Availability
Any combination of accessories may be ordered when the FT4 Powder Rheometer is purchased. Additional items may be procured later as and when required.
Accessory Kits
Three sizes of Accessory Kit are available, each contained in its own accessory case. The three sizes are:
- 25mm diameter testing vessels. Used for evaluating samples that are available in small volume.
- 50mm diameter testing vessels. The standard size recommended for most applications.
- 62mm diameter testing vessels. Accommodates large volume samples such as very low bulk density powders, when flow energies are especially low.
Vessels
The borosilicate glass vessels in which powders are tested are available in 3 bore sizes: 25mm, 50mm and 62mm diameter made from precision bore tube with flame polished ends. Lengths vary to provide various sample volumes. All have a unique identification number.
Blades
Three sizes are available to allow testing in the above testing vessels. Blade diameters are normally 23.5mm, 48mm and 60mm, allowing approximately 1mm tip clearance between blade and the vessel wall.
Blades are made of hardened stainless steel to a precise shape and size.
Compaction Pistons
There are two types of piston used for compacting or compressing powder samples:
- Solid piston
- Vented piston – incorporates a sintered stainless steel mesh to allow air to escape from the compressed powder.
Three sizes of these compaction pistons are available to suit the various testing vessels. Compaction pistons are used for consolidating powders prior to shear testing, for compressibility testing and for applying a consolidating stress during permeability testing. Fig 1 shows results from compressibility testing.
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| Fig. 1: Bulk compression of initially conditioned samples as a function of applied normal stress |
Aeration Bases
These are used when air is required to be fed through the powder bed during testing. They incorporate a sintered stainless steel mesh which supports the powder sample but allows air to pass through.
Aeration bases are used for all aeration tests as well as for permeability testing.
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| Fig. 2: Pressure drop through powder bed at constant 2mm/s air velocity as a function of applied normal stress |
Aeration Control Module - measuring the flowability of aerated powders
The effect of aeration on powder flowability
The bulk properties of all powders are affected by the presence of entrained air, as it fills the spaces between the particles, or in the case of cohesive powders, is incorporated into agglomerates of fine particles. The amount of air present determines how the solids interact with each other and this has a direct impact on the flow properties.
The addition of air occurs naturally when powder is moved freely - for example, when discharging from a hopper. In other situations, air is added as part of the process, such as when powder coatings are transported in a coating system. When aerated, the contact between particles is reduced and less energy is required to move the powder. This reduction is described by the Aeration Ratio (AR), which can be measured automatically using the Aeration Control Module on the FT4 Powder Rheometer.
The value of the AR reflects the cohesiveness or stickiness of the powder. Powders that are cohesive do not allow air to pass through readily, and the powder bulk does not become uniformly aerated, but escapes by forming channels or vertical pathways. The resulting reduction in energy, and therefore in AR, is small.
In less cohesive powders, air is able to permeate through the bulk of the powder, separating the particles from each other and hence the reduction in energy is large. In extreme cases, toners or powder coatings for example, a powder may fluidise and behave as a fluid requiring only a small amount of energy to produce flow.
The Aeration Control Module - what it can do
The ACU module is controlled directly by the FT4’s control system allowing the following types of automated test programs to be run:
- Aeration or fluidisation assessments to determine response to increasing levels of aeration - up to fluidisation velocities if appropriate for the powder - see Fig 3 below.
- De-aeration testing to determine how readily a given powder releases entrained air following aeration
- Permeability measurements derived from measurements of pressure drop across a powder bed during aeration tests - see Fig 2 above.
Advantages of automating the measurement
- The Aeration Control Module is fully controlled by the FT4 control system and requires no operator involvement once the test has commenced
- Provides an accurate and reproducible measurement of aeration and de-aeration characteristics – typically to an accuracy of +/- 1%
- Operator errors are eliminated – a particularly important factor when dealing with small differences in powder properties
- The test sequence is fully programmable, ensuring that each stage is carried out at the precise testing rates and times specified, to maximise reproducibility of results
- An automated test can be set up to run as a QC measure close to the plant
Carrying out an automated aeration test
After selecting an aeration program from the library of test programs and setting up the sample, the automated test can be started and completed without further operator involvement. In special cases, a new program may be prepared by using the program editor. For example, extra conditioning or more testing and aeration stages might be needed.
