Key points
Within industry, powders are widely recognised as being more of a challenge when compared to working with liquids or gases. Powders are commonly associated with batch-to- batch variability, problems with storage and transportation, and inconsistent behaviour in unit operations.
This is not surprising, given that powders consist of solid particles, gases (usually air), and liquid (usually water), and their complex behaviour is determined by the interactions of each of these phases.
Particle size, shape, density, surface texture, hardness, etc., as well as gas and moisture content, and other factors such as the influence of consolidation, shear rates, and temperature, mean that specifying a powder by physical properties of particles alone, will not accurately reflect the range of behaviours it may exhibit.
It is essential that any powder characterisation techniques employed are also influenced by the range of factors that can affect behaviour.
This article examines the potential benefits of uniaxial shear testing for process industries in this context. It considers the parameters generated and reports the results of a study on a wide range of common industrial powders to assess the sensitivity and repeatability of a new uniaxial instrument.
Biaxial shear cell powder testing
Biaxial shear testing provides reliable, assured measurement of powder properties to quantify how measily a static, consolidated powder transitions into flow.
Compared with traditional techniques, such as angle of repose, flow through an orifice, and tapped density methods, it enables close control of test conditions to measure and compare the flowability of powders under moderate to high stress, such as those present during storage and some manufacturing processes.
Biaxial shear cell testing measures the force required to shear a consolidated powder plane relative to another, with the shear stress measured at a range of consolidation stresses.
The data are used to construct a yield locus, and Mohr’s circle analysis can then be applied to determine parameters such as Major Principal Stress (MPS), Unconfined Yield Strength (UYS) and Flow Function (FF or ff c ).
A major limitation of this approach is the dependence on a mathematical model to derive these parameters, including UYS, which can lead to them being under- or over-reported, or not reported atall if the data does not fit the model.
Uniaxial testing – simple, direct measurement of Unconfined Yield Strength
The recent introduction of a new uniaxial powder tester (Figure 1) overcomes limitations associated with biaxial shear cell instruments, by providing rapid, simple and direct measurement of fundamental flow properties.
Uniaxial testing measures the force required to break or fracture a free-standing column of consolidated powder – the uniaxial Unconfined Yield Strength (uUYS) – as a function of the Major Principal Stress (MPS) imposed during consolidation.
The directly measured uUYS values are analogous to the UYS extrapolated from biaxial shear cell data and provide a robust and reliable way to directly rank the flowability of powders.
Free-flowing powders with much weaker tensile forces between the particles tend towards lower uUYS values while cohesive powders, with relatively strong inter-particulate forces, tend towards a higher uUYS.
Uniaxial testing is not a new concept, but has not been widely adopted for several reasons:
- For data to be reliable and repeatable, uniform density must be achieved throughout the powder column.
- To ensure a consistent and representative fracture, an optimum height/diameter ratio for the consolidated powder column must be achieved for powders with varying levels of compressibility.
- The confining vessel must be removed with minimal impact on the consolidated column.
The Advanced Uniaxial Powder Tester, overcomes these challenges by incorporating a number of technological advances:
- A double-ended compaction system to ensure a more uniform stress field throughout the column.
- A variable volume vessel to allow initial fill quantity to be adjusted depending on compressibility.
- A low impact sleeve and easy removal mechanism.
The test takes several minutes to complete and involves filling the sleeve with powder and automatically consolidating it using a computer-driven vented piston to a defined normal stress (a)’ reducing the stress to zero and removing the sleeve (b); determining the fracture point of the column, as the piston is moved down again at a constant speed, to directly measure the uUYS (c)
Repeatable and sensitive results
12 materials were evaluated using the new uniaxial powder tester (Figure 3). The materials were representative of a range of industries, including pharmaceutical, food, and chemical.
Each sample was evaluated at 3 major principal stress (MPS) values, (20, 60 and 100 kPa) with 5 repeats performed at each stress level. Where materials did not form viable powder columns at these MPS values, alterative values were used. MPS hold times, and consolidation and failure speeds, were kept consistent across the range of tests.
Clear differences are shown in the Flow Function curves for the samples tested, with microcrystalline cellulose (MCC) yielding the highest uUYS values throughout. It is also interesting to compare, for example, the results for Limestone and Detergent 2.
Detergent 2 has a lower uUYS at lower MPS values but is significantly more sensitive to stress levels, generating higher uUYS values at 100 kPa and therefore likely to be problematic in high stress process environments such as large hoppers.
The repeat tests on the 12 materials also demonstrate high levels of repeatability, as indicated by the error bars on individual data points. It is also worth noting that all materials, including the more free- flowing powders, were successfully consolidated to form free-standing columns.
The Compressibility profile for each powder also exhibits high repeatability (typically less than 5% RSD), suggesting that the uniaxial tester successfully achieves consistent consolidation throughout the column at various MPS levels (Figure 4). Although not shown, the Consolidated Bulk Density and H/D values also display the same high levels of repeatability.
The Compressibility profiles also illustrate that this single factor cannot be relied on solely to indicate flowability. For example, the Compressibility profiles for MCC and CMC 1 show little differentiation while their uUYS values are markedly different, reinforcing the perspective that flowability cannot be inferred from other properties
Three of the sample powders were also tested by another user to assess reproducibility. The high levels of repeatability demonstrated by both users in all tests performed, and the reproducibility of measurements between the users, is shown in Figure 5.
Until now, the potential benefits offered by uniaxial powder testing have been offset by the challenges of reliably constructing a uniformly-compacted, free-standing powder column.
The emergence of the first commercial uniaxial powder tester addresses these challenges with the result being a simple but robust instrument that directly and rapidly measures uUYS.
In comparison to other simple techniques, it offers high levels of repeatability, reproducibility and sensitivity, and provides efficient ranking of flowability across a diverse range of materials.
These capabilities make it a viable option for the quality control and assurance of raw materials and finished products across industry and a valuable tool in troubleshooting manufacturing processes.
