The most commonly used form of drug administration is the tablet. This is due to its many advantages: it is cost effective to manufacture, and easy to handle, dispense and administer to patients. There are also other subtle advantages, such as the ability to formulate controlled release and the phenomenon of increased stability of the drug due to its design as a solid powder form, as opposed to a liquid form. A tablet formulation normally consists of several excipients, each with its own particular function, either related to the manufacturing process of tablet or to the desired physicochemical properties of the tablet, such as mechanical strength, increased stability, or improved disintegration and dissolution. During direct compression tablet manufacture , powders are compressed in a die to yield solid tablets. In order to achieve uniformity of mass and content amongst tablets, the powder must be able to flow well enough from the hopper into the die; thus flow of powders is of critical importance in direct compression tablet manufacture. Obtaining a free-flowing powder, as opposed to a cohesive powder, requires that the correct balance be achieved between the driving forces, such as gravity, particle mass, and angle of inclination of the bed, and the drag forces, such as adhesive, cohesive and surface forces. The latter may be influenced by the amount of moisture present during storage of the powder.
Microcrystalline cellulose (MCC), a commonly used excipient in the pharmaceutical industry, is available in a range of particle sizes. The objective of this study was to investigate the effect of moisture on the flow properties of six different grades of MCC having different particle sizes and size distributions, both as a single excipient and in a direct compression formulation containing a significant amount of MCC. The MCC powders, and the formulations were subjected to flow measurements testing, including bulk, poured and tapped densities, height of conical pile and flow rate through a defined orifice diameter; the values obtained were used to calculate various flow indices. Following testing on Day 0, around 400 g of the different MCC grades and formulations were stored in a stability chamber, where high humidity was induced. The relative humidity found in the chamber was recorded using a data logger. The flow properties of the MCC grades and the different formulations were tested on Day 14 and 35.
From the results obtained, it was observed that powder flow is dependent on particle size, with the worst flow being achieved by MCC grades with the smallest particle size, and hence larger surface area for cohesive forces to form. The flow properties of both the MCC and the formulations deteriorated with prolonged exposure to humid conditions, this being apparently due to agglomeration and inter-particle interactions mediated by the moist environment; increases in mean particle sizes were observed both by sieving and microscopy. It was thus concluded that relative humidity control during storage of MCC and its formulations is important in order to ensure that the flow properties of the powders are not adversely affected.