Quality Assurance-Physical
Powder Testing
The widespread use of powders in the pharmaceutical industry has led to a proliferation of test methods for measuring powder flow which is Powder Testing.
The new harmonised chapters in the Pharmacopoeias on Powder Flow (USP Chapter <1174> and Ph.Eur. 2.9.36) list four well defined methods for powder testing aimed at trying to bring about some degree of standardisation within the 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 Pharmacopoeias – flow through an orifice, angle of repose and shear cell – in a single, cost effective unit.
For compressibility index and Hausner ratio testing, we offer a series of tapped density testers and the bulk density tester (Scott Volumeter), detailed monographs for which feature in USP Chapter <616> and Ph.Eur. Chapter 2.9.15.
1) 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.
The cylinder attachment comprises a stainless steel cylinder having a capacity of 200 mL and comes complete with a set of 20 interchangeable stainless steel disks (each having 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) each of which can be secured to the bottom of the cylinder by means of a collar provided for this purpose.
The cylinder attachment can be used in two ways (a) to carry out quantitative flowability tests based on mass vs time or (b) to determine the intrinsic flowability of the powder concerned in the form of a flowability index based on comparative measurements.
In certain circumstances 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 as opposed to a cylinder.
The angle of repose is the angle (relative to the horizontal base) of the conical pile produced when a granular material is poured on to 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.
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 angle of repose can be determined by reading off the height of the powder cone in mm from the digital display of the height gauge and dividing the reading by 50.
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 test is based on measuring the force required to shear a circular disk through a prepared sample of bulk material. 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.
The acrylic disk sealing the bottom of the test cell is now removed and load steadily applied to the test sample by pouring sand thought a funnel into a container of appropriate proportions until such time as the sample fails (shears).
The results are expressed in terms of bulk density, shear strength and , if appropriate, estimate of the device outlet required.
1) Bulk Density
The bulk density of powders can be extremely difficult to measure since the slightest disturbance may result in a change in the results.
This is the result of the relationship between the particles that constitute the powder bulk. This same relationship affects the ability of the powder to flow.
The bulk density of a powder is measured in a Scott Volumeter. It may be described as the density of the powder “as poured” into a measuring vessel.
Tapped density, on the other hand, is the density attained after “tamping down”: this is normally measured using an instrument that lifts and then drops a measuring cylinder containing the powder through a fixed distance.
2) Tapped Density
The Tapped Density Testers Series JV have been designed to measure the tapped density of powders, granules and similar products in accordance with USP Chapter <616> Method 2 and Ph.Eur. Chapter 2.9.15.
This technique is particularly useful in powder flowability studies and also in determining the amount of settlement during transit to optimise pack sizes e.g., washing powders.
Tapped density is achieved by mechanically tapping a measuring cylinder (i.e. raising the cylinder and allowing it to drop a specified distance under its own weight) containing the sample under test.
The tapped density in grams per mL can be calculated by dividing the sample weight by the final tapped volume. Measures of the ability of the powder to flow and its compressibility can now be given in the form of the Hausner ratio (Tapped Density/Bulk Density) and the Compressibility Index (Tapped Density – Bulk Density/Tapped Density) x 100).