Adjustable variables include:
- Air velocity at each stage
- Number of aeration stages
- Number of test stages
- Number of conditioning stages
- Usual range of test conditions – blade helix angle, speed and traverse height
- Usual hardware options – vessel and blade sizes
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| Fig. 3: Typical aeration profiles of a toner and a spray dried lactose |
This graph shows the quite different aeration characteristics of: a toner with particles microns in size; and a spray-dried lactose which has uniform, spherical 100 micron particles. The toner responds rapidly to aeration, whereas the highly porous lactose does not. However, both materials eventually fluidise. The rate at which the initial flow energy (BFE) is initially reduced, called the Aeration Sensitivity, is 0.41 mms-1 for the toner and 0.012 mms-1 for the lactose.
The small error bars on the chart represent the standard deviation, which is consistently low showing the repeatability of the test using the automated aeration module. This repeatability is key to gaining a good understanding of a powder's aerated flow properties, especially when comparing two or more similar powders, for example when looking for batch variation.
Description and specification
The Aeration Control Module interfaces to the FT4 Powder Rheometer and its software by the Universal Serial Bus, utilising the System Modules Interface Board. This allows test program software to directly control air velocity in the test vessel as the test progresses.
The Aeration Control system comprises this main control module and the porous aeration bases through which air is fed into the bottom of the testing vessel during testing. The control module can regulate air supply between 2 ml/min and 5000 ml/min to an accuracy better than 1%.
Availability
The Aeration Control Module is available at the time of purchase of a new FT4 Powder Rheometer or as a retro-fit to existing FT4 systems.
Shear Cells to measure shear properties
Shear Cell Modules
The three sizes of Shear Cell modules for the FT4 Powder Rheometer allow the shear properties of powders to be measured. Using a shear cell on the high-specification FT4 platform allows the automatic generation of accurate and reproducible shear information which complements the dynamic flow date.
Why Shear?
Since its invention by Jenike in the 1960's, the shear cell has been used to provide a measure of the shear properties of a compacted powder. The data produced have certainly helped engineers to design better plant. However, the technique provides only limited information about powder flowability and has remained technically challenging. Successful measurement has depended to a large degree on the instrumentation used and the skill and experience of the operator. In addition, potentially useful data on the shear strength of a powder in its unconsolidated state have proved elusive with traditional measurement systems.
Freeman shear cells
The FT4’s high-specification hardware platform combined with the precise control and extensive analysis capabilities of its software, have allowed the design of a highly sensitive, reproducible and fully automated shear measurement system. Shear strength in both consolidated and unconsolidated samples can be measured.
The Freeman shear cells offer:
- Fast, automated operation
- Independence from the operator
- High reproducibility
- Sample conditioning before test
- Three sizes of shear cells - 85ml, 10ml and 1ml sample size
- Testing of unconsolidated powder
- Small sample volume - down to 1ml
- Sophisticated data analysis
- Comprehensive data plotting and formatting
- Force control and position control modes available
- Precision measurement of axial and rotational position
- Hopper design methodology and software
Which shear properties can be measured?
Shear strength data complement the standard FT4 measurements. Traditionally shear testing has been used mainly for testing cohesive powders when consolidated - simulating conditions at the outlet of a hopper. The FT4 shear cells however can measure cohesive and non-cohesive powders when consolidated or non-consolidated.
Primary data generation provides the yield locus (see report below). This is generally reproducible to around 1%, depending on the material. Yield loci for six different powders pre-consolidated to 9kPa are shown are Fig 1. Other parameters may be derived from the yield locus that are especially valuable with respect to the Jenike approach to the design of bins and silos. These include:
- Unconfined yield strength
- Major principal stress
- Flow function (the ratio of the above)
- Internal angle of friction
- Cohesion – yield strength of previously consolidated powder
These parameters are derived from the yield locus either directly or by using a Mohr Circle construct.
