Liquid phase migration in the extrusion and squeezing of microcrystalline cellulose pastes

Extensive movement of the liquid phase relative to the solids in solid–liquid pastes during extrusion forming is an undesirable process phenomenon. The impact of formulation and flow pattern on liquid phase migration (LPM) during extrusion of model pharmaceutical pastes (40–50 wt% microcrystalline cellulose/water) has been investigated by ram extrusion through square-entry and 45° conical-entry dies, and by lubricated squeeze flow (extensional flow). Threshold velocities for LPM were observed in both configurations. Squeeze flow testing showed that dilation during extension can cause LPM, while ram extrusion featured both dilation effects and drainage due to compaction. The threshold velocities observed in the two configurations agreed when presented as characteristic shear rates. The threshold velocity increased with paste solids content.