Prospects of Using Boron Powders As Fuel. III. Influence of Polymer Binders on the Composition of Condensed Gasification Products of Model Boron-Containing Compositions

The effect of polymer binders 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 the gas generator of a ducted rocket engine. The physicochemical transformations of boron particles during combustion wave propagation in a solid propellant and during the interaction with the decomposition products of oxidizer and binder were investigated. The binder components were oligodiene urethane and polyether urethane rubbers plasticized with various substances: phthalic acid dimethyl ester, transformer oil, and dioctyl sebacate. Samples of successively complicated composition with the same rubber-to-plasticizer ratio were studied. After thermal analysis of plasticizers and rubbers, plasticized binders and compositions of these binders with boron were investigated and the combustion parameters of model compositions obtained by adding a third component—ammonium perchlorate—were determined. It was found that the thermal decomposition 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 collected from the gas phase directly from the burning surface and those remaining in the form of a porous framework after combustion 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 gas generators of ducted rocket engines. The possibility of the formation of boron suboxide (B6O) crystals during combustion of boron-containing compositions is shown for the first time.