Effect of polymer binder on dispersion stability of dense non-colloidal pastes

Improving the processing of dense paste composites is pivotal for production of material formulations that consistently meet performance criteria, integrate with new technologies, and are environmentally conscious. There is a need in the field to define formulation-processing relationships that can be leveraged to support the formulation development and processing of dense pastes. The key challenge of interest to this work is the formation of heterogeneities in the particle spatial distribution during processing. Dense pastes undergo particle network deformation under the complex shear and pressure forces experienced during processing. Gravitational forces and applied stresses that exceed the viscoelastic yield stress can cause irreversible particle migration and the formation of heterogeneous particle microstructures. Such heterogeneities, if persistent after solidification into the final composite, may affect mechanical properties and performance. Herein, the influence of polymer molar mass on heterogeneity formation during paste processing is evaluated via settling experiments and rheological testing. The effects of polymer molar mass are deconvoluted from the effects of viscosity using viscosity-matched binder solutions of aqueous polyvinylpyrrolidone or polyethylene oxide and bimodal suspensions of non-colloidal glass particles with a total solids content of 61.4 vol%. We show that low molar mass binders at high concentrations provide improved stability against settling and shear induced migration, which is attributed to balanced viscous dissipation and elasticity. Elucidating the formulation-processing relationships between polymer formulation parameters and paste stability during processing provides insight into the complex rheology of dense pastes, and enables informed polymer selection during formulation development.