Unexploded ordnance (UXO) resulting from munitions failures creates significant safety and environmental hazards. Smaller munitions, such as BLU-97 bomblets, are notorious for their tendency to dud. A typical sub munition fill, such as PBXN-107, uses cyclotrimethylenetrinitramine (RDX) in an acrylate binder as the main explosive component. RDX is persistent in the environment and is prevalent on and near Department of Defense (DoD) training ranges. The Environmental Protection Agency (EPA) has issued a drinking water guideline limit for RDX of 2 μg/L.
As a potential solution to the persistent environmental problems of UXO, cyclicN,N′-dinitrourea derivatives may provide an attractive replacement for currently used energetics like RDX. Many cyclicN,N′-dinitrourea derivatives have similar explosive performance parameters to RDX but are more hydrolytically reactive. The objective of this project was to develop and characterize an energetic fill formulation incorporating cyclicN,N′-dinitrourea derivatives as a replacement for currently used energetics like cyclotrimethylenetrinitramine (RDX). This energetic fill would readily decompose into benign products when exposed to moisture (i.e., high humidity or precipitation).
Three cyclicN,N′-dinitrourea derivatives were investigated for use in self-remediating energetic fills; tetranitroglycoluril (TNGU), hexanitrohexaazatricyclododecanedione (HHTDD), and 2,4,8,10-tetranitro-2,4,8,10-tetraazaspiro[5.5]undecane-3,9-dione (TNSUK). TNGU was chosen for scale-up because of its straightforward synthesis, rapid degradation under humid conditions, relative stability in dry conditions, and explosive performance greater than RDX (detonation pressure and detonation velocity. TNGU has been successfully scaled-up to the 400 gram per batch level. The physical properties (purity, density, morphology) and safety properties (impact, friction and electrostatic sensitivity) of TNGU have been characterized. As-synthesized, TNGU crystals have a small particle size and a needle-like crystal morphology that is typically undesirable for plastic bonded explosive (PBX) formulations. Recrystallization efforts examined a variety of conditions to produce larger, lower-aspect-ratio morphologies.
Basic compatibility testing of TNGU with other explosive formulation ingredients has shown that TNGU is compatible with conventional acrylate binders such as lauryl methacrylate (LMA) and 2-ethylhexyl acrylate (EHA). The TNGU/EHA PBX has been scaled up to 50 g. The TNGU/LMA PBX has been scaled up to 250 g. A proton high-resolution magic-angle-spinning nuclear magnetic resonance (1H HR-MAS NMR) technique has been developed to characterize the hydrolyzability of TNGU in PBX formulations. Safety properties, thermal stability, and detonation velocity have also been evaluated.
The ability of the TNGU PBX to self-remediate, even in a hydrophobic binder system, was demonstrated. Ultimately, the TNGU PBX formulations were unable to pass Vacuum Thermal Stability (VTS) testing. We hypothesize that the characteristic that made it attractive as a self-remediating explosive, its rapid hydrolyzability, also made it susceptible to unacceptable degradation under VTS test conditions. Unfortunately, the TNGU PBX’s inability to pass VTS testing indicates that it is unlikely to be developed as an energetic fill in confined ordnance items unless the gas generation at elevated temperatures can be mitigated.
HHTDD degraded much faster than TNGU in water, so we believe that it would also perform poorly in VTS testing. HHTDD was also difficult to synthesize and purify, making it an unattractive candidate for scale-up. HHTDD was not pursued further.
Current synthesis strategies for TNSUK are more complicated (a four step synthesis) than the TNGU synthesis with a low yeild, but still acceptable. TNSUK was synthesized at the 5 gram scale. A small scale (5 gram) TNSUK PBX mix was prepared and preliminary characterization was completed A preliminary hydrolysis experiment was performed using the techniques developed for the TNGU PBX, confirming that the method could be successfully adapted to monitor the progression of hydrolysis in a TNSUK PBX. Further development and characterization of TNSUK and a TNSUK PBX may result in a hydrolyzable PBX with acceptable properties. If TNSUK is pursued with a follow-on effort, its complex synthesis and low-yield will have to be mitigated.
This project demonstrated the feasibility of a high performance self-remediating PBX using a conventional binder system. The development and ultimate implementation of a successful self-remediating PBX would prevent future contamination of military test and training ranges while allowing the continued use of live munitions. We were unable to develop an acceptable TNGU PBX, but the methods developed and lessons learned should facilitate the more efficient development of an acceptable self-remediating PBX.