System Options
Full automation of powder testing using the FT4 Powder Rheometer provides accurate, reproducible determination of a range of critical parameters. The elimination of user variability allows confident discrimination of even small differences between powders, while unattended operation frees analysts for other tasks.
Built around the core dynamic measurement system are the following fully automated measurement options:
|
||||||||||||||||
|
||||||||||
Shear Cell |
The Freeman Automated Shear Cell modules for the FT4 Powder Rheometer allow the assessment of powders in terms of shear strength. Using a shear cell on the high-specification FT4 platform allows the automatic generation of accurate and reproducible shear information. This is in addition to the comprehensive flowability data provided by a standard FT4 measurement, and broadens the application of the system, bringing closer our goal of a universal powder tester.
Why shear? |
Since its invention in the 1960's, the shear cell has been used to provide a measure of the forces in 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 depends 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 advantages |
Back
to top |
The FT4’s patented high-specification hardware platform combined with the precise control and extensive analysis capabilities of its software, have for the first time 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.
- Fast, automated and independent of the operator
- Highly reproducible
- Sample conditioning before test
- Applicable to unconsolidated powder
- Small sample volume (as low as 3ml)
- Sophisticated data analysis
- Comprehensive data plotting and formatting
- Force control and position control modes available
- Precision measurement of axial and rotational position
Describing powder flow properties |
Shear strength data complement the standard FT4 measurements. For some materials and packing conditions shear strength is an important indicator of flow behaviour. Traditionally, shear cell assessments have been on consolidated powders. However, the FT4 is capable of the fine control necessary for testing unconsolidated or "stress free" powders.
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
- Internal angle of friction
- Cohesion – shear strength at yield when normal stress is zero
- Flow function (the plot of unconfined yield stress versus major principal stress)
These parameters are derived by extrapolation of the yield locus and therefore are less precise and useful for describing flow properties. (The extrapolation towards zero normal stress is necessary because data points at stress levels below about 30% of the pre-consolidation stress are impossible to obtain. This is due to the self generated normal pressure resulting from the action of shearing.)
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. Fig1 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 zero or 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.
Fully automated testing |
The Freeman shear cell accessory is delivered with a library of testing programmes. To perform a test the user simply selects the required programme, 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 10kPa and then sheared at 9, 6 and 3kPa with pre-consolidation to 10kPa 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.
The small volume shear cell |
A recent development is the fully automated, small volume shear cell which can be used to measure volumes down to 3 ml or less, depending on the characteristics of the powder. This completely new module 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.
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.
Aeration Control Module - measuring the flowability of aerated powders |
The effect of aeration on powder flowability |
To some extent the bulk properties of all powders are affected by the presence of entrained air, as it fills the spaces between the 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 especially 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 and hence the reduction in energy is large. In extreme cases, toners or powder coatings for example, a powder may fluidise such that it behaves as a fluid and requires 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 programmes to be run:
- Aeration or fluidisation assessments to determine response to increasing levels of aeration - up to fluidisation velocities if appropriate for the powder
- De-aeration testing to determine how readily a given powder releases entrained air following aeration
- Porosity measurements derived from measurements of pressure drop across a powder bed during aeration tests
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 |
The screen shot below gives an insight into how an automated aeration test can be set up and indicates the number of variables that can be easily and finely adjusted. The first aeration control stage is highlighted in yellow (on the right of the image) and all the adjustments to be made to this step can be carried out using the controls on the left-hand side of the screen. If the powder being tested requires different methodology, extra conditioning, testing and aeration stages can be added or adjusted quickly and easily. For example, more cohesive powders will require higher aeration levels to become fully aerated - this means simply adjusting the required air velocity on the left-hand side of the image below. Also, on pouring the powders into the vessels, some may become partially aerated and may require additional conditioning cycles to ensure consistent initial levels of entrained air.
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
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. For the lactose, an air velocity of 8 mm/s has reduced the energy from 1200 mJ to just 30 mJ, giving it an average aeration ratio (AR) value of 40. 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 Freeman Technology Powder Rheometer software and to the FT4 by the Universal Serial Bus, utilising the System Modules Interface Board. This allows test programme 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 5 ml/min and 5 L/min to an accuracy better than 1%.
The porous discs through which air is fed into the powder sample, are sintered stainless steel mesh with a nominal grid size of 60 micron. The aeration bases containing these discs are available in three sizes – 25 mm, 50 mm and 62 mm diameters.
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.
Wall Friction Module |
Why measure wall friction? |
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. |
|
 Wall friction module showing rotary housing and array of discs with various surface finishes
|
The wall friction module and measurements |
The wall friction module allows this frictional resistance to be measured experimentally. A 48mm diameter 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 practise the complete test and the analysis are fully automated after the sample has been prepared and the programme started. The measurements take a few minutes after this.
The wall friction module includes a set of 4 discs made of 304 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 |
The graphs below describe the results of tests using 304 stainless steel discs having a range of surface finishes from quite rough, 80 grit (2.5micron) to a mirror finish (0.12micron). These four surfaces were evaluated to determine their friction characteristics in relation to four different powders - limestone, finely milled lactose, coarsely milled lactose and talcum. The rotational speed was 12º/min for all tests.
The graphs below show how the friction angle for the 120 grit stainless varied for the 4 materials (upper graph) and also how it varied for limestone in contact with the different surface finishes (lower graph).
