For decades, manufacturing plastic-bonded high explosives, or PBXs, has relied on legacy processes like slurry coating. In this method, explosive crystals are mixed with a binder, a polymer that helps hold the material together, to form small granules called prills. Those prills are then pressed into dense explosive parts.
This process is difficult to control, inefficient at times, and often results in batch-to-batch variability. That variability matters because PBX detonation and mechanical performance depend not just on chemistry, but also on particle size, shape, porosity and how the material responds during pressing.
Replacing variability with predictability
A team of researchers at Lawrence Livermore National Laboratory (LLNL) is working on an initiative called Project MAHEM - A Modern Approach to HE (high explosive) Manufacturing - to address those weaknesses by building a stronger scientific foundation for explosive formulation and pressing.
The project, supported by LLNL's Laboratory Directed Research and Development program, seeks to better understand the relationships between feedstock, processing, structure and performance, while also exploring more modern manufacturing methods in industries like pharmaceuticals and food processing. The team's goal is to create a more responsive and predictable framework for developing explosive materials that meet the nation's nuclear deterrence mission.
"In an ideal world, the development of a new PBX would start with clearly defined requirements for initiation, performance, safety, and mechanical properties, and then we would work backwards to deliver a qualifiable product in a responsive manner," noted Kyle Sullivan, Project MAHEM principal investigator. Not only would our product meet all of the requirements, but the specifications would be clearly articulated with confidence in the specified bounds and the pathway to scale-up logically laid out. In reality, this is far from the case today."
As part of their HE modernization effort, the team recently published a paper in Propellants, Explosives, and Pyrotechnics that demonstrates the potential for using extrusion-spheronization, a widely used mechanical process for making uniform pellets or granules, to create mock prills with PBX-like compositions. They believe this is the first time this specific technique has been used with high explosives.
"Extrusion-spheronization is a great approach that allows us to use new methods to make 'old' materials," explained LLNL scientist Dylan Kline. "Few industries have quality-control standards as stringent as pharmaceuticals, and because their products must be incredibly well-characterized and controlled, the process used to make them must be as well."
To test this technique, the team used an extruder and spheronizer modified for remote operations in LLNL's High Explosives Applications Facility, which is designed for safe handling of energetic materials. To reduce the likelihood of any unintentional accidents, they used a PBX-like formulation made of 95% insensitive high explosive and 5% polymer binder.
First, they used a planetary centrifugal mixer to mix the explosive powder with a binder dissolved in solvent, which created a wet lacquer and made the mixture workable with a kinetic-sand texture. The wetted powder was then pushed through an extruder to form small, elongated cylinders resembling sprinkles. The "sprinkles" were then tumbled in a spinning chamber that rounds the pieces into near-spherical particles.
The resulting prills were then characterized using particle-size imaging, scanning electron microscopy, and computed tomography to examine their size, surface morphology, and internal structure. Finally, the prills were pressed into cylindrical test parts and their density and compressive strength were measured. This allowed the team to link prill properties to final performance.
After testing several solvent mixtures, they found that a 75% propyl acetate/25% butyl acetate mixture gave the strongest pressed parts, but the main driver of mechanical performance appeared to be prill size and shape rather than the solvent chemistry itself. These results accomplish one of the project's main goals: to reliably and repeatably make prills using a process that was not as sensitive to small process variations as traditional methods.
When modernization meets automation
While the team successfully used extrusion-spheronization to produce PBX prills, they were still having to work in a "batch" mode. This means that someone would need to prepare a batch with the mixer, load it into the extruder, leave the room, perform the extrusion operation, come back in to reconfigure the equipment for spheronization, leave the room again, and then perform the spheronization.
To improve this tedious, manual process, the LLNL MAHEM project sponsored a group of mechanical engineering students at the University of California, Santa Barbara (UCSB) to develop an autonomous extruder-spheronizer as part of their capstone project. The project, called SPHERO-EX, won first place at UCSB's design competition, demonstrating that their automation method can produce prills with the same quality as LLNL's manual methods.
"This project pushed our engineering skills to their fullest potential, said UCSB mechanical engineering student Ethan Kwan. "It was amazing to watch our design evolve throughout the year, and it was immensely satisfying to see the build come together during the final stages of Capstone."
"I commend Dylan, Justin, and the Project MAHEM team for inspiring the next generation of engineers and helping students reach their full potential," added Sumita Pennathur, UCSB mechanical engineering professor. "The students' first-place finish is a testament to what can happen when a motivated team is paired with exceptional LLNL mentors. It was an honor to serve as their faculty advisor, and I hope collaborations like this continue for years to come."
Not only does the students' automation technique allow more material to be processed at one time, but it also reduces the downtime between processing stages. Most notably, because scientists no longer have to keep entering and exiting the room, it effectively decreases the amount of time they are exposed to explosive material. Given the unique safety considerations required when working with HE, this is a major benefit.
"Our research at LLNL has proven that the extrusion-spheronization process is effective for producing PBX prills and UCSB has shown that our process is amenable to automation," said Kline. "The clear next step is to adopt this technique and scale it up so that we can make larger batches."
At a larger scale, extrusion-spheronization could transform the future of HE manufacturing, modernizing decades-old processes once and for all.
-Shelby Conn
