Solid rocket motor propellants and explosives used within the U.S. Department of Defense (DoD) historically have been formulated to provide the most performance over all other operational requirements. Performance requirements needed to be met in order to defeat real and potential threats. As such, a high degree of risk to both the environmental and sensitivity hazards associated with the resulting weapon platforms was accepted. Often, reaction products resulting from the ingredients selected to provide the needed energy were overlooked with respect to toxicity and associated sensitivity. In order to achieve burning rate and performance objectives, the current fielded minimum signature rocket propellants need to incorporate metal catalysts (primarily lead compounds) and the nitramine RDX. As is well known, chronic or acute exposure to lead has serious health effects on immune, cardiovascular, and central nervous systems. RDX is also known to be acutely toxic to humans. Many studies have shown that accidental ingestion or inhalation has led to central nervous system disorders.

The overall main objective of this SERDP Exploratory Development (SEED) project was to develop and demonstrate the next generation propellants for use in weapons systems that display a more acceptable environmental impact and toxicity by introducing cutting edge energetic materials as replacements for toxic lead catalysts and the nitrogenous poly-cyclic RDX explosive.

This limited-scope project was directed towards demonstrating a novel binder system using an energetically functionalized polyphosphazene backbone augmented with insensitive energetic fillers as the RDX replacement. The investigation was conducted to ascertain the viability of developing a new class of minimum signature propellants using nitrato/azido functionalized polyphosphazenes that provide an energetically dense polymer backbone with significantly superior low-temperature properties compared to existing energetic polymers. In addition, the application of a polyphosphazene energetic (PPZ-E) polymer system with a polymer backbone that contains higher enthalpy functional groups (–ONO₂, -N₃) would allow reaction kinetics and heats of combustion to control burn-rate parameters rather than catalytic kinetics, thus eliminating the need for lead or any other metal burn-rate modifiers. Similarly, RDX nitramine content would also be replaced with significantly more thermodynamically stable energetics that respond more favorably to various stimuli. These energetics would include 1,1-Diamino-2,2-dinitroethylene (FOX-7), Guanylurea dinitramide (FOX-12), or similar ingredients stabilized by hydrogen bonding, overlapping Pi orbitals, and resonance structures.

Many attempts were made to achieve an appropriately cross-linked binder in order to test the project hypotheses, but the resulting formulations would not crosslink effectively. In order to perform the study of record, the investigators chose to use a co-polymer system using similar energetic polymers that would take part in traditional urethane chemistry to augment the polyphosphazene polymers. Although this approach is not ideal when studying the effects of the PPZ-E materials, it was successful in providing cross-linked propellants that subsequently were submitted for testing.

Two alternative energetic polyphosphazene polymers (nitrato and azido functional groups) were tested as potential replacement polymer backbones in rocket motor propellant formulations. After an intensive study to acquire viable propellant candidates, the formulations were characterized via safety/thermal, burning rate, and decomposition properties.

Thermally, the materials behave well and show no signature peaks (early endotherms, large shifts) suggesting incompatibility with the other ingredients in the formulation. Basic safety testing was conducted to assess propellant response to insult or stimuli via impact, friction, and electrostatic discharge. Both propellants performed well within accepted safe handling conditions and showed improved impact results when compared to traditional AP and/or minimum signature propellants. The burning behaviors of both propellants also were within accepted metrics of traditional propellants, with the azido candidate burning exceptionally well.

The decomposition products of two propellant formulations containing polyphosphazene energetic binders were analyzed using Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry (STMBMS) to determine whether the formulations emit hazardous fluorine- and/or phosphorus-containing compounds during thermal decomposition. The measurements showed a multi-stage decomposition process that includes evaporation of BTTN and reactions involving the CHNO compounds and CHNO side-chains of the polyphosphazene binders. The experiments were unable to detect any fluorine- or phosphorus-containing compounds and it appears that the P=N backbone of the polyphosphazene binder does not undergo significant thermal decomposition under the conditions of these experiments.

This SEED project constituted only a feasibility demonstration on whether the energetically functionalized polyphosphazene backbone would perform as a viable substitute for propellants without proving more toxic in nature relative to existing materials. Although the material showed no evolution of hydrogen fluoride gases or problematic phosphoric species (up to the limits of the experimental detection system), it is noted that the analysis was never fully subjected to pressure and temperatures traditional to rocket motor firings. Secondly, a recommendation was made to the manufacturer that in order to fully demonstrate the polymer superiority, the cure issues would need to be addressed. It should be noted that the manufacturer had already taken this task at hand (upon early notification and consultation with the Naval Air Warfare Center Weapons Division) and successfully claimed a hydroxylated functional polyphosphazene polymer for more traditional cure chemistries.