The introduction of an instrument offering a new powder testing technique opens up fresh possibilities for optimising powder handling processes. Tim Freeman, Managing Director, Freeman Technology, considers the merits of uniaxial testing, which include low equipment costs and fast measurement, and explores the benefits it offers to the powder bulk handling industry.
Traditional approaches to powdercharacterisation within the bulk solids handling industry range from the truly manual – a subjective visual or textural assessment – to tests such as angle of repose and Carr’s Index. In recent years, the need for greater relevance to powder processing has drawn attention to the limitations of these methods, and those at the forefront of bulk powder handling have embraced instruments such as the FT4 Powder Rheometer (Freeman Technology) and
(biaxial) shear cell analysis. However, the investment costs associated with such systems – in terms of equipment, training and ongoing manpower – are significant and have sometimes been a hurdle.
The launch of a new uniaxial powder tester offers the bulk powder handling community new choices for powdercharacterisation. Combining low upfront costs, fast measurement times and good repeatability, the Uniaxial Powder Tester (UPT, Freeman Technology) has the potential to extend routine,high valuetesting within the bulk solids handling industry in order to improve the effectiveness of equipment design and operation.
Uniaxial testing: a straightforward technique…
Uniaxial testing has long been recognised for its potential to offer simple, straightforward powder characterisation. In basic terms it measures the stress required to break or fail a consolidated column of powder, thereby directly measuring the unconfined yield strength (UYS) of a powder as a function of a preconsolidation or major principle stress, σ1. The uniaxial UYS (uUYS) is similar to the UYS, a parameter derived by extrapolation in biaxial shear testing, one of the most widely deployed powder characterisation methods. Due to differences in the consolidation and failure protocols applied by the two techniques, the absolute values for uUYS and UYS are not always identical, but the two parameters will rank the flowability of powders in a closely similar way.
Figure 1: Uniaxial testing involves the construction of a uniformly consolidated powder column and its subsequent fracture through the application of a compressive force.
Figure 1 shows the simple procedures that underpin uniaxial measurements. The first step is to produce a stable column by loading the sample into a cylinder and applying a known consolidation stress σv (a). Detaching the cylinder then leaves a free-standing column (b) that is subsequently fractured through the application of a steadily increasing compressive stress (c). The stress at which the column fractures equals the uUYS of the powder for the given preconsolidation stress.
By enabling direct measurement, uniaxial testing eliminates errors typically associated with determining UYS via the extrapolation process involved in biaxial testing. Variability of just 10% in the extrapolation process can magnify into changes in UYS values of greater than 100% so this is an important gain. Time savings in data analysis are also substantial.
… suitable for a wide range of materials.
Realising the benefits of uniaxial testing has required a major step forward in powder testing technology. In recent decades powder testing instrumentation has become progressively more precise and automated, offering an enhanced platform for the development of uniaxial testing protocols. The launch of the UPT exemplifies this progress, bringing new options for routine powder testing.
A key challenge with uniaxial testing, especially for more free-flowing materials, is the initial construction of a free-standing consolidated powder column. This issue has been directly addressed in the UPT, partly by implementing double-ended consolidation, which ensures better uniformity of consolidation through the powder column and extends the applicability of the instrument to a broad range of materials.
Figure 2: uUYS data for six different powder samples measured by uniaxial testing shows the range of materials for which it is suitable.
The UPT has demonstrated its versatility through the measurement of six different powder samples:
- CRM116 Limestone (Commission of the European Communities, 4mm, angular)
- Microcrystalline Cellulose (Avicel PH101 – FMC Europe N.V., Belgium, 50mm, irregular)
- Commercial Talc Powder (20mm, platelets)
- Methyl Cellulose (Metolose 90H, Shin Etsu, Japan, 83.4µm, fibrous)
- Lactose 1 (Lactohale LH200, DFE Pharma, Germany, 5-160µm, tomahawk)
- Lactose 2-(Respitose ML006, DFE Pharma, Germany 2-45µm, tomahawk)
These materials were selected on the basis of their flowability - which ranges from cohesive (e.g. Lactose 2) to the easy-flowing (e.g. Lactose 1 and talc) - and for their industrial relevance. The results show that the uniaxial technique can characterise and differentiate samples that span a wide range of behaviour, confirming its ability to support applications across the bulk powder handling industries.
Comparing uniaxial testing to other available techniques
Like a number of powder testing techniques, uniaxial testing can clearly be used for a range of materials and it quantifies powders using an established metric. This then raises the question of how the practicalities of measurement compare with the alternatives, and the potential benefits of adoption.
Generally speaking, when compared with uniaxial testing:
- Biaxial shear cell analysis is a slower measurement, has higher equipment cost and a higher training burden. However, biaxial shear cell data remains essential for the application of hopper design methodologies and has the advantage of allowing testing under lower consolidation stresses. This can make the resulting data more relevant to certain process environments.
- Simple techniques, such as Angle of Repose, Tapped Density and Flow through an Orifice, offer comparable testing times and lower equipment costs. However, uniaxial testing delivers greater repeatability and improved process relevance, making it more effective for QA/QC and process troubleshooting. For example, unlike Flow through an Orifice methods, uniaxial testing is not prone to generating unusual or null results for certain free-flowing or cohesive materials such as colloidal silica.
Table 1 summarises the relative merits of these three powder testing techniques which are all routinely used by the bulk powder handling industry, providing guidance for a rigorous appraisal of the alternatives.
Table 1: A summary of the relative benefits and practicalities of uniaxial, shear cell and simple powder testing techniques
In summary, uniaxial testing meets industrial requirements for quick powder testing by relatively non-expert analysts, with the additional benefit of low equipment costs, and high process relevance.
The commercial introduction of instrumentation for uniaxial testing brings a new technique for the bulk powder handling industries that is straightforward and well suited to routine application within the industrial environment. Measurement times are typically in the order of a few minutes per sample, and training requirements and equipment costs are low. The generated parameter, uUYS, is similar to that measured indirectly via biaxial shear cell analysis and measurements are highly repeatable. These advantages make uniaxial testing highly worthy of consideration when assessing the options for introducing powder testing in an industrial setting and enhancing existing techniques in order to drive up the efficiency of bulk powder handling processes.