The Composition C4 (C4) and hand-moldable M112 demolition blocks are used by all Department of Defense (DoD) services and are consumed at the rate of several hundred thousand pounds per year. Since 1950, C4 has been produced by a water slurry process production process that is time, energy, and labor intensive. The project demonstrated the manufacture of an alternate C4 composition (called PAX-52) by using a twin screw extrusion (TSE) process. The improved process was based on a “green” formulation which replaced the C4 binder system with a silicone polymer (Polydimethylsiloxane) and replaced the research department explosive (RDX) nitramine with the far more eco-friendly High Melt Explosive, Her Majestys Explosive (HMX).
Twin screw extrusion is a continuous manufacturing platform relying on screw elements assembled on two parallel shafts, with the two screws enclosed in a figure-8 barrel with openings for feeding of the solid and liquid ingredients, devolatilization via the application of vacuum and a die for the shaping of the high-solids energetic suspension. The TSE technology presented difficult challenges for the processing of energetic materials so the development of multiple important scientific and technical capabilities prior to its demonstration were required to meet those challenges.
A 500-pound batch of PAX-52 material was produced using the TSE at Picatinny Arsenal and then post-processed-packaged into 257 M112 demolition blocks. The production run met all expectations for steady-state continuous processing with no waste in the production of homogenous, well-formed bulk blocks. The PAX-52 material was characterized against a select panel of energetic qualification tests and found to be within tolerance for all key C4/M112 quality and performance specifications. In particular PAX-52 was found to be cap sensitive and powerful enough to pass the steel plate cutting test.
The DoD may only have to bear a small price premium for the superior PAX-52/M112. The TSE method validated manufacturing cost savings through reductions in labor, energy and process aid waste (water and solvent). A cost assessment which included labor, energy and materials, showed a production cost differential nearly twice that of C4 on a normalized basis, which at face value makes PAX-52 a pricey alternative to C4; however, the project looked past the obvious pound-for-pound HMX-to-RDX price differential to investigate several over-shadowed cost burdens associated with the current technology that would be eliminated by the alternative technology. These other costs (internal, external, and contingent) associated with the impacts C4 production has on ecotoxicity and climate change, human health and occupational, water, land and energy usage, and more, are studied by a Life Cycle Assessment and compared to PAX-52. Here, a Sustainability Analysis assessed Life Cycle Costs (internal to DoD) and impacts on resource availability, human health, and environmental quality (external costs) of both technologies and found PAX-52 to be significantly more sustainable with a lower Total Cost of Ownership.
The PAX-52 TSE technology demonstration garnered positive feedback from the current C4 user community including; combat engineers, ordnance demolition units and future munitions developers. Each had an interest to see PAX-52 at a readiness level that lead to a technology transfer. The technology highlighted for the stakeholders the feasibility of a one-step M112 production process and the PAX-52 product met all their performance and physical requirements; however, their full acceptance of the PAX-52/M112 was deferred until it matched the cost of the C4/M112.
J. He, S. Lee and D. Kalyon, “Shear viscosity and wall slip behavior of dense suspensions of polydisperse particles”, arXiv: 1806.01900 [cond-mat.soft] (2018) is currently prepared for submission to the journal Science.