Introduction:- The poured and tapped bulk density techniques are used to characterize powder flow-ability. Poured bulk density is determined by pouring a sample of known mass into a graduated cylinder, and measuring the volume occupied. Tapped density is then determined by measuring the volume occupied by the sample after it is subjected to a prescribed amplitude and frequency of tapping over a prescribed period of time using a tapped density tester. These quantities are often used to calculate the Hausner Ratio and Carr Index, which are dimensionless measures of powder flow-ability.
ASTM D7481-09:Standard Testing Procedure:-
- Ensure that the measuring scale is zero.
- Ensure that the graduated cylinder to be used is clean and dry.
- A 100 mL graduated cylinder should be used for a median particle size less than 1.3 mm.
- A 250 mL graduated cylinder should be used for a median particle size less than 1.7 mm.
- Weigh the selected cylinder to the nearest 0.1 g.
- Poured Bulk Density:-
- Pour the material carefully into the graduated cylinder.
- If using very fine powder, it may be necessary to first pass the material through a screen to break up agglomerates.
- Read the volume on the graduated cylinder.
- Measure the combined mass of the sample and graduated cylinder to the nearest 0.1 g and record this value.
- Tapped Bulk Density
- After performing steps 1 through 3 as indicated above, tap the cylinder using a tapped density tester.
- As specified in the ASTM standard, the tester should vertically drop the cylinder a distance of 14 ± 2 mm at a rate of 300 times per minute.
- An alternative method described in the ASTM standard is to vertically drop the cylinder a distance of 3 ± 0.3 mm at a rate of 250 taps per minute.
- Tap the cylinder 500 times and record the volume from the graduated cylinder.
- Tap the cylinder an additional 750 times and again record the volume
Calculations:- To calculate the poured bulk density, determine the mass of the sample by subtracting the mass of the graduated cylinder from the combined mass of the cylinder and the powder sample. Then divide the mass of the sample by the untapped volume occupied by the sample.
The tapped bulk density is calculated by dividing the mass of the sample by the final tapped volume of the sample.
USP-NF Standard Testing Procedure
- Poured Bulk Density:-
- If necessary, pass the sample through a 1 mm sieve to break up agglomerates prior to testing.
- Weigh approximately 100g of powder and record the mass of the sample.
- Add the sample to a 250 mL graduated cylinder, taking care to not compact the powder bed.
- If the powder has a bulk volume greater than 250 mL, a smaller sample size should be used.
- If the powder has a bulk volume between 50 and 100 mL, a 100 mL graduated cylinder may be used.
- Read the volume, V0, occupied by the sample to the nearest unit marked on the graduated cylinder.
- Calculate the poured bulk density as
- Tapped Bulk Density:-
- A mechanical tapping device should be selected that is capable of providing the following requirements.
- 250 ± 15 taps per minute at a drop height of 3 ± 0.2 mm
- 300 ± 15 taps per minute at a drop height of 14 ± 2 mm
- Perform steps 1 through 3 from the poured bulk density procedure above.
- Tap 10, 500, and 1250 times for the same sample, and record the corresponding volumes V10, V500, and V1250.
- Calculate the tapped density in terms of the sample mass m, and the tapped volume Vt
Measures of Powder Compressibility:- Because the interparticulate interactions influencing the bulking properties of a powder are also the interactions that interfere with powder flow, a comparison of the bulk and tapped densities can give a measure of the relative importance of these interactions in a given powder. Such a comparison is often used as an index of the ability of the powder to flow, for example the Compressibility Index or the Hausner Ratio.
The Compressibility Index and Hausner Ratio are measures of the propensity of a powder to be compressed as described above. As such, they are measures of the powder ability to settle and they permit an assessment of the relative importance of interparticulate interactions. In a free-flowing powder, such interactions are less significant, and the bulk and tapped densities will be closer in value. For poorer flowing materials, there are frequently greater interparticulate interactions, and a greater difference between the bulk and tapped densities will be observed. These differences are reflected in the Compressibility Index and the Hausner Ratio.
Compressibility Index = 100 (V0 – Vf)/V0
V0: unsettled apparent volume
Vt: final tapped volume
Hausner Ratio = V0/Vt
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