POWDER TECH NOTE 24
GEOMETRIC (Envelope) DENSITY WITHOUT MERCURY
One of the most common density measurements involves
the determination of the geometric space occupied within the
envelope of a solid material, including any interior voids,
cracks or pores. This is called geometric, envelope or bulk
density. And only equals true density when there are no
internal openings in the material being measured.
If the material has a uniform rectilinear or spherical shape,
the volume it occupies can be calculated from measurements
by caliper or ruler. However, a great majority of measurements
involve complex shapes in formed materials such as ceramics,
powder metals, molded polymers and granulated or pelleted
products. In this case, the unknown volume may be
determined by immersing it in liquid and measuring the
volume of liquid displaced, which is the Archimedes principle.
For decades*, liquid mercury has been used to measure
geometric (envelope) density because its non-wetting nature
prevents its entry into small voids. However, if there are
problems with mercury amalgamation, safe handling or
disposal facilities, liquids such as hot wax**, oil, kerosene or
water have been used. In the latter case, the sample is usually
coated with a polymer spray to seal openings and an
appropriate correction is made in each measurement.
*ASTM C493-93
TAP DENSITY
Each particle of a solid material has the same true density
after grinding, milling or processing, but more geometric space
is occupied by the material. In other words, the geometric
density is less, approaching 50% less than true density if the
particles are spherical.
Handling or vibration of powdered material causes the
smaller particles to work their way into the spaces between the
larger particles. The geometric space occupied by the powder
decreases and its density increases. Ultimately, no further
natural particle packing can be measured without the addition
of pressure. Maximum particle packing is achieved.
Under controlled conditions of tap rate, tap force (fall) and
cylinder diameter, the condition of maximum packing
efficiency is highly reproducible. This tap density
measurement is formalized in the British Pharmacopoeia
method for the Apparent Volume, ISO 787/11 and ASTM
standard test methods B527, D1464 and D4781 for tap density.
(Over)
**ASTM B527-93
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PN 59000-24
POWDER TECH NOTE 24
POWDER PYCNOMETRY
MATERIALS
At ambient pressure, only voids in solid materials greater
than 14.5 µm can be penetrated by mercury.
That being the case, a free flowing powder having a narrow
particle size distribution centering on 14.5 µm will enter or be
excluded from cylindrical, slit, wedge, slit/wedge and
constricted (ink bottle) openings in much the same manner as
liquid mercury.
Quantachrome provides a particle size reference material,
PP 14, a pycnometric powder that is well suited to be
employed as described above.
Automated tap density determinations are performed either
by the Quantachrome Autotap or two sample Dual Autotap.
These units accept standard graduated cylinders from 10 to
1000 ml. During tapping, cylinder rotation helps to maintain a
level powder surface for accurate readings. Automatic
operation is controlled by a preset number of taps up to 9999.
Both tap units conform to the tap density standards cited
herein.
Further, if the pycnometric powder has a known tap
density, the volume displaced by the unknown sample is easily
calculated from the tap density of the combination of a
pycnometric powder reference + unknown sample.
Vp = Vmix + [Vmix – V0] (m0/ms)
Where:
Vp = geometric (envelope) volume, unknown [cc/g]
Vmix = sum of volumes, reference + unknown [cc/g]
V0 = pycnometric powder volume, reference [cc/g]
m0 = mass of reference [g]
ms = mass of unknown [g]
This alternative method of powder pycnometry has been
demonstrated to agree within <1% with the method of mercury
immersion pycnometry1.
Autotap and Dual Autotap
[1] C.A. Leon y Leon, et al., A Comparison Between Particle Densities determined by
Mercury Displacement and Solid Pycnometry, Powder Technology, submitted (1997)
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PN 59000-24