Most manufacturers handling powders certainly would agree that achieving a better understanding of the materials' characteristics and how they change during processing could enhance the efficiency and safety of their operations. Fortunately, vendors are making solid progress in demystifying some key factors affecting powders.
For instance, Freeman Technology, Tewkesbury, U.K., and Medford, N.J., is looking beyond the physical properties of the particles and treating powder as a bulk entity. Using its FT4 powder rheometer, the company can evaluate and understand all the parameters that influence process performance.
"Our approach is to simulate the stresses, flow regimes and conditions that a powder may be subjected to in a process, for example, aeration, moisture absorption and electrostatic charging, and directly quantify its response," says Jamie Clayton, operations manager.
One project involved dynamic testing to meaningfully quantify differences in flow behavior between two different samples of sodium nitrite. The aim was to get the same properties in the second sample as in the first, commercially available, one.
A combination of dynamic powder testing and automated imaging showed the process for making the second sample needed modification to produce coarser particles with a smoother regular shape (Figure 1).
"The results here are not specific for sodium nitrite and in general could be true for a wide range of different powders. However, our experience tells us that when working with powders there isn't a one-size-fits-all solution; there is no such thing as a 'good' or 'bad' powder. The real question is, 'What is powder flow?' because the answer will depend on the material in question, the process concerned and what the formulation scientist or process engineer needs to achieve," notes Clayton.
For example, the properties of powder necessary for efficiency in a bulk bag filling process may significantly differ from those required when manufacturing a pharmaceutical tablet. A common preconception in both cases is that the most-free-flowing powder is needed. However, in reality, a free-flowing powder can introduce its own problems such as dusting, segregation and flooding. So, the key is to identify powders that best suit a particular process and accurately quantify their properties.
"In some unit operations, such as tableting or pneumatic conveying for example, a more cohesive powder may actually be conducive to better results. In all circumstances though, it's important to recognize that any application will consist of a series of stages and, in order to achieve efficient processing and a high-quality final product, a given powder must be compatible with all stages within the process," he adds.
Another recent project focused specifically on controlling the impact of humidity on the powder flow properties of lactose and microcrystalline cellulose (MCC). "The main driver for this work was not only to gain a greater understanding of how humidity affects powder flow properties, but also to examine the general assumption that moisture is always detrimental to powder flow. The results illustrated how small quantities of adsorbed moisture can actually lead to an improvement in certain conditions," Clayton explains.
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.