How to Measure the Physical Properties of Powders and Granules

Almost every manufacturing industry has to deal with powders and granules at some stage during production. Depending on the nature of your powders, you will have different testing priorities. For example, companies that require the flow of powders through hoppers will be interested in their caking and consolidation behavior, particularly after a period of storage. Exporters of panko breadcrumbs will need to know their fracture behavior before and after frying to ensure consistent quality. Protein powder manufacturers may be interested in agglomeration properties and mouthfeel. Cosmetics companies producing powder compacts such as eyeshadow and blusher will be motivated to find an optimum hardness – low enough for a payoff on the brush, high enough to prevent damage during transit.

Knowing a powder’s characteristics and the factors which impact on them is essential.  This allows the optimization of formulation, equipment and incorporates quality control principles throughout production, thereby assisting in process design, performance evaluation, and troubleshooting. Stable Micro Systems have developed a wide range of tests to analyze and quantify the physical properties of powders using a Texture Analyser, ranging from the flowability of powder ingredients to the physical properties of a final product.

Typical Texture Analyser Tests for Powder and Granule Products

How to Measure Compaction and Tabletting Properties

Many industries require the use of powder compaction during forming processes. Several parameters may be of interest when a powder is put under compression, and Stable Micro Systems offer several testing solutions. The simplest test method is the use of an Indexable Powder Compaction Rig, allowing low compaction force testing of multiple powder samples.

Once the first sample is centralized with the testing probe, subsequent samples can simply be moved along via a ‘quick-click’ mechanism which aligns the next test ready to go, increasing sample throughput. The compaction properties of powders can be assessed using target force or target distance modes. Many powder compacts are formed to a target distance and so this test can be used to assess the force required to reach this distance. However, new advances in tableting machines allow compression to a target force (resulting in powder compacts with constant porosity). Going to a target force using this rig will result in a measurement of the cake’s height.

In addition, by looking at the loading, hold, and unloading periods of a hold until time graph (stress relaxation), the information on compressibility (Carr index), relaxation, stiffness, and elastic recovery can be calculated. The loading period involves both elastic and plastic deformation, the hold period is generally purely plastic, and the unloading period is generally purely elastic.

Indexable Powder Compaction Rig and typical graph

How to Measure Flowability of Compacted Powder

In industries that handle powders on a regular basis, it is very important to understand how a powder or granular material responds to pressure. In storage, the weight of powder in a container exerts pressure on the particles at the bottom. If the powder has good flow behavior it will not consolidate and will flow out of the silo or hopper without sticking – this is very desirable. The longer a powder is stored for, the more likely it is to form a cake in its hopper and refuse to flow without further assistance.

A good measure of a powder’s flowability is the force required to cause it to flow. A simple well-established method is the measurement of unconfined yield stress.  It can be used to analyze the flow behavior of free-standing powders and the change in their behavior with different consolidation stresses and times. The measurement is made by filling a chosen weight of powder sample into a tube and a compression piston then applies a chosen consolidation force for a specified time. After consolidation, the tube is slid upwards above the consolidation probe and held up via a support disc before the probe moves back down to compress the freestanding column of powder, which “yields”.

Unconfined Yield Stress Rig and a flow function curve plotted across various regions of flow behavior

Formulae are built into Exponent software which is able to collect the required parameters and calculate the unconfined yield stress. Unconfined yield stress has little meaning as a measurement in itself as it depends on the consolidation stress (a powder more heavily compressed is likely to flow less freely and have higher yield stress). Consequently, the “flow factor” is plotted and used to compare between samples. The larger the flow factor, the more easily the powder will flow after any given consolidation pressure.

How to Measure Vertical Shear Strength

Vertical Shear Strength Rig

Another traditional textbook method of measuring a powder’s bulk resistance to the initiation of flow is the Powder Vertical Shear Rig. This also helps to imitate the consolidation that occurs under the powder’s own weight when the outlet is shut and the feeder is switched off. It is when the outlet is opened and the feeder started again that the blockages can build up.

The likelihood of a blockage forming depends on both the properties of the powder and the conditions surrounding it (humidity, temperature, stress state, hopper geometry). The vertical shear test provides a method to test the commencement of the flow of a powder from a packed state, imitating a filled hopper. This test can also help to determine the correct hopper outlet size for reliable flow.

During the test, a known mass of powder is transferred to the main body of the rig and compressed to a known force to create a uniform cake of powder. A trapdoor below the powder cake is released, exposing a circular surface of the cake. A probe slightly smaller than the hole then pushes a plug of the powder cake through the bottom of the rig. The powder is put into an almost perfect shear state (where the force is parallel with the powder movement).

Formulae are again built into Exponent software to calculate the sample’s vertical shear strength, along with information on the cake height reached after the hold period, giving a measure of compressibility.

How to Measure Granule Compaction

In many cases, the properties of the powder particles or granules themselves are of interest. There are two main ways that these can be tested – in a ‘bulk’ test in the Granule Compaction Rig, or by testing a single particle at a time during a cylinder compression test. Granule testing is important to provide an indicator of the tableting potential of some materials, or a measure of friability or compaction strength of granules.

 Granule Compaction Rig and typical graph

The Granule Compaction Rig provides a circular testing area into which the sample is contained before compression with a cylinder probe of similar diameter to the testing area. Normally, the maximum force and energy (area under the curve) are taken and used as an indication of hardness. The higher the force and energy required to compress, the more difficult it will be to form the granules into tablets.

A cylinder compression test can provide a means of measuring the progressive swelling of granules underwater, or can be used simply to compress (and possibly fracture) an individual granule. If the probe height is calibrated before the test, the height of the granule can be recorded, which is useful when granule dimensions are of importance. A large number of tests must be made to have a realistic set of measurements as granules often vary in their geometry. However, this type of test is very quick to perform.

 Single Granule Compaction test and typical graph

Being able to measure powder and granule flow is of prime importance to manufacturers, but as quality standards continue to rise, few can afford to stop there. Finished product testing is essential to ensure correct performance and safeguard reputation, sales, and market share.

For a full summary of typical texture analysis tests that can be performed on powder and granule products:

This information has been sourced, reviewed and adapted from materials provided by Stable Micro Systems Ltd.

For more information on this source, please visit Stable Micro Systems Ltd.