Compaction Behavior of Spray-Dried Alumina

Compaction behavior and resultant porosity of spray-dried alumina were examined over a range of pressures from 18 to 345 MPa. The variability in pressed density and pore size was measured as a function of spray-dried granule size, binder concentration, and powder moisture content. An exponential behavior was found between pressed density and compaction pressure, density increasing with the logarithm of the compaction pressure. Pore size also displays an exponential behavior, pore diameter decreasing with the log of increasing pressure. The distribution width about the median pore size was noted to decrease approximately in a log-log response to increasing compaction pressure. The behavior of the compaction and porosity curves is related to the degree to which the polymer binder is plasticized which, in turn, affects the pressure at which the spray-dried granules begin to crush. This apparent yield pressure, although dependent on amount of binder and powder moisture, is intrinsically dependent on the water/binder ratio. Finally, by analogy to soil mechanics, an empirical equation relating compact density, yield pressure, and slope is presented.