Cohesion, or the shear strength at zero consolidation stress of a previously consolidated powder, is a particularly useful powder characteristic: it is a measure of how well the material forgets or remembers that it has been consolidated. Fig 4 shows clearly how the cohesive group of powders (the top three) have high values of cohesion – i.e. the intercept on the vertical axis.
Of particular use is the cohesion value of an unstressed or conditioned powder. The FT4 can control force sensitively to allow the maintenance of near zero consolidation stress during a shear test. This enables measurement of the cohesion at near zero pre-consolidation. Measuring the cohesion of conditioned or stress-free powders provides a very useful flowability parameter. The Freeman conditioning process that precedes all testing is especially important here since the results very much depend upon uniform and reproducible packing.
Fig. 4: Yield loci of six different powders after pre-consolidation to 9kPa
The 1ml shear cell enables users to extend the range of shear testing applications on the FT4 to include those for which only very small amounts of sample are available. This encompasses many pharmaceutical actives, and materials, such as early stage formulations, where a wide range of tests must be carried out on limited supplies.
Fully automated testing
The Freeman shear cell accessory is delivered with a library of testing programs. To perform a test the user simply selects the required program, loads and prepares the sample and then clicks 'run'.
The following stages are fully automated:
- The standard Freeman conditioning cycle prepares the sample for testing
- In a 'pre-shear' stage, the powder bed is sheared in order to maximise the major principal stress
- For powder characterisation, a single series of tests at a particular pre-consolidation stress may be sufficient. For example, a powder may be pre-consolidated to 9kPa and then sheared at 7, 6, 5,4 and 3kPa with pre-consolidation to 9kPa between each test
- More comprehensive, multi-stage testing to produce a range of yield loci corresponding to different pre-consolidation stress levels. Usually a given sample will be suitable for repeated shear testing to obtain the required set of yield loci. In this case the entire evaluation is automated.
Shear testing data and Mohr Circle Analysis
Torque and force data are automatically converted into shear stress and consolidation stress in real time. The yield loci data, including cohesion measurements, may be quickly derived in preferred formats using the Data Analysis software provided.
A typical report (shown below) provides the yield loci profiles, the Mohr circles describing the unconfined yield strength, the major principal stress and a table of all derived parameters.
Wall Friction Module
Why measure wall friction?
Wall friction module showing rotary housing and array of discs with various surface finishes
In all processing machinery, powder is required to flow within containing surfaces such as a hopper, a chute or a pipe. The frictional resistance between these surfaces and the powder moving over it can be important in determining the smoothness or consistency of the powder transfer process. This frictional resistance will depend upon the powder characteristics, the material of which the containing surface is made, its surface finish and any surface treatment used. The flow characteristics of the system and possibly whether flow will occur at all, depends upon the geometry, the flow properties of the powder and the wall friction.
The wall friction module and measurements
The wall friction module allows this frictional resistance to be measured experimentally. A wall friction disc of the appropriate material and finish is used to compress a powder sample whilst the disc is rotated to measure the frictional resistance. A step sequence of predetermined normal stresses is applied whilst the disc is rotated and the frictional torque is measured at each stress level. The shearing stress is calculated from the measured frictional torque and the wall coefficient of friction is determined from the slope of the shear stress versus normal stress data.
In practice the complete test and the analysis are fully automated after the sample has been prepared and the program started. The measurements take a few minutes after this.
The wall friction module includes a set of 3 discs made of 316 stainless steel and having various surface finishes. The disc design is simple so that further discs may be easily made of other specific materials and surface conditions if required.
Example application data
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| Fig. 6: Shear stress as a function of applied normal stress when rotating a stainless steel disc (316 with 1.2 micron surface finish) against five different powders. |
| Powder | Wall Friction Angle |
|---|---|
| Limestone, CRM116 | 32.9 |
| Respitose, ML001 | 31.0 |
| MCC, PH101 | 29.1 |
| Lactose, FlowLac 100 | 18.2 |
| Glass beads, 50 micron | 12.4 |
Wall friction angle for each powder type
