Effect of host particle size on the modification of powder flow behaviours for lactose monohydrate following dry coating

Our previous work demonstrated that the powder flowability of a cohesive lactose sample can be improved substantially using a dry coating technique. Our study reported here aims to investigate the influence of host particle size on the modification of powder flowability following dry coating process. Four commercial lactose monohydrate powders with different particle sizes were coated by an intensive mechanical process or mixed using a conventional tumbling process, both with magnesium stearate. All four untreated lactose samples showed a relatively poor powder flow. After dry coating, poured and tapped densities of all the lactose samples increased, while Carr indices and Hausner ratios decreased substantially. The angle of repose values were reduced to a notable extent only for particles with a median size larger than about 7 I?m after dry coating. Both specific energy (SE) and cohesion values of lactose samples, measured by a powder rheometer system, decreased substantially after coating. In contrast, no apparent changes in powder flow were evident for conventionally mixed batches, except that in the dynamic powder rheometry measurement, a relatively small change in SE was observed. This study demonstrated that for the finer particles examined, cohesive forces were more influential in the powder bed after the surface treatment and resulted in a relatively poor flow. However, for the larger powders studied, the cohesive inter-particle forces could be overcome after this dry coating, whereby satisfactory flow could be obtained. This study indicated that the host particle size was a critical factor in influencing the modification of cohesive powder flowability.