BULK DENSITY:- 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. Hence, the bulk density depends on both the density of powder particles and the spatial arrangement of particles in the powder bed. The bulk density is expressed in grams per millilitre (g/ml) although the international unit is kilogram per cubic metre (1 g/ml = 1000 kg/m3) because the measurements are made using cylinders. It may also be expressed in grams per cubic centimetre (g/cm3). The bulking properties of a powder are dependent upon the preparation, treatment and storage of the sample, i.e. how it was handled. The particles can be packed to have a range of bulk densities and, moreover, the slightest disturbance of the powder bed may result in a changed bulk density. Thus, the bulk density of a powder is often very difficult to measure with good reproducibility and, in reporting the results, it is essential to specify how the determination was made.
Method A:- Measurement in a graduated cylinder
Procedure
Pass a quantity of powder sufficient to complete the test through a sieve with apertures greater than or equal to 1.0 mm, if necessary, to break up agglomerates that may have formed during storage; this must be done gently to avoid changing the nature of the material. Into a dry graduated cylinder of 250 ml (readable to 2 ml), gently introduce, without compacting, approximately 100 g of the test sample (m) weighed with 0.1% accuracy. Carefully level the powder without compacting, if necessary, and read the unsettled apparent volume (V0) to the nearest graduated unit. Calculate the bulk density in (g/ml) using the formula m/V0. Generally, replicate determinations are desirable for the determination of this property. 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 test sample, such that its untapped apparent volume is 150 ml to 250 ml (apparent volume greater than or equal to 60% of the total volume of the cylinder); the mass 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.
Method B:- Measurement in a volumeter Apparatus
The apparatus consists of a top funnel fitted with a 1.0 mm sieve. 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 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 cubical (16.39 ± 0.20 ml volume with inside dimensions of 25.400 ± 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 cubical 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 side of the cup and determine the mass (M) of the powder to the nearest 0.1%. Calculate the bulk density (g/ml) using the formula M/V0 in which V0 is the volume of the cup and record the average of three determinations using three different powder samples.
Method C:- Measurement in a vessel Apparatus
The apparatus consists of a 100 ml cylindrical vessel of stainless steel.
Procedure
Pass a quantity of powder sufficient to complete the test through a 1.0 mm sieve, if necessary, to break up agglomerates that may have formed during storage and allow the obtained sample to flow freely into the measuring vessel until it overflows. Carefully scrape the excess powder from the top of the vessel as described for Method B. Determine the mass (M0) of the powder to the nearest 0.1% by subtraction of the previously determined mass of the empty measuring vessel. Calculate the bulk density (g/ml) using the formula M0/100 and record the average of three determinations using three different powder samples.
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