An Introduction to Industrial Powder Characterization

In this interview, Dr. Jeff Kenvin, Fellow Group Leader, Micromeritics Instrument Corporation and Mr Jamie Clayton, Operations Director, Freeman Technology Ltd talk to AZoM about powder characterisation, and the capabilities of Freeman Technology and Micromeritics in the field.

Can you provide a brief introduction to Micromeritics and Freeman Technology and the relationship between the two companies?

Jeff Kenvin (JK): Micromeritics Instrument Corporation provides solutions for material characterization, focusing in five core areas:  density; surface area and porosity; particle size and shape; powder characterization; and catalyst characterization/process development. The company was founded in 1962 and is headquartered in Norcross, Georgia, USA; worldwide Micromeritics has more than 300 employees. In June 2018 we acquired Freeman Technology to strengthen our offering with respect to powder characterization as part of a defining strategy of delivering distinctive, integrated analytical solutions in industrially vital areas.

Jamie Clayton (JC): Freeman Technology, which is headquartered in Gloucestershire, UK, specializes in systems for measuring the flow properties of powders and has almost 20 years’ experience in powder characterization. Our flagship product is the FT4 Powder Rheometer®, a universal powder tester, that offers sensitive, highly differentiating multi-faceted bulk powder characterization for the optimization and control of powder behavior in development, formulation, scale-up and manufacture.

What is the industrial relevance of powder characterization? Which sectors face particularly demanding challenges in terms of powder handling or performance?

JC: Estimates suggest that around 80% of industrial products are either sold as powders or handled in powder form during their manufacture. Powder characterization therefore has widespread industrial relevance and there is a high degree of commonality in the challenges faced. The efficient manufacture of high-performance products relies largely on developing an understanding of the relationships between process and product behavior and relevant powder characterization data.

Surface area of excipients after being pretreated in a dry environment at several temperatures from ambient to 120 °C

The pharmaceutical industry exemplifies the challenges faced with formulators required to optimize powder properties for very different product types. For example, a tableting formulation must flow consistently at a relatively high rate under the low to moderate stress conditions that exist as the formulation flows into the feedframe, and under the forcing conditions applied for die filling. Compressibility characteristics impact performance in the press and the stability of the finished tablet. For dry powder inhaler formulations, in contrast, aerosolization is the defining requirement and the response of the powder to air is crucial. The surface area of particles for example can significantly influence bulk powder behavior and investigations show how this property can change dramatically with respect to temperature. The best powder test methods provide data that rationalize behavior under a range of very different conditions.

JK: New industries such as additive manufacturing (AM) present us with fresh powder handling challenges but even in established industries the ongoing drive to improve performance calls for increasingly advanced characterization. For example, innovations in catalytic materials have the potential to improve the efficiency and sustainability of fundamental processes such as fluid catalytic cracking (FCC). For FCC catalysts, bulk powder properties influence fluidization behavior in the reactor and reformer, but we need to characterize and manipulate particle properties with precision to control intimate contact between reactants and the catalyst at the molecular level.  For a catalyst, properties such as surface area and porosity define mass transfer within the particle, how easily reactants and products can transfer to/from active sites, and the localized environment that prevails during reaction. Optimizing these characteristics is critical when it comes to breaking new ground in terms of selectivity and turnover.

Unified method for the comprehensive pore size distribution of a material.  In this example, a hierarchically porous molecular sieve exhibits micro, meso, and macro porosity; the unified method provides an improved estimate of porosity via the simultaneous use of liquid intrusion and gas adsorption data.

The temperature programmed surface reaction (TPSR) of an alcohol may be used to demonstrate the dehydration of isopropanol too propene and the TPSR provides the number of catalytically active sites from the quantity of propene produced

How do Micromeritics and Freeman Technology products fit together when it comes to powder characterization?

JK: As the preceding answers make clear, when it comes to optimizing powder products it is important to consider both particle and bulk powder properties. Micromeritics and Freeman Technology are a perfect fit in this respect with the majority of our products quantifying particle properties…

JC: …while our focus is bulk powder properties, primarily powder flowability, but also other bulk characteristics such as compressibility, permeability and bulk density.

JK: We offer instrumentation to characterize particle form, properties such as particle size and shape and density, which can all directly impact product performance. And beyond that we have systems that probe the internal structure of powder particles, quantifying specific surface area and porosity, which again can be performance defining. All these properties may usefully form part of a specification for a powder product, depending on the strength of their influence on product performance. They may also impact bulk powder properties.

JC: For example, powder flowability is routinely correlated with particle size and shape. Finer powders tend to be less free-flowing than coarser analogues; more spherical particles exhibit better flow properties than those that are less regularly shaped. Flowability can also be impacted directly by particle porosity and density, and by surface topography, which impacts the strength of particle-particle interactions. However, flowability cannot be predicted from particle properties alone, the impact of variables such as those listed can only be quantified by measurement. The resulting data can be vital for process optimization or for understanding the effect that bulk properties can also have on product performance.  In combination Micromeritics and Freeman Technology have the technology to measure all the variables of interest to generate the data needed to confidently control powder performance.

Can you provide some examples illustrating how particle properties define the performance and value of powder products?

