Twin-Screw Extrusion of Nano-Alumina–Based Simulants of Energetic Formulations Involving Gel-Based Binders

A 7.5-mm twin-screw extruder was developed specifically for the processing of energetic formulations involving nanoparticles. Prior to extrusion of energetic formulations, simulants of CMC, water, and alumina nanoparticle gels were extruded. Quantitative measures of degree of mixedness (statistics of concentration distributions) were obtained on samples processed with the twin-screw extruder and with conventional processing methods using wide-angle X-ray diffraction (WAXD) and thermo-gravimetric analysis (TGA) and were corroborated with microscopy. Twin-screw extrusion process generated more homogeneous mixtures of nanoparticles than conventional (intensive batch) mixing technologies and the use of surfactants further improved the homogeneity. With increasing homogeneity the suspension exhibited lower elasticity and shear viscosity. Overall, the results of this study emphasize the important roles played by the surface properties of rigid particles, the interfacial tension between the particles and the binder, and the rheological behavior of the binder. In the absence of properly selected binder and surfactant(s), the processing of nanoparticles, without agglomeration, is difficult to achieve. This finding may be relevant to the evaluation of past efforts, which have aimed to improve the ultimate properties of energetic formulations by incorporating nanoparticles.