BULK DENSITY AND TAPPED DENSITY OF POWDERS
The bulk density of a powder is the ratio of the mass of an untapped powder sample and its volume including the contribution of the interparticulate void volume. The bulk density is expressed in grams per mL (g/mL) although the international unit is kilograms per cubic meter (1 g/mL = 1000 kg/m3) because the measurements are made using cylinders.
The bulk density of a powder is determined by measuring the volume of a known weight of powder sample, that may have been passed through a sieve (usually greater than or equally to 1 mm), into a graduated cylinder (Method I), or by measuring the mass of a known volume of powder that has been passed through a volumeter into a cup (Method II) or a measuring vessel (Method III). Method I and Method III are favored.
Commonly used 250-mL cylinder (readable to 2 mL) and approximately 100 g of test sample. If the powder density is too low or too high, such that the test sample has an untapped apparent volume of either more than 250 mL or less than 150 mL, it is not possible to use 100 g of powder sample. Therefore, a different amount of powder has to be selected as the test sample, such that its untapped apparent volume is150–250 mL (apparent volume greater than or equal to 60% of the total volume of the cylinder); the weight of the test sample is specified in the expression of results.
For test samples having an apparent volume between 50 mL and 100 mL, a 100-mL cylinder readable to 1 mL can be used; the volume of the cylinder is specified in the expression of results. Tapped density is obtained by mechanically tapping a graduated measuring cylinder or vessel containing a powder sample.
A 250-mL graduated cylinder (readable to 2 mL with a mass of 220 ± 44 g) commonly used for TD measurement. Carry out 10, 500, and 1250 taps on the same powder sample and read the corresponding volumes V10, V500, and V1250 to the nearest graduated unit. If the difference between V500 and V1250 is less than or equal to 2 mL, V1250 is the tapped volume. If the difference between V500 and V1250 exceeds 2 mL, repeat in increments such as 1250 taps, until the difference between succeeding measurements is less than or equal to 2 mL. If it is not possible to use a 100-g test sample, use a reduced amount and a suitable 100-mLgraduated cylinder (readable to 1 mL) weighing 130 ± 16 g. If the difference between V500 and V1250 is less than or equal to 1 mL, V1250 is the tapped volume.
Using bulk and tapped density powder cmpressiblity can be easily measured by Carr's Index and/or Hausner ratio.
Compressibility Index Formula
Compressibility Index = 100 (VB – VT)/VB [VB – Bulk volume; VT – Tapped volume];
Hausner Ratio Formula
Hausner Ratio = VB/VT [VB – Bulk volume; VT – Tapped volume]
True density - is the mass of a particle divided by its volume, excluding open and closed pores. It's measured by gas pycnometer.
Reference:
- USP-NF 〈616〉
