powders. This process transforms individual components into a homogeneous formulation. One of the most important material properties for the mixing process is the formulation viscosity which is affected by particle size and morphology, solvent, concentration of ingredients and temperature. The homogenization influences further operations such as extrusion whereby inhomogeneous pastes can cause difficulties by the adjustment of the propellant geometry. Mixing technologies which are conventionally applied for homogenizing propellant powder formulations are mixing systems for high viscous materials as kneading and extrusion processes. Kneaders can be horizontal or vertical executed but always use internals as for example mixing blades, to homogenize and hence, the internals are in contact with the materials. Homogenization, mixing time, low energy input and reproducibility are significant criteria for the mixing process. Beside their advantages in safe handling and uniformly kneading, conventional kneaders require long process times to achieve homogenization. For example, kneading times can reach from minutes up to several hours to homogenize the formulation of the propellant powder. Innovative mixing processes for the homogenization of gun propellant formulations are planetary and resonance acoustic mixing systems. The speedmixer is a planetary mixer which uses centrifugal force to homogenize the formulation. Thus, no internals are required whereby it is a non-contact mixing process that is likely applicable for sensitive materials. By adjusting the mixing speed and time, the speedmixer can homogenize a wide spectrum of viscous formulations. Another novel mixing technology is resonance acoustic mixing which is also a non-contact mixing technology. The LabRAM is a resonance acoustic mixer with a vertically vibrating platform which oscillates at high acceleration to homogenize the formulation at its mechanical resonance frequency. Centrifugal and acoustic mixing systems are characterized by short process times for homogenization.
In this study novel mixing technologies are investigated for the homogenization of gun propellant formulations. To evaluate centrifugal and acoustic mixing technologies, this work compares the mixing of a conventional kneader, speedmixer and LabRAM in terms of solvent content, mixing time and process parameters. In this study different formulation were used, based on a low vulnerable energetic propellant powder (LOVA). Particularly, comparison of the different mixing technologies was conducted with an inert LOVA-based formulation whereby energetic materials were replaced by non-energetic ingredients. The mixing technologies were further applied to homogenize the energetic LOVA-formulation. In order to characterize the mixing processes the rheological behavior of the formulations was investigated by capillary rheometry. The viscosity correlates thereby with the homogenization.