Understanding the Different
Behaviors of Powders
Defining Confined and Unconfined Flow
Challenges with Confined and Unconfined
Techniques for Quantifying the Flowability of
Measuring Confined Flow Using Basic Flow Energy
Measuring Unconfined Flow Using Specific Energy
Understanding Cohesivity in Classification of
Understanding the Interparticle Forces Between
Differences Between Cohesive and
The Mechanisms of Powder
Technology is a specialist company pioneering the measurement and
understanding of powders and their flow properties. Founded in 1989, the company
developed the novel, patented technology that forms the core of its Powder
Rheometer system at its design and manufacturing centre in Worcestershire,
UK where all manufacturing takes place in an ISO 9001:2008 accredited
environment. Research into understanding powder behaviour is central to the
company's business strategy.
Technology's business is researching powder behaviour and designing
instrumentation for powder characterisation. The FT4 Powder
Rheometer is a universal powder tester that is really three instruments in
one – combining a powder rheometer with a shear cell capability and a
compression tester. This allows comprehensive characterisation of powders of all
types, reflecting the complexity of powders in contrast to traditional single
number assessments of flowability.
Understanding the Different Behaviors of Powders
can exhibit very different behaviors depending on the conditions to which they
are subjected during storage, handling or processing. For example, powders inside
a hopper or bin are confined, whereas at the outlet of the hopper and during
discharge, they are unconfined and free to flow under gravity. In unconfined
conditions the powder may break into lumps formed of agglomerated powder held
together by cohesive forces, or perhaps separate into individual particles if
the material is non-cohesive. In the latter case, sufficient aeration may
fluidize the powder with potentially serious consequences. The same powder
confined in a hopper may release its air and become very resistant to flow after
a certain period of confinement. In production such differences in the behavior
of any given formulation can be very challenging – especially if the material
characteristics are not well suited to the processing conditions. One of the
keys to successful processing is to match a powder to the demands that a unit
operation will place upon it.
This paper examines the mechanisms associated with powder flow,
contrasting the way in which materials behave when confined and unconfined.
Understanding this important aspect of behavior is a significant step towards
knowledge-based processing, as advocated by QbD.
Defining Confined and Unconfined Flow of Powders
When a sample of powder is swirled in a jar, or tipped onto a surface, it
exhibits unconfined flow behavior – flow is induced by gravitational forces
only. The particles move freely or bond together to form larger agglomerates,
depending on the characteristics of the material. Such conditions often apply in
industrial processes – when powder is poured out of a sack into a hopper, is
pneumatically conveyed, or flows freely into an empty die, for example.
In contrast, when a powder is confined and forced to flow in a certain way,
the particles are less free to move, and the powder behaves quite differently.
It may extrude or ooze into the available volume, or simply lock up in a
formation that is essentially immobile. During processing, confined powder
flow occurs in molding operations, when powder is forced into a partially
full die or mold and during pelletization when a powder feed is extruded into a
stable product form.
Challenges with Confined and Unconfined Powders
that flows freely when unconfined is likely to be much more problematic when
confined and, perhaps less intuitively, the converse is also true. Cohesive
powders that flow poorly when unconfined can move relatively easily under
more forcing conditions. It is therefore important when matching process and
powder to understand what conditions will prevail in the unit operation, and
also the characteristics of the material. So, how can these aspects of flow
behavior be investigated reliably and what does testing tell us about the
mechanisms governing powder flow?
Techniques for Quantifying the Flowability of Powders
The quantification of flowability is a topic that has exercised the minds of
powder processors for decades. There are many different ways of assessing flow
behavior but arguably the most sensitive is dynamic characterization with a
powder rheometer. Here the principle is to measure the energy needed to cause
three dimensional flow, established by rotating a blade through the sample to
generate a prescribed flow pattern. The torque and force acting on the blade are
measured to determine parameters such as Basic Flow
Energy (BFE) and Specific Energy (SE). These
flow energies describe the resistance to flow of the powder in the confined and
unconfined state respectively. Conditioning the sample before measurement
ensures that it is analyzed in a consistent baseline state, which is one of the
reasons why the technique delivers such a high level of reproducibility.
Measuring Confined Flow Using Basic Flow Energy (BFE)
Energy (BFE) is measured by rotating the blade downwards through the
sample. This applies a compacting motion, each particle interacting with its
neighbor but unable to move far because of the container walls. This is why the
value of BFE correlates well with confined flow behavior.
Measuring Unconfined Flow Using Specific Energy (SE)
Energy (SE), in contrast, is measured by rotating the blade upwards
through the sample. The powder particles are free to move up and over the
twisting blade so measurements reflect the ease with which the material will
flow when unconfined. The observed correlation between SE and shear
strength , the parameter measured during shear testing, highlights the
ability of this method to quantify cohesivity.
Understanding Cohesivity in Classification of Powders
Cohesivity is one of the most commonly used terms in the
classification of powders. With a cohesive
powder the particles have a greater tendency to stick to one and another,
and often to other surfaces as well. These forces of attraction may be strong
because of particle size or particle shape, or some combination of these and
other factors. Generally speaking the strength of interparticle forces increases
with decreasing particle diameter, and irregularly-shaped particles have a
greater tendency to lock together than those that are more spherical. The least
powders tend to have spherical, relatively large particles.
Understanding the Interparticle Forces Between Cohesive Powders
The strong interparticle forces of more cohesive
powders give rise to a range of distinctive characteristics. Such powders
have low permeability, high compressibility and, of course, high shear strength.
