The widespread use of powders in the pharmaceutical industry has led to a proliferation of test methods for measuring powder flow.
The new harmonised chapters in the Pharmacopoeia on Powder Flow (USP Chapter and Ph. Eur. Chapter 2.9.36) list four well-defined methods for powder testing aimed at trying to bring about some degree of standardisation within the existing test methodology:
- Flow through an orifice
- Angle of Repose
- Shear Cell
- Compressibility Index and Hausner Ratio
The new Flowability Tester BEP2 provides a range of options for testing pharmaceutical powders including three of the four methods quoted in the pharmacopoeia – flow through an orifice, angle of repose and shear cell – in a single, cost effective unit.
In addition to providing the test methods detailed in the harmonized pharmacopoeia chapters, it is also suitable for flowability testing according to Ph.Eur. 2.9.16.
An optional balance/timer simplifies time vs mass testing.
The BEP2 is an easy to use, small footprint instrument with interchangeable cylinder, funnel, angle-of-repose and shear cell attachments. A description of each attachment can be found below.
Cylinder Attachment (Flow Through an Orifice)
Measuring the ability and the time taken for a powder to flow through an orifice of known size is a useful method of quantifying powder flow.
At the same time, it is important to recognise that the ability of the powder to flow through the orifice can be affected by factors other than the characteristics of the powder itself.
Such factors include the shape and material employed in the construction of the powder container, the diameter and height of the powder bed and the shape of the orifice concerned.
The Pharmacopoeia suggest that the use of a circular cylinder as the powder container encourages powder over powder flow as opposed to powder over container wall, minimising any effect brought about by differences in the material used to produce the powder container. As the title suggests, this technique is only suitable for materials that flow – not cohesive materials. Assuming this to be the case, then the Pharmacopoeia suggests that providing:
- The height of the powder bed (the ‘head’) is much greater than that of the orifice
- The diameter of the opening is greater than 6 times the diameter of the particles
- The diameter of the cylinder is greater than 2 times the diameter of the opening then any difference in results brought about by either powder bed or orifice can be considered negligible. The cylinder attachment has been designed to take all of these factors into account.
The cylinder attachment comprises a stainless steel cylinder measuring 76 mm long x 57 mm i.d. and having a capacity of 200ml. The bottom of the cylinder is sealed with a collar designed to accept discs having various orifice diameters.
The attachment comes complete with a set of 20 interchangeable stainless steel discs each containing a precision drilled hole in the centre covering the following sizes:
4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20,22, 24, 26, 28, 30, 32, 34 and 36 mm.
A shutter covers the hole during filling. This can be smoothly removed without vibration to allow the powder to flow through the selected hole. The cylinder attachment can be used in two ways:
- To carryout quantitative flowability tests based on mass vs time
- To determine the intrinsic flowability of the powder concerned in the form of a flowability index based on comparative measurements.
Funnel Attachment (Flow Through an Orifice)
In certain instances where, for example, the purpose of the test is to simulate flow in a hopper or other production situation, it may be preferable to use a funnel in the form of a truncated cone.
The funnel attachment is based on the stainless steel flow funnel and nozzle described in the European Pharmacopoeia (EP) Chapter 2.9.16 for flowability. It has a capacity of approx. 400ml. The attachment is supplied with three nozzles corresponding to aperture sizes of 10, 15 and 25 mm respectively.
Both funnel and nozzles are manufactured from pharmaceutical grade 316 stainless steel. The nozzles can be quickly interchanged using the connecting nut provided for that purpose.
The opening at the bottom of the funnel is secured by means of an adjustable shutter which is closed during the filling operation. The test is carried out in a similar manner to that of Method A (Mass vs Time) of the cylinder attachment
Balance Timer Attachment
By adding a balance and a timer linked to a micro-switch located on the shutter mechanism, it is now possible to conduct time vs mass tests using either cylinder or funnel methods without the need for an external stopwatch.
The balance/timer option allows the use of the unit in 4 modes:
- Determination of the flow time of a predetermined sample weight
- Determination of the flow time of a predetermined sample volume
- Determination of the weight of sample in a predetermine time
- Plot of time against sample weight (weight/time)
Angle of Repose Attachment
The angle of repose is the angle (relative to the horizontal base) of the conical pile produced when a granular material is poured onto a horizontal surface. It is related to the density, surface area and coefficient of friction of the material concerned.
The angle of repose attachment comprises a 100 mm diameter circular test platform together with a digital height gauge, having a range of 0-300 mm and an accuracy of 0.03 mm. The test platform has a protruding outer lip in order to retain a layer of powder upon which the cone is formed. Surplus powder is collected in a tray below the test platform. For this particular test, the funnel is normally equipped with a special 10 mm i.d. nozzle mounted 75 mm above the test platform.
If necessary, the contents may be stirred to assist in the powder flow. The tangent of the angle of repose (in degrees) can be determined by reading off the height of the powder cone in mm from the digital display of the height gauge and dividing it by 50.
Shear Cell Attachment
Shear cell methodology is widely used in the pharmaceutical industry to determine the flow properties of fine grained powders and bulk solids and how they will behave in bins, hoppers, feeders and other handling equipment. The ability of a material to flow through such devices is dependent on the bulk density of the material and its shear strength.
The shear cell employed with the BEP2 comprises a cylindrical chamber (manufactured from clear acrylic) measuring 140 mm i.d. and 32.5 mm high and capable of holding 500 ml of the sample. In the floor of the chamber, there is a 100 mm hole which can be sealed during the consolidation process using an acrylic disc provided for this purpose.The test is based on measuring the force required to shear a circular disc through a prepared sample of bulk material. It comprises two stages:
- Sample consolidation (bulk density measurement)
- Failure inducement (shear strength)
The sample is first subjected to a consolidated load such that the bulk density of the material can be determined – ideally, this should be similar to the loads experienced by the material in practice.
Alternatively, a standard reference can be employed e.g. 10 kilos. The acrylic disc sealing the bottom of the test cell is now removed and load steadily applied to the test sample by pouring sand through a funnel into a container of appropriate proportions resting on top of the sample until such time as the sample fails (shears).
The results should be expressed in terms of bulk density, shear strength and if appropriate, estimate of device outlet required.