The effect of mixing on the extrusion-spheronisation of a micro-crystalline cellulose paste

The effect of mixer shear strain rate on the performance of a model micro-crystalline cellulose pharmaceutical paste undergoing extrusion-spheronisation was studied using a laboratory scale planetary mixer and a screw-based mixer. The maximum shear strain rate in each mixer was estimated. Five pastes were prepared, one each at 97 and 265 s(-1) in the planetary mixer, and one each at 304, 988 and 2220 s(-1) in the screw mixer. The rheology of the pastes was quantified by Benbow-Bridgwater characterisation based on ram extrusion. Each paste was spheronised and pellet size and shape distributions obtained by automated size analysis. Mixer type (rather than shear strain rate) was found to have the strongest influence on the paste properties, with the screw-mixed material having a higher yield strength and forming smaller pellets with a narrower size distribution when spheronised under identical conditions.