JK: Specifications for metal powders for AM provide some interesting examples of how particle properties define performance and value. AM processes such as powder bed fusion and binder jetting require the spreading of powder in layers just tens of microns thick with close particle packing essential in terms of the quality of the finished component. Particle size and size distribution influence packing behavior and the rate at which the powder melts or sinters, during the manufacturing process. Finer particles are desirable on both counts, with particle size specifications for metal AM powders for powder bed fusion typically in the 15 – 45 µm region.

Particle shape also influences packing behavior, and flowability, with more spherical particles commanding a premium. Poor surface quality and the presence of satellites can compromise the performance of otherwise regular particles. In terms of density, the preference is towards fully dense powders, since these fuse to form a finished component with well-controlled porosity. Specific surface area data can be useful for elucidating sintering behavior and, in the case of binder jetting, binder interactions. Recycling powders is essential for the sustainability of the industry, so particle properties are rigorously assessed and compared for both virgin and recycled materials.

And can you do the same for bulk powder properties?

JC: AM powders are an interesting example from the perspective of bulk powder properties too, particularly flowability. Though fine particles are advantageous from the perspective of packing and sintering, the resulting powders can have poor flow properties. Poor flowability is not compatible with efficient AM which relies on smooth, consistent powder flow across the build platform. The feedback we have from our AM customers is that they use dynamic powder flowability measurements, alongside specifications for variables such as particle size, to successfully identify powders that will print in a given machine. For example, ExOne a leader in binder jetting technology, uses flowability testing to assess customer powders, the suitability of alternative supplies and when developing new qualified materials.

One of the ways to improve the flowability of a fine powder is to exert tighter control over particle shape, via the manufacturing process. More spherical metal powders, with enhanced fluidity, are available, but at greater cost, so being able to determine whether these are required has a direct impact on profitability. In an analogous way, a flowability specification can be extremely valuable for determining the impact of changes in the powder arising from recycling, thereby supporting the development of optimized strategies for re-use.

If you could offer one piece of advice about powder characterization what would it be?

JC: The most important thing to recognize when it comes to powders is that they are not inherently ‘good’ or ‘bad’, but rather, differently suited to specific applications. For example, the optimal bulk powder properties for a powder coating used in a fluidized bed process will differ from those of a cosmetic powder destined for a compact. Powders consist of solid particles, gas (typically air) and liquid (usually low levels of water), with behavior governed by interactions between the three phases. Multi-faceted powder testing generates dynamic, shear and bulk properties that capture this behavior and that can be correlated with performance to determine which characteristics are critical for any given process. Testing powders in this way is therefore an effective way of setting bulk powder specifications.

JK: When you have the idea of powders and particles clearly differentiated then it becomes straight forward to appreciate that every powder product will have a unique set of particle and bulk powder properties that optimally defines its performance. My advice would be to think about exactly what defines the value of your product as a prelude to establishing a really robust specification. We can quantify particles and powders in a wide range of different ways but clearly, they are not all going to be equally insightful or relevant for each product.  Market leaders tend to be those that invest the time and effort in identifying which parameters define performance and establishing correlations between the two. Such understanding provides a secure platform for vital activities such as process optimization and supply chain management.

Going forward, what are the plans for Micromeritics and Freeman Technology?

JC/JK: Going forward we are now in a unique position to explore the relationships between particle properties and bulk powder behavior which is interesting and exciting for us, but more importantly of significant potential benefit for our customers. We have best in class technology in a number of key areas combined with unrivalled and highly complementary application expertise. Bringing those things together will allow us to learn faster and enhance our combined offering to customers.

About Dr. Jeff Kenvin

Dr. Kenvin received his B.S. degree in Chemical Engineering from Drexel University in Philadelphia , Pa. He received his Ph.D. degree in Chemical Engineering from the Georgia Institute of Technology in Atlanta, Ga.

His areas of expertise are in the characterization of porous materials and catalysts using physical and chemical adsorption and in mercury porosimetry.

While employed by the Mobil Research and Development Company as a member of the Mobil Lubes – Process and Products Research Division, Dr. Kenvin managed research and development projects involving the commercialization of new hydrotreated base oils. He was the innovator of the use of chemometrics to define base oil quality standards. These chemometric models subsequently led to the development and implementation of novel techniques for characterizing composition and optimizing refinery lube operations, and the technology was successfully implemented in all Mobil lube refineries. In 1994, Dr. Kenvin received the Products Division – Technology Award for his contribution to the Mobil Corporation.

Later, as a member of the Hoechst A.G. North America Next Generation Process-Polymer Group, Dr. Kenvin directed leveraged-research at the Georgia Institute of Technology. His research involved the identification of new technologies for the process and development of low-cost terephthalic acid and novel monomers for improving the properties of PET (polyester terephthalate), including the application of super-acid catalysis for the carbonylation of toluene.

Dr. Kenvin has been invited to present scientific lectures at more than 100 seminars and regularly participates in short courses on particle technology. He also serves on international advisory boards and is an officer of the ASTM D-32 committee on catalysis.

About Jamie Clayton

Jamie Clayton is Operations Director at powder characterisation company Freeman Technology, based at the company’s headquarters in Tewkesbury, UK. He graduated from University of Sheffield with a degree in Control Engineering and is responsible for overall management of company activities, including the R&D, production, sales and customer support teams. During his time with the company, Jamie has worked as a mentor with several academic groups and is an active member of ASTM F42. Jamie is also a regular contributor to conferences and workshops on the topic of powder rheology and works closely with clients on the application of the company’s technology.