The objective of this SERDP Exploratory Development (SEED) project is to develop a series of low-toxicity cast-cure binder systems for propellant and explosive formulations. The binder systems will eliminate the use of diisocyanates, phthalates, and other chemical compounds known to pose health and environmental issues. Preliminary experiments will demonstrate the compatibility of these new binders with common explosive materials, as well as the feasibility of the formulations for meeting processability, performance, and sensitivity requirements. Longer term goals include: (1) replacement of the hydroxyl-terminated polybutadiene (HTPB) in a qualified formulation with a binder identified through this project; (2) tuning the chemistry and structure of these low toxicity options to identify binder properties that provide optimum propellant/explosive performance; and (3) developing strategies to enhance the energetic output of these binders by grafting organic explosive components to the polymer.
This project will develop three low toxicity candidate binder systems for potential replacement of the diisocyanate cured HTPB. The candidates include: (1) HTPB elastomer cured with maleic anhydride functionalized polybutadiene (as opposed to diisocyanate cross-linker) in the presence of a nontoxic sebacate-based plasticizer; (2) poly(propylene-glycol) based elastomers cross-linked through epoxy-amine reactive chemistry, also with a nontoxic sebacate-based plasticizer; and (3) lightly cross-linked silicone elastomers cured in the presence of non-toxic silicone oil plasticizer. HTPB based resins are broadly utilized as a binder in cast-cure formulations. While the specific details of each formulation can vary, this binder type is generally composed of HTPB that is cross-linked with a di-functional isocyanate in the presence of a polymer plasticizer. Commonly utilized polymer plasticizers are phthalates, which can pose environmental and health hazards. Currently used diisocyanate curing agents are toxic, expensive, and extremely sensitive to humidity levels during processing. Therefore, isocyanate cured HTPB resins are subject to performance fluctuations, in addition to health and environmental concerns. Each of the low toxicity binder candidates to be developed in this SEED project exhibits chemical versatility that enables tuning over a broad range of mechanical properties (i.e., modulus, glass transition temperature, adhesive behavior). As such, it permits systematic investigation of the binder properties on polymer bonded explosive (PBX) sensitivities, performance, and processing. In addition, these binders possess functional groups enabling modification of the polymer to enhance energetic output, as the project advances.
The expected benefits of this project are the elimination of toxic diisocyanate and phthalates chemistry through the entire weapon lifecycle from synthesis and processing stages through to less toxic by-products resulting from unexploded ordnance (UXO) range contamination. In addition, the work will identify critical mechanical performance requirements for use in formulation binder systems that can guide future research to develop energetic polymer binders. (Anticipated Project Completion - 2015)