Measurement of flowability of lubricated powders by the vibrating tube method

The evaluation of lubricity or flowability of pharmaceutical powders is important for consistent production and quality control of drug products. However, there have been only a few studies on quantitative measurements of the properties of lubricated powders.

Magnesium stearate (MgSt) and sodium stearyl fumarate (SSF) were used as lubricants. Lubricated powders were prepared by adding lubricants to spray-dried lactose under different conditions. To evaluate flowability, the vibrating tube method was used. In this method, the vibration amplitude of the tube is increased at a constant rate, and the mass of the powder discharged from the tube is recorded. Flowability profiles, i.e. the relationships between the mass flow rate and vibration acceleration, were obtained experimentally. To characterize static and dynamic friction properties of powders, critical vibration acceleration required to make powder particles flow and the average mass flow rate were determined.

Addition of 0.5% MgSt was sufficient for the reduction of static friction between particles. Blending time of the lubricants had little effect on the average mass flow rate of lubricated powders. On the other hand, addition of SSF resulted in an increase in static friction at the beginning of blending, and after a certain blending time, flowability improved. The combination of MgSt and SSF improved both static and dynamic friction properties irrespective of the blending time.

The vibrating tube method can be used to evaluate the flowability properties of lubricated powders, and the experimental results provide useful information on the production of pharmaceutical solid dosage forms.