The bulk density is an economically, commercially, and functionally
important property. When shipping powders over long distances the producers are
of course inter-ested in a high bulk density in order to reduce the shipping
volume. A high bulk density also saves packing material and storage
capacity.
For some powders the aim is a low bulk density, obtained by agglomeration,
for optical reasons, or because of requirements to instant powder
production.
Bulk density is defined as the weight of a given volume of powder and is
expressed in g/ml, g/100 ml, or g/l. The reciprocal value is the bulk volume
which is expressed in ml/100 g or ml/g. The bulk volume is usually used when a
graduated cylinder glass is used for the determination. The volume of 100 g of
powder is then measured in the cylinder. As to the other method giving the bulk
density, the weight of the powder in a 100 ml cylinder is measured. Both
results can naturally be converted to the other expression. See Fig. 104. The
value may either be expressed as tapped 0 times (loose), tapped 10 times
(poured), 100 times, or 1250 times. Various types of equipment can be used for
the tapping. Also manual tapping is used. The intensity of the tapping is
naturally influencing the value.
Fig. 104
Determination of powder bulk density
The bulk density of milk powders is a very complex property, as it is a
result of several other properties. However, the primary factors determining
the bulk density are:
- Particle density, given by:
- the solids density, a function of
product composition
- the content of occluded air in the particles
- Amount of interstitial air, i.e. air between particles (agglomeration)
- Flowability
These properties are discussed in the following.