Flow Properties of Powders
The use of powders in the pharmaceutical industry has generated a variety of methods for characterizing powder flow. Not surprisingly, scores of references appear in the pharmaceutical literature, attempting to correlate the various measures of powder flow to manufacturing properties.
The development of such a variety of test methods was inevitable; powder behavior is multifaceted and thus complicates the effort to characterize powder flow. The purpose of this chapter is to review the methods for characterizing powder flow that has appeared most frequently in the pharmaceutical literature. In addition, while it is clear that no single and simple test method can adequately characterize the flow properties of pharmaceutical powders, this chapter proposes the standardization of test methods that may be valuable during pharmaceutical development.
Four commonly reported methods for testing powder flow are
(1) angle of repose,
(2) compressibility index or Hausner ratio,
(3) flow rate through an orifice, and
(4) shear cell.
In addition, numerous variations of each of these basic methods are available. Given the number of test methods and variations, standardizing the test methodology, where possible, would be advantageous. With this goal in mind, the most frequently used methods are discussed below. Important experimental considerations are identified and recommendations are made regarding the standardization of the methods. In general, any method of measuring powder flow should be practical, useful, reproducible, sensitive, and yield meaningful results. It bears repeating that no one simple powder flow method will adequately or completely characterize the wide range of flow properties experienced in the pharmaceutical industry. An appropriate strategy may well be the use of multiple standardized test
methods to characterize the various aspects of powder flow as needed by the pharmaceutical scientist.
ANGLE OF REPOSE
The angle of repose has been used in several branches of science to characterize the flow properties of solids. The angle of repose is a characteristic related to inter-particulate friction or resistance to movement between particles. The angle of repose test results is reported to be very dependent upon the method used. Experimental difficulties arise as a result of the segregation of material and consolidation or aeration of the powder as the cone is formed. Despite its difficulties, the method continues to be used in the pharmaceutical industry, and a number of examples demonstrating its value in predicting manufacturing problems appear in the literature.
The angle of repose is the constant, threedimensional angle (relative to the horizontal base) assumed by a conelike pile of material formed by any of several different methods (described briefly below).
Basic Methods for Angle of Repose
A variety of angle of repose test methods are described in the literature. The most common methods for determining the static angle of repose can be classified on the basis of the following two important experimental variables:
1. The height of the “funnel” through which the powder passes may be fixed relative to the base, or the height may be varied as the pile forms.
2. The base upon which the pile forms may be of fixed diameter or the diameter of the powder cone may be allowed to vary as the pile forms.
Variations in Angle of Repose Methods
In addition to the above methods, the following variations have been used to some extent in the pharmaceutical literature:
The drained angle of repose is determined by allowing an excess quantity of material positioned above a fixed diameter base to “drain” from the container. The formation of a cone of powder on the fixed diameter base allows the determination of the drained angle of repose.
Dynamic angle of repose is determined by filling a cylinder (with a clear, flat cover on one end) and rotating it at a specified speed. The dynamic angle of repose is the angle (relative to the horizontal) formed by the flowing powder. The internal angle of kinetic friction is defined by the plane separating those particles sliding down the top layer of the powder and those particles that are rotating with the drum (with roughened surface).
Angle of Repose General Scale of Flowability
Although there is some variation in the qualitative description of powder flow using the angle of repose, much of the pharmaceutical literature appears to be consistent with the classification by Carr
which is shown in Table 1. There are examples in the literature of formulations with an angle of repose in the range of 40 to 50 that were manufactured satisfactorily. When the angle of repose exceeds 50 , the flow is rarely acceptable for manufacturing purposes.
Experimental Considerations for Angle of Repose
Angle of repose is not an intrinsic property of the powder; i.e., it is very much dependent upon the method used to form the cone of powder. The following important considerations are raised in the existing literature:
The peak of the cone of powder can be distorted by the impact of powder from above. By carefully building the powder cone, the distortion caused by impact can be minimized.
The nature of the base upon which the powder cone is formed influences the angle of repose. It is recommended that the powder cone be formed on a “common base,” which can be achieved by forming the cone of powder on a layer of powder. This can be done by using a base of fixed diameter with a protruding outer edge to retain a layer of powder upon which the cone is formed.
