Anthony A. Parker

Settling Behavior of Alumina Dispersions and Resultant Green Body Characteristics: Solvent, Binder, and Dispersant Competition in Model Systems

The chemical functionality of binders, dispersants, and solvents will influence competitive adsorption/desorption behavior on alumina powder, and hence will affect both slurry and ultimate green tape properties. Given that multiple competitive interactions are common to most dispersions, it is usually difficult to ascertain mechanisms from simple settling density experiments. However, this problem can be partially overcome with the choice of model systems that minimize the number of competitive processes. This criterion is met to a first approximation with a model system of toluene solvent, polystyrene binder, and a C8 aliphatic dispersant with an anchor group of variable functionality. Plateau adsorption concentrations from settling experiments in toluene show that surface coverage for efficient dispersants is typically achieved at about 6μmoles/m 2 . Less efficient dispersants adsorb at higher plateau concentrations, and are eventually displaced after repeated washing steps in toluene. The settling densities of dispersions prepared with C 8 dispersants are consistently higher than densities achieved in toluene alone, and as seen in a case study with n-octylsilane, the settling densities are independent of the presence of polystyrene. Green bodies cast from a polystyrene/Al 2 O 3 slurry also show increased densities in the presence of n-octylsilane dispersant. However, despite the apparent low levels of interaction between polystyrene and n-octylsilane in the dispersion state, solid state NMR and dynamic mechanical results show that the solid state molecular motional behavior of polystyrene is strongly affected by the presence of n-octylsilane dispersant.

NMR Studies of Solvent Dynamics in High Solids Alumina Slurries

Solid state 13 C NMR techniques have been used to study the mobility of both toluene and ethanol in slurries containing 70% alumina by weight. A comparison of relative signal intensities from cross polarization experiments, and from single excitation experiments under static conditions (no sample spinning) shows that a portion of the solvent molecules are immobilized with “solid” characteristics, while a majority of the molecules exist in mobile or “liquid” like environments. The immobilized solvent is rigid enough to cross polarize with protons, and hence it is characterized by long correlation times, and by a high viscosity. The remainder of the solvent is too mobile to cross polarize, and hence it is characterized by a lower viscosity. The results suggest that solvent is partitioned with the most viscous components being bound near the particle surfaces. These experiments and calculations will be discussed along with the implied effects of solvent partitioning on bulk rheological behavior.