Particle Mysteries Take a Powder

Insights promise to enhance processing

By Seán Ottewell, Editor at Large

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The two materials were selected for study because they represent powders commonly used in industry and also exhibit varying degrees of hygroscopicity. The results demonstrated that even the small quantities of moisture adsorbed by powders considered to be hydrophobic could significantly impact flow properties.

"I think we always knew that assuming moisture adsorption is always detrimental and that so-called hydrophobic powders aren't affected by humidity was over-simplistic, but this study allowed us to prove this and demonstrate how influential even minor changes in moisture content can be. It was particularly relevant that some of the greatest changes were seen across a relative humidity range that would be typical of most processing environments," he notes.

The effect of humidity on powders can be very complex; the test results show how properties such as cohesion can prevail in certain circumstances while mechanical friction or interlocking can dominate when the powder is subjected to different stress levels (Figure 2). This underscores why a multivariate testing approach is required to simulate the various stages within a specific process and understand how the powder responds.

A further study has looked at factors that influence blending kinetics, a subject still very poorly understood. Based on work carried out as part of a Ph.D. research project at the University of Birmingham, Birmingham, U.K., the study correlated positron emission tomography (PET) data with flow energy measurements. The results suggest that dynamic data may be a reliable predictor of blending behavior and, consequently, useful for developing and optimizing blending processes.

While PET primarily is used in academia and isn't an established industrial tool, Clayton believes it provides a novel comprehensive method for evaluating mixing efficiency to determine optimum parameters such as speed and mixing time to achieve uniform dispersion.

"In this study we see how two different mixtures require different process equipment settings to achieve a predefined level of acceptance. These different requirements are likely to be due to the different physical properties of the constituent powders but after applying a range of powder characterization techniques, only the FT4 was able to identify and quantify these differences," he explains.

The company says its reports and white papers — particularly those related to subjects such as caking or humidity — are generating a lot of interest from process engineers and formulation scientists across a wide range of industries.

"As a company we have experienced great success in the pharmaceutical industry where our technology lends itself well to recent initiatives such as process analytical technology and quality by design. However, the FT4 is used across a diverse range of industries including chemical, food, agriculture, powder coatings and toners. One of our existing users has recently demonstrated how minor variations in the raw ingredients and upstream process can have a major impact on the properties of household detergent tablets," notes Clayton.

Analysis of particle size and shape present their own challenges when it comes to understanding how different operating conditions affect various materials. The main push from its customers is to gain insights for improving product quality and consistency, notes Malvern Instruments, Malvern, U.K. Automation can play a key role in achieving these aims, it stresses.

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