Many Department of Defense (DoD) weapons systems make use of energetic fills cured with isocyanates. In addition to the variety of potentially severe health hazards these isocyanates pose to workers during manufacturing, the strong urethane bonds that result from their use can make environmentally friendly demilitarization of energetic fills difficult or impossible. Many of these weapons must be treated through open burn/open detonation or incineration ruling out any recycle, recovery, or reuse (R3) of the energetic fills. To address these problems, this project seeks to replace traditional isocyanates with curatives built around a non-toxic and thermally-reversible Diels-Alder scaffold. Composite materials have been cured using Diels-Alder based moieties since the 1980s, and recent work has demonstrated the promise of such an approach in propellant and explosive applications. Substitution of traditional isocyanates with a thermally-reversible species will remove the problems of both isocyanate toxicity and harsh or incomplete demilitarization. The cure reversion at high temperatures initiated by the reversible curatives will result in grain liquefaction allowing for recovery of the energetic fills.
This project will examine two propellant types for their compatibility with Diels-Alder based reversible curatives: (1) a high-performance high-burn-rate smoky propellant comprised of non-polar HTPB binder, and (2) a minimum signature propellant comprised of polar polycaprolactone binder. While the polymers will be crosslinked using Diels-Alder based chemistry (a dienophile reacting with a diene), the pre-polymer HTPB and polycaprolactone species must be functionalized with a furfuryl-based diene. Two different chemistries to functionalize the polymers with dienes will be examined—an ether-based linker and an ester-based linkage. Once the prepolymers are properly functionalized with a furfuryl-based diene, a dieneophile will then be used to complete the crosslinks between the polymer chains. Four different dieneophile candidates will be examined—two commercially available aromatic species and two alkyl-based dienophiles that must be synthesized. Each of these cure chemistries will be evaluated beginning at the 5g scale to ensure safety and compatibility with the high performance and minimum signature propellant candidates. Preliminary assessment of the mechanical properties will begin at the 5g scale as well. Once safety and compatibility has been established, the propellant mixtures will be scaled to 100g where assessment of the mechanical properties and burning rates will be completed using JANNAF half dogbones and the low pressure window bomb, respectively. The mechanical properties and burning rates will be examined both at room temperature and at elevated temperature. In addition to traditional propellant metrics, the demilitarization possibilities will be explored by dissolution of the propellants in both organic and aqueous conditions. The likelihood of demilitarization success will be judged on percent recovery of the solids under both solvent conditions.
While the search for new environmentally friendly and high performing curative technologies to replace isocyanates in propellant and explosives has proven difficult, recent small-scale formulation work suggests “drop in replacements” may be possible, enabling non-toxic performance improvements. This project will evaluate reversible Diels-Alder scaffolds in both a polar minimum smoke type binder and a non-polar high performance binder. If successful, this work could result in a new generation of high performing non-toxic curatives that do not use isocyanates and enable safe demilitarization and complete recovery of the solid fills. The new toxicity and mechanical properties data generated will provide formulation scientists with additional considerations for these new and promising curative technologies. (Anticipated Project Completion - 2015)