New pellet-making method for explosives promises safer, more controlled manufacturing

For decades, the manufacturing of plastic-bonded high explosives (PBXs) has relied on traditional methods like slurry coating, where explosive crystals are combined with a polymer binder to form granules. These granules are then pressed into final explosive parts. However, this process is challenging to control, can be inefficient, and frequently leads to variations between batches. Such variability is critical because the detonation and mechanical performance of PBXs are influenced by factors beyond chemistry, including particle size, shape, and porosity, as well as how the material behaves during pressing.

To address these limitations, a team at Lawrence Livermore National Laboratory (LLNL) has initiated Project MAHEM (A Modern Approach to HE Manufacturing). This project aims to establish a stronger scientific foundation for formulating and pressing explosive materials. Supported by LLNL's Laboratory Directed Research and Development program, the initiative seeks to deepen the understanding of the connections between feedstock, processing, structure, and performance. It also explores modern manufacturing techniques borrowed from industries like pharmaceuticals and food processing, with the ultimate goal of developing a more responsive and predictable framework for creating explosive materials essential for the nation's nuclear deterrence mission.

As part of this modernization effort, researchers have demonstrated the potential of extrusion-spheronization, a process widely used for producing uniform pellets or granules, to create mock prills with PBX-like compositions. This marks what is believed to be the first application of this technique to high explosives. The method involves extruding a mixture into small, elongated shapes that are then tumbled to form near-spherical particles.

To test the technique, the team utilized modified extrusion and spheronization equipment within LLNL's High Explosives Applications Facility, ensuring safe handling of energetic materials. They used a PBX-like formulation consisting of 95% insensitive high explosive and 5% polymer binder. The process began with mixing the explosive powder and binder in a solvent to create a workable mass. This mass was then extruded into small cylinders, which were subsequently rounded into near-spherical particles. Characterization of these prills involved particle-size imaging and microscopy to analyze their size, surface, and internal structure. The prills were then pressed into test parts, and their density and compressive strength were measured to correlate prill properties with final performance.

Experiments revealed that a specific solvent mixture (75% propyl acetate/25% butyl acetate) resulted in the strongest pressed parts, although prill size and shape appeared to be the primary drivers of mechanical performance. This outcome fulfilled a key project objective: the reliable and repeatable production of prills using a method less sensitive to minor process variations than traditional techniques. While successful, the current implementation of extrusion-spheronization still operates in a batch mode, requiring manual steps and reconfigurations between extrusion and spheronization, which the MAHEM project is working to improve.