Dry particle coating for improving the flowability of cohesive powders

Several dry processing techniques are used to coat cohesive cornstarch powder with different size silica particles. For nanosized silica guest particles, FESEM images show that both the magnetic assisted impaction coater (MAIC) and the hybridizer (HB) produce particles that are significantly more uniformly coated than using either a V-shape blender or simple hand mixing. Image analysis confirms that MAIC and HB provide higher surface coverage for the amount of guest material (flow aid) used. The improvement in flowablity of coated cornstarch is determined from angle of repose measurements using a Hosokawa powder tester. These measurements show that nanosized silica provides the best flowability enhancement, whereas mono-dispersed 500-nm silica does not improve the flow properties of cornstarch at all. This observation agrees with a simple theoretical derivation based on the original Rumpf model, which shows that the flowability improvement is inversely proportional to the guest particle size for a given host particle size or size of surface asperities. Experimental results also indicate that surface-treated hydrophobic silica is more effective in improving the flowability of cornstarch particles than untreated hydrophilic silica. An increase in processing time using MAIC and the V-blender also improves the flowability of the cornstarch since the guest particles are more deagglomerated and better dispersed, the longer the processing time.