<616> BULK DENSITY AND TAPPED DENSITY

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BULK DENSITY AND TAPPED DENSITY

The bulk density of a solid is often very difficult to measure since the slightest disturbance of the bed may result in a new bulk density. Moreover, it is clear that the bulking properties of a powder are dependent on the “history” of the powder (e.g., how it was handled), and that it can be packed to have a range of bulk densities. Thus, it is essential in reporting bulk density to specify how the determination was made.

Because the interparticulate interactions that influence 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. The bulk density often is the bulk density of the powder “as poured” or as passively filled into a measuring vessel. The tapped density is a limiting density attained after “tapping down,” usually in a device that lifts and drops a volumetric measuring cylinder containing the powder a fixed distance.

BULK DENSITY

Bulk density is determined by measuring the volume of a known mass of powder sample that has been passed through a screen into a graduated cylinder (Method I) or through a volume-measuring apparatus into a cup (Method II).

Method I—Measurement in a Graduated Cylinder

Procedure— Unless otherwise specified, pass a quantity of material sufficient to complete the test through a 1.00-mm (No. 18) screen to break up agglomerates that may have formed during storage. Into a dry 250-mL cylinder introduce, without compacting, approximately 100 g of test sample, M, weighed with 0.1% accuracy. If it is not possible to use 100 g, the amount of the test sample and the volume of the cylinder may be modified and the test conditions specified with the results. Select a sample mass having an untapped apparent volume of 150 to 250 mL. A 100-mL cylinder is used for apparent volumes between 50 mL and 100 mL. Carefully level the powder without compacting, if necessary, and read the unsettled apparent volume, Vo, to the nearest graduated unit. Calculate the bulk density, in g per mL1, by the formula:

(M) / (Vo).

Generally replicate determinations are desirable for the determination of this property.

Method II—Measurement in a Volumeter

The apparatus (Fig. 1),

Fig. 1. Scott Volumeter.

conforming to the dimensions in ASTM B 329-90 (Scott Volumeter)2, consists of a top funnel fitted with a 1.00-mm (No. 18) screen or the screen opening specified in the individual monograph. The funnel is mounted over a baffle box containing four glass baffle plates over which the powder slides and bounces as it passes. At the bottom of the baffle box is a funnel that collects the powder and allows it to pour into a cup of specified capacity mounted directly below it. The cup may be cylindrical (25.00 ± 0.05 mL volume with an inside diameter of 30.00 ± 2.00 mm) or a square (16.39 ± 0.05 mL volume with inside dimensions of 25.4 ± 0.076 mm).

Procedure— Allow an excess of powder to flow through the apparatus into the sample receiving cup until it overflows, using a minimum of 25 cm3 of powder with the square cup and 35 cm3 of powder with the cylindrical cup. Carefully scrape excess powder from the top of the cup by smoothly moving the edge of the blade of a spatula perpendicular to and in contact with the top surface of the cup, taking care to keep the spatula perpendicular to prevent packing or removal of powder from the cup. Remove any material from the sides of the cup, and determine the weight, M, of the powder to the nearest 0.1%. Calculate the bulk density, in g per mL, by the formula:

(M ) / (Vo)

in which Vo is the volume, in mL, of the cup. Generally replicate determinations are desirable for the determination of this property.

TAPPED DENSITY

Tapped density is achieved by mechanically tapping a measuring cylinder containing a powder sample. After observing the initial volume, the cylinder is mechanically tapped, and volume readings are taken until little further volume change is observed. The mechanical tapping is achieved by raising the cylinder and allowing it to drop under its own weight a specified distance by either of two methods as described below. Devices that rotate the cylinder during tapping may be preferred to minimize any possible separation of the mass during tapping down.

Method I

Procedure— Unless otherwise specified, pass a quantity of material sufficient to complete the test through a 1.00-mm (No. 18) screen to break up agglomerates that may have formed during storage. Into a dry 250-mL glass graduated cylinder (readable to 2 mL) weighing 220 ± 44 g and mounted on a holder weighing 450 ± 10 g introduce, without compacting, approximately 100 g of test sample, M, weighed with 0.1% accuracy. If it is not possible to use 100 g, the amount of the test sample may be reduced and the volume of the cylinder may be modified by using a suitable 100-mL graduated cylinder (readable to 1 mL) weighing 130 ± 16 g and mounted on a holder weighing 240 ± 12 g. The modified test conditions are specified with the results. Carefully level the powder without compacting, if necessary, and read the unsettled apparent volume, Vo, to the nearest graduated unit.

Mechanically tap the cylinder containing the sample by raising the cylinder and allowing it to drop under its own weight using a suitable mechanical tapped density tester that provides a fixed drop of 14 ± 2 mm at a nominal rate of 300 drops per minute. Unless otherwise specified, tap the cylinder 500 times initially and measure the tapped volume, Va, to the nearest graduated unit. Repeat the tapping an additional 750 times and measure the tapped volume, Vb, to the nearest graduated unit. [note—Fewer taps may be appropriate, if validated, for some powders.] If the difference between the two volumes is less than 2%, Vb is the final tapped volume, Vf . Repeat in increments of 1250 taps, as needed, until the difference between succeeding measurements is less than 2%. Calculate the tapped density, in g per mL, by the formula:

(M ) / (Vf).

Generally replicate determinations are desirable for the determination of this property.

Method II

Proceed as directed under Method I except that a suitable mechanical tapped density tester that provides a fixed drop of 3 mm (±10%) at a nominal rate of 250 drops per minute is used.

MEASURES OF POWDER COMPRESSIBILITY

The Compressibility Index and Hausner Ratio are measures of the propensity of a powder to be compressed. As such, they are measures of the relative importance of interparticulate interactions. In a free-flowing powder, such interactions are generally less significant, and the bulk and tapped densities will be closer in value. For poorer flowing materials, there are frequently greater interparticle 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— Calculate by the formula:

Hausner Ratio— Calculate by the formula:

1 The density of solids normally is expressed in g per cm3 and that of liquids is normally expressed in g per mL; however, because powder volumes are measured in cylinders graduated in mL, bulk and tapped densities will be expressed in g per mL. By definition, the mL and the cm3 are equivalent volumes.

2 Apparatus may be purchased from scientific supply companies and is usually described as a “Scott, Schaeffer and White Paint Pigment Volumeter.”

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USP32–NF27 Page 226

Pharmacopeial Forum: Volume No. 31(3) Page 909