Effect of powder substrate on foaml drainage and collapse: implications to foam granulation

Foam granulation is a relatively newer wet granulation process whereby foamed binder solutions are added to powders in a mixer. It is essential to understand the effect of powder substrate on foam drainage and half-life, which are relevant to nucleation and agglomeration during foam granulation. Hydroxypropyl methylcellulose (HPMC) and hydroxypropyl cellulose (HPC) foams were characterized. Anhydrous lactose and stearic acid were selected as model soluble and insoluble substrates, respectively. The effect of these substrates on foam stability was measured by foam drainage and collapse time and microscopic observations. Both HPMC and HPC foams were similar in foam quality and foam density. Lactose destabilized both HPMC and HPC foams and foam drainage and collapse times were reduced two to four times in the presence of lactose. On the contrary, stearic acid did not significantly change foam drainage and collapse times. Microscopically, lactose exhibited rapid wetting within 15 s upon contacting the HPMC and HPC foam beds, whereas stearic acid remained unwetted even after 8 min and collapse of the foam beds. Substrate solubility can influence foam-substrate interaction. On the basis of this, we suggest potential mechanisms of nucleation and agglomeration of soluble and insoluble substrates during foam granulation.