They are more prone to forming agglomerates and the resulting 'structure’ traps
air within the bed, which is not easily released. It can be quite difficult to
aerate a cohesive material since these agglomerates are not easily broken down
and when air does penetrate it has a tendency to channel rather than flow more
uniformly around individual particles, leaving flow properties largely unchanged
(see figure 1).
Figure 1. Aeration of cohesive and non-cohesive
So, in summary, with a cohesive bed the forces of attraction between
particles are relatively high, giving poor flowability. The bed resists aeration
so this poor flowability is not readily alleviated by blowing air through the
material, although continuous aeration may improve performance. Unless actively
consolidated, cohesive material will tend to entrain a great deal of air,
reducing its bulk density and making it potentially highly compressible.
Quite differently, with less cohesive materials the forces of attraction
between particles are weak to begin with and gravitational forces dominate. If
the powder is aerated, flow energy becomes extremely low, very quickly.
Particles are readily separated by the air, which substantially reduces the
frictional or shear forces between them, so that the material behaves
increasingly like a fluid rather than a powder. Less cohesive materials have
lower compressibility and higher permeability. Such materials are generally
closely packed and contain little entrained air, apart from the substantial void
space between the particles. This means that compressibility is limited by the
elastic and plastic properties of the particles. Permeability tends to be high
because air can flow easily through the void spaces. However, vibration can
promote closer packing of particles in a non-cohesive powder bed increasing both
resistance to flow and bulk density.
Differences Between Cohesive and Non-Cohesive Powders
These fundamental differences between cohesive and
non-cohesive powders have another important consequence with respect to
processing. Cohesive powders release their air reluctantly – usually by
being consolidated over time. On the other hand non-cohesive
powders, especially those greater than 50 µm in size, release entrained air
easily – sometimes so quickly that it compromises flowability.
The Mechanisms of Powder Flow
Comprehensive characterization of two powders at either end of the cohesivity
spectrum – spray dried lactose and finely milled lactose (Table 1) - allows us
to investigate the mechanisms governing flow behavior.
Table 1. Data for finely milled and spray dried
Finely Milled Lactose
Particle size (D50) (µm)
Basic Flowability Energy, BFE (mJ)
Specific Energy, SE (mJ/g)
Shear Stress* (kPa)
% volume change at 18 kPa pressure (Compressibility)
% volume change due to 100 taps
Permeability – pressure drop across powder bed at 11 kPa and 2 mm/s air
Finely milled lactose is clearly the more cohesive material, as
would be expected from its size and shape. When unconfined it has the higher
flow energy (SE) so under these conditions it will flow less easily than
the spray dried lactose; however, under compaction or confined conditions the
situation reverses. The BFE of the finely milled lactose is less than half that
of the spray dried lactose. Why is this?
When the blade pushes down through the finely milled lactose in a compacting
motion it compresses the material. The finely milled lactose, because it holds
air, is relatively easy to compress, the whole bed having an almost 'spongy’
quality to it. The movement of the blade impacts only a small portion of the
sample since the induced displacement can be absorbed quite easily. Here then
the 'flow zone’ is relatively small so the energy needed to move the powder (the
With spray dried lactose the flow zone is very much larger. The spherical,
130 µm (D50), spray dried lactose bed becomes tightly packed, remaining
permeable due to the large void space, but experiencing high inter-particle
friction throughout the flow zone. In this zone, the bed locks up and becomes
difficult to shear as the blade forces its way through the powder. Therefore a
large fraction of the bed moves as the blade rotates giving rise to a high BFE value
(figure 2). Under compaction, beds of this type can easily lock up due to
mechanical interlocking and wedging mechanisms which make shearing (flow), very
Figure 2. Forced flow behavior of non-cohesive and
From a processing point of view these two powders will obviously behave very
differently. The spray dried lactose will flow freely when unconfined and given
the chance will fluidize, but under forced flow conditions it will perform
poorly. Such powders tend to perform well in a tablet press, flowing freely into
the die and forming a strong tablet; compressive forces are transmitted
effectively through the powder plug. In extrusion applications, on the other
hand, performance will be poor. Studies confirm that high pressures are required
to push even a small amount of spray dried lactose through a die, and extrusion
tends to be erratic.
Conversely the finely milled lactose, and similar cohesive
powders, flow quite poorly when unconfined, so flow additives could be
useful to achieve acceptable flowability for many processing steps. However, the
finely milled lactose can flow well when extruded, providing the entrained air
is not removed by excessive squeezing.
An understanding of the mechanisms affecting flow properties is essential in
order to achieve optimal powder processing. Matching powder
properties with the demands of a given unit operation minimizes the
potential for processing problems, designing in manufacturing performance. As
industry increases its focus on processing the need for relevant
characterization at an early stage increases.
More-cohesive powders flow relatively poorly when unconfined but
under the right conditions, may be extruded. In contrast, less-cohesive
materials tend to flow more freely when unconfined but can easily lock up under
forcing conditions, in which case they are able to withstand significant applied
pressure. Dynamic powder characterization coupled with an analysis of
the demands of the process allows the developer to avoid a mismatch between
powder and plant. Building quality into the manufacturing process in this way is
highly beneficial in terms of long-term production economics and an important
goal in the current climate.
Source:Investigating the confined and unconfined flow behavior of
For more information on this source please visit Freeman