Effect of Poor Solvent Ratio on Rheological Behavior of Alumina Suspension and Properties of Green Sheets

The influence of solubilization of the solvents for PVB binder on the rheological behavior and properties of alumina green sheets was investigated. Suspensions of alumina particles were prepared with various ratios of toluene (poor solvent) and n-butanol (good solvent) including PVB binder. Binder solutions of concentration 20 wt% were opaque at less than 5 wt% of n-butanol ratio, where turbidities, viscosities, TI values and thread-forming properties of binder solutions were large. These results indicate that the binder molecule exhibits a thread-like shape in poor solvent. The amount of binder adsorbed on alumina particles in suspension increased with decreasing the ratio of n-butanol, suggesting that binder at the adsorbed state on alumina particles is more stable than at the state of drifting in the poor bulk solvents. And this result suggests that occupation area of thread-like binder is small in poor solvent. Viscosities, TI values, sedimentation volumes and thread-forming properties of suspensions increased with decreasing the n-butanol ratio. Bulky cohesion structures of alumina particles in suspension with high TI values are supposed to be maintained in sedimentation cakes. However, the packing degree of green sheet after drying was high. The number of binder segments expanding into bulk solution is expected to be fewer, because hydroxyl groups in PVB binder are adsorbed on alumina particles via hydrogen bonding in poor solvent. Expanding binder segment is easy to shrink while drying. Distance between alumina particles decreases, and, as a result, it is thought that the packing degree of alumina particles became high.