Prospects of using boron powders as fuel. III. Influence of polymer binder on the composition of condensed gasification products of model boron-containing compositions

The effect of polymer binder on the thermal behavior, combustion, and composition of condensed gasification products of model boron-containing compositions was studied to determine the optimal fuel composition for a ducted rocket gas generator. The physicochemical transformations of boron particles during the passage of a combustion wave in a solid propellant composition and the interaction with the decomposition products of the oxidizer and binder were investigated. The binder components were oligodienurethane and polyetherurethane rubbers, plasticized with various substances: dimethyl ester of phthalic acid, transformer oil, and dioctyl sebacate. Samples of a consistently complicated composition were studied while maintaining the ratio between the rubber and plasticizer. After thermal analysis of plasticizers and rubbers, plasticized binders and compositions of these binders with boron were investigated and then the combustion parameters of model compositions obtained by adding a third component, ammonium perchlorate, were determined. It was found that the thermal destruction of the more heat-resistant binder and the melt formation of boron oxide largely overlapped and proceeded simultaneously. Features of the combustion of compositions with various binders were identified by high-speed video recording. The composition and microstructure of condensed gasification products, both taken from the gas phase directly from the burning surface and those remaining after combustion in the form of a porous skeleton, were studied in detail by electron microscopy and thermogravimetry. Based on a comparative analysis of experimental data, it is concluded that it is preferable to use plasticizers and rubbers with reduced thermal stability in inert binders for compositions with amorphous boron designed for ducted rocket gas generators. The possibility of the formation of boron suboxide $(\mathrm{B}_6\mathrm{O})$ crystals during combustion of boron-containing compositions was shown for the first time.