Recommended Procedure for Angle of Repose
Form the angle of repose on a fixed base with a retaining lip to retain a layer of powder on the base.
The base should be free of vibration. Vary the height of the funnel to carefully build up a symmetrical cone of powder. Care should be taken to prevent vibration as the funnel is moved. The funnel height should be maintained approximately 2–4 cm from the top of the powder pile as it is being formed in order to minimize the impact of falling powder on the tip of the cone. If a symmetrical cone of powder cannot be successfully or reproducibly prepared, this method is not appropriate. Determine the angle of repose by measuring the height of the cone of powder and calculating the angle of repose, α, from the following equation:
To analyze and advance powders with respect to flowability and to de-sign powder taking care of hardware like storehouses, feeders, and stream advancing de-indecencies, it is important to know the mechanical properties – the supposed stream properties. In the current paper it is illustrated which actual boundaries de-recorder the stream properties of a powder or a mass strong, and how these para still up in the air tentatively.
Information on the stream properties of a powder or a mass strong is important to plan storehouses and other mass strong taking care of gear so that no stream issues (stream hindrances, isolation, sporadic stream, flooding, and so on) happen. Besides, quantitative data in regards to flowa-bility of mass items is required, for example as a component of relative tests (for example impact of stream specialists or different augmentations on stream conduct) and quality control. The stream properties rely upon a few boundaries, e.g., • molecule size appropriation, • molecule shape, • substance creation of the particles, • dampness, • temperature. It is unimaginable to expect to decide hypothetically the stream conduct of mass solids in reliance of these boundaries. Regardless of whether this were conceivable, the cost for the assurance of all boundaries of impact would be extremely high. Along these lines it is essential, and furthermore easier, to discourage mine the stream properties in suitable testing gadgets. The current paper manages all sort of particulate solids, which are additionally called mass sol-ids, powders, or pulverizes. In the accompanying the overall articulation “mass strong” is utilized for this large number of items.
Flow of Particulate Solids (Powder Flow)
The stream conduct of powders and granulated solids varies profoundly from that of fluids and gases. The key distinction between the two sorts of stream originates from the way that the stream properties of particulate solids rely upon their condition of pressure and at times on their set of experiences of pressure.
(Note: In this segment, the term ‘powder stream’ is utilized for ‘stream of particulate solids’, for quickness. The vast majority of the conversation, be that as it may, is pertinent likewise to particles outside the powder range, like gems and granules.)
The powder stream is significant in various spaces of food designing and innovation. Some are recorded underneath:
•Charging and releasing of capacity canisters, containers and vehicles for mass vehicle
•Solid activity of feeders
•Stream of powders in gauging, dividing and bundling hardware
•Dosing powder items in programmed apportioning and distributing hardware
•Compaction (solidifying) of powder items away
•Pneumatic passing on
Factors Influencing Flow Properties of Powders
Stream properties of powders are essentially mechanical properties. The practices of mass powders are especially like the non-Newtonian liquids because of its plastic stream and dilatancy. The emphasis of particles is impacted by the alluring powers, for example, vanderwaals, molecule size, shape, porosity, dampness content and mass thickness.
Exceptionally fine particles have alluring powers, for example, Van Der Waals, electrostatics, surface strain and so forth and coming about attachment of powder and helpless stream properties. As molecule size expands, stream of the powders additionally increments and later certain level fills in as free streaming. Suitable mix of coarse and fine particles or as such choice of molecule size circulation further develops the stream properties.
Surface Texture of The Particles
Smooth surface of molecule expands the stream properties, but the unpleasant surface is the explanation of helpless flowability because of grating. Level and stretched particles give high porosity and low stream property. Conversely, high thickness and low porosity give great stream property. Powder stream properties can be improved by handling the granules in round shape.
The higher how much dampness, more prominent the danger of attachment and grip. The instrument, which can assist with understanding the impact of dampness on the powder stream properties like electrostatic charge. Fusing ideal stream added substance, for example, magnesium stearate, powder and so on further develops the powder flowability.
The streaming point is impacted by a wide arrangement of boundaries: the grating between the grains, the state of the grains, the firm powers (van der Waals, electrostatic and narrow powers) between the grains.
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