(367c) Polymer Property Impacts on Processing Propellant Composites

Improving the way we process propellant composites is pivotal for the production of novel material formulations that meet performance demands, integrate with new technologies, and are more environmentally conscious. There is a need in the field to define fundamental formulation-processing relationships that can be leveraged to address the challenging nature of developing and processing highly loaded particulate composites. The key challenge of interest to this work is the formation of inhomogeneities in the particle spatial distribution upon shaping of the dense paste propellant precursor. Dense pastes undergo particle network deformation under applied stresses above the yield stress that often cause irreversible particle migration and the formation inhomogeneities. These inhomogeneities are then cured into the propellant grain and can hamper mechanical properties and performance. Herein, inhomogeneity formation after significant deformation is examined via rheological thixotropy testing and is investigated as a function of binder formulation. Model suspensions of inert glass microparticles were employed and the total solids content was kept consistent at 61.4 vol% of an efficient bimodal distribution of small and large glass spheres. Polymer concentration and molecular weight in the binder were systematically varied using polyvinylpyrrolidone (PVP) polymers dissolved in water. Thixotropy tests show that increasing polymer content effectively reduces inhomogeneity formation; and that viscosity is not the only predictor for thixotropic behavior in dense pastes. This observation may be due to polymer entanglements acting to stabilize the particle network. Elucidating the formulation-processing relationship between polymer formulation and thixotropic behavior will fill fundamental knowledge gaps and help enable informed polymer selection during formulation development.