One important characteristic is tapped bulk density, or simply tapped (tap) density: that is, the maximum packing density of a powder (or blend of powders) achieved under the influence of welldefined, externally applied forces. The minimum packed volume thus achieved depends on a number of factors including particle size distribution, true density, particle shape and cohesiveness due to surface forces including moisture. Therefore, the tap density of a material can be used to predict both its flow properties and its compressibility (see right sidebar as to how). These are just two of the many parameters which are important in the overall tabletting process - which requires that loose powders be compacted into a durable solid form with the correct mechanical strength, porosity and dissolution characteristics – and in capsulefilling performance. :The tapped density is obtained by mechanically tapping a graduated cylinder containing the sample until little further volume change is observed. The tapping can be performed using different methods. The tapped density is calculated as mass divided by the final volume of the powder. The interparticulate interactions that influence the bulking properties of a powder are also the interactions that interfere with powder flow. It is therefore possible to gain information about the relative importance of these interactions in a given powder by comparing the bulk and tapped densities, and such a comparison can be used to index the ability of the powder to flow.
The compressibility index and Hausner ratio are measures of the products ability to settle, and permit an assessment of the relative importance of interparticulate interactions. In a free-flowing powder these interactions are less significant and the bulk and tapped densities will be closer in value. For poorly flowing materials, there are greater interparticulate interactions and a greater difference between the bulk and tapped densities will be observed. The differences are reflected in the compressibility index and Hausner ratio. Bulk density is the amount of powder by weight that is present in a defined volume. It is usually expressed as g ml?1 and is obtained by measuring the volume of a fixed weight of powder after it has been tapped for a defined number of times. A high bulk density is very important in packaging and transportation, and is desirable as it can significantly reduce costs. The bulk density is influenced by a range of factors. These include the amount of air entrapped in the powder particles (occluded air), the overall density of the particle (determined by the composition), the air between the individual powder particles (interstitial air), the particle size distribution and the particle shape.