A Rheological Study of the Effect of Bentonite on Heated Emulsions

Abstract

Summary The rotational viscometer having a variable speed drive can be used to give a viscosity curve expressed in fundamental units. Two viscosity curves which will characterise the consistency of a pharmaceutical cream base, at a given temperature, can be obtained in a reasonable time for control purposes. The consistency of autoclaved emulsions prepared with and without bentonite has been determined quantitatively and the results tabulated. Emulsions prepared without bentonite showed shear‐rate thinning both before and after autoclaving. A change in type of viscosity from shear‐rate thinning to uniform plastic flow occurred when an emulsion containing bentonite was autoclaved. The entire viscosity curve is necessary to characterise shear‐rate thinning whereas uniform plastic flow can be characterised by two factors: “plastic viscosity” and yield value.