The Purdue Energetics Research Center (PERC) has developed a "switchable explosive", which can enhance its safety during handling and transportation.

Steve Son, Alfred J. McAllister Professor of Mechanical Engineering, worked with Metin Ornek, research scientist at Zucrow Labs, and a team of other researchers to create the explosive. They started with RDX, a common explosive, encased in a rubberized polymer binder. They then injected thermally-expandable microspheres (TEMs).

“TEMs have a thermoplastic shell and are filled with a low boiling point hydrocarbon,” said Son. “When heated, the hydrocarbon vaporizes and the TEMs expand to 60 to 80 times their original size. An RDX-based rubberized explosive is perfect for this application, because the polymer allows for expansion.”

Heating the explosive in a specific way results in microstructural changes, which causes the TEMs to become hot spot locations. When shocked, these hot spots sustain a detonation wave fully throughout the material. Without that heating process, the TEMs do not detonate unintentionally. This is what creates the switchable explosive.

Their research has been published in Propellants, Explosives, and Pyrotechnics.

The team began by mixing the explosives, changing the amount of TEMs in each sample. Afterward, they were left with five different samples containing TEMs, and one baseline sample without any TEMs. They heated each sample (either to 95°C or 125°C), held that temperature for 30 minutes, and then cooled them back to their original temperature.

Following that, micro CT was used to study the microstructural differences of the unheated and heated samples. The 95°C samples increased in size and in porosity due to the expansion of the TEMs. However, the heated 125°C sample showed little to no expansion, as the TEMs deflated back to their original size due to the higher temperature. This process of expanding and deflating was also confirmed through hot-stage microscopy, a powerful analytical technique that allowed the researchers to look at changes in the material.

The samples were then tested to see if they could sustain a detonation wave. A brass witness plate and high speed video were used to conclude whether each sample was successful. Each sample was ignited and none of the 95°C samples were able to sustain a detonation wave fully. However, the 125°C samples had different results.

High speed imaging of the TEMS unheated and heated at 125°C

“The high temperatures allowed the TEMs to completely expand, melt, rupture, and deflate within the explosive formulation,” said Ornek. “This caused critical hot spots to form, allowing the sample to detonate fully throughout the material.”

The sample heated at 125°C was able to successfully sustain a detonation wave, but the unheated sample was unable to, proving a switchable explosive could be created.

“The inability of the unheated sample at 125°C to detonate further confirms that until the material is heated and the TEMs expand, the material will not detonate unintentionally. This makes it safer to store and transport,” said Son.

This research was sponsored by the Army Research Office and was accomplished under Cooperative Agreement Number W911NF-22-2-0170. The high-speed camera was acquired with DURIP Award No. FA9550-16-1-0315 (Dr. Martin Schmidt, Program Officer).

Writer: Julia Davis, juliadavis@purdue.edu

Source: Steve Son, sson@purdue.edu & Metin Ornek, mornek@purdue.edu

Switchable RDX-Based Rubberized Explosive with Thermally-Expandable Microspheres
Joesph R. Lawrence, Metin Ornek, Robert E. Ferguson, Diane N. Collard, Steven F. Son
https://doi.org/10.1002/prep.202400053
ABSTRACT: Improving the safety of explosive materials through the synthesis of insensitive explosives has been studied extensively. However, little work has focused on creating switchable explosives. A switchable explosive is normally insensitive to detonation, and therefore safe to handle and transport, but can be sensitized when needed to create a functional explosive. Similarly, it may be desired to desensitize an explosive to prevent its function. This study examined the ability to create a switchable 1,3,5-trinitro-1,3,5-triazinane (RDX)-based rubberized explosive using thermally-expandable microspheres (TEMs). The addition of TEMs to the explosive formulation allowed for microstructural changes and potential hot spot locations to form as the microspheres expanded. Small voids (less than about 10μm) are more likely to be critical hot spots when shocked, and likewise larger voids are less likely to ignite successfully (sub-critical) when shocked. Consequently, both sensitization and desensitization are possible. The rubberized explosive considered here with unexpanded microspheres was unable to sustain a detonation for the size used, but after specific heating followed by cooling to produce small voids, a detonation was achieved. The TEMs addition to the RDX-based rubberized explosive resulted in an explosive that is detonation insensitive when unheated but becomes a functional explosive after it is sensitized through heating. This paves the way to create insensitive explosive formulations with on-demand switchable detonation function through the incorporation of thermally-expandable microspheres. Desensitization was also demonstrated with specific heating of TEMs in an initially detonable explosive charge. Finally, we also demonstrated that deflagration can be affected by heating TEMs.