Tap Density

The bulk density of a powder depends on how closely individual particles pack together. The bulk density is affected not only by the true density of the solids, but by the particle size distribution, particle shape and cohesiveness. It is an important property in packaging and powder handling.

Handling or vibration of powdered material can overcome the cohesive forces and allow particles to move relative to one another and so smaller particles can work their way into the spaces between the larger particles. The total volume occupied by the powder decreases and its density increases. Ultimately no further natural particle packing can be measured without the addition of pressure and maximum particle packing has been achieved!

Under controlled conditions of tap rate, drop height and container size, the condition of maximum packing efficiency is highly reproducible. This tap density measurement is formalized, using graduated measuring cylinders, in the British Pharmacopoeia method for Apparent Volume, ISO 787/11 and ASTM standard test methods B527, D1464 and D4781 for tap density, and others.