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Many pyrotechnics, propellants, and explosives are comprised of a polymeric binder that holds energetic solids in a plastic matrix. The binder serves many roles in these materials. Initially, the polymer can aid in processing whether it is cast, pressed, or extruded. Furthermore, the polymer mechanically holds all the ingredients together, serving as a structural element literally binding together the final material. This role is especially critical in rocket propellants because cracks and voids in the propellant will lead to motor grain failure, often with catastrophic results. From a safety perspective, the binder physically coats the energetic solids in these materials, thus providing a buffer to minimize the physical and chemical interaction of reactive solids with each other. This lowers the electrostatic discharge, impact, and friction sensitivity of the final material. Finally, and related to rocket propellants, the binder serves as a fuel when the hydrocarbon polymer is combusted by the oxidizer. However, the binder generally diminishes the performance (detonation pressure and velocity) of most explosives. To improve the performance of explosives with significant binder content, and to increase the energy density of propellants, energetic polymers are needed.
While there are energetic binders available the safety benefits of increasing binder content are lost because these materials contain either organic azides or nitrate esters. These functional groups are chemically unstable, easily ignited, and generally create reactive fragments on aging. In fact, propellants that utilize nitrate esters require expensive monitoring programs to ensure both adequate safety properties and performance as the propellant ages. The cost of such monitoring is often cited as one reason most modern explosives do not to use nitrate esters as binder materials. Furthermore, the energetic groups are pendant moieties attached to the polymer, but not incorporated into the polymer backbone. This impairs the physical properties of these polymers and causes the formulator to need a higher weight percent of binder in order to achieve adequate coating. In short, there is a need for improved energetic binders to address safety, performance, aging, and processing requirements.
While tetrazoles are somewhat less energetic than azides or nitrates, the bis-alkyltetrazoles are more thermally stable and substantially less chemically reactive. Higher percentages of these binders could be used without negative safety consequences. Further, the energetic functionality is built into the polymer backbone, minimizing the total moles of pendant atoms. This is anticipated to yield a binder with superior physical properties.
Given this, Navy researchers have developed a process for making energetic cast cured binders by using tetrazole polyols and isocyanate resins for making multifunctional tetrazole based cross-linked polymers. Several formulations have been made and tested – each is disclosed in the patents.
- Better safety and performance characteristics
- US patents 7,517,997; 7,557,220; and 8,053,582 available for express licensing