Fine powders developed to meet certain product performance targets often suffer from inconsistent and unpredictable flow that can lead to caking and in-process blockages. Therefore, processors frequently include additives in a blend to enhance flow properties. Although typically incorporated to improve process efficiency, additives also can substantially influence final product quality.
When optimizing the use of flow additives, it's important to recognize their impact on behavior may extend beyond a simple improvement in flowability. Choosing the most-appropriate grade of flow additive for a particular blend and incorporating it at an optimal level for the application also are crucial. This article provides some guidance on the use of powder additives, outlining issues useful to assess as part of blend development. Experimental data illustrate how multifaceted powder characterization can help this development process.
USING FLOW ADDITIVES
Flow additives ease powder flow by physically lubricating interparticulate movement and by disrupting the cohesive bonds between particles within the powder. Reducing or breaking interparticulate forces, such as electrostatic or Van der Waals interactions, allows particles to move more easily with respect to one another, thereby enhancing flowability. A corresponding reduction in the adhesive forces between a powder and a material of construction can ease movement within process equipment and storage vessels. As a result, additives in certain circumstances simultaneously may serve both as lubricants and flow enhancers.
Used effectively, flow additives can substantially increase manufacturing efficiency by maintaining consistent flow through the process and preventing unplanned shutdowns due to machine blockages. They also can impart superior performance to a finished product, either directly, e.g., better flow properties for a fine milk powder, or indirectly, such as consistent tablet weight resulting from a smoothly operating tableting press.
To select the most-appropriate flow additive, powder processors must consider a wide range of variables. In some applications, very specific end-product requirements may dictate choice. For instance, certain grades of hydrophobic silica aren't permitted in food applications — so, in such cases, choosing an appropriate additive relies on a detailed understanding of final product use and the regulatory framework that governs it.
However, a processor also should take many other factors into consideration. Indeed, making the optimum choice requires fully determining how an additive will affect the manufacturing process and influence critical quality attributes of the final product.
While most flow additives are selected on the basis of their ability to improve flow, the question remains as to what constitutes "improved flow" in a given process. Furthermore, an additive simultaneously and unintentionally may substantially affect a range of other powder properties that contribute to process performance. Compressibility, permeability, response to consolidation and, indeed, the ability to aerate, all may change with the inclusion of just small quantities of flow additive; these potentially may impact, possibly detrimentally, many aspects of behavior in a process. Therefore, optimizing the use of flow additives requires a comprehensive understanding of exactly how the additive will influence both process performance and product quality.
MULTIVARIATE POWDER ANALYSIS
It is helpful to consider powders as multicomponent systems comprising solids (the particles), gases (air entrained between the particles) and water, often in the form of moisture. Powder behavior depends upon complex interactions between these components as well as external variables, such as the environmental conditions experienced during processing. (For specifics on the impact of moisture, see: "Optimize Humidity for Effective Powder Handling.") Individual processes subject powders to various stresses and flow regimes, making it important to identify the properties that dictate performance in any specific operation.
For example, the low-stress dynamic conditions that prevail in a fluidized bed differ dramatically from the high-stress static conditions imposed on a powder stored under its own weight in a hopper. Therefore, a unique set of powder properties will define performance in each case. Comprehensively defining how a flow additive will influence powder behavior consequently requires a multivariate approach to powder analysis.
Dynamic powder properties directly quantify flowability and have proven application in optimizing manufacturing practices. Dynamic measurement involves rotating a precision-engineered blade through an accurately controlled volume of powder along a prescribed path. Measuring the axial and rotational forces acting on the powder determine its resistance to the motion of the blade. The resulting data then are used to generate flow parameters such as basic flowability energy (BFE) and specific energy (SE). Dynamic properties can be measured for powders in consolidated, moderate-stress, aerated or even fluidized states, allowing the generation of data that directly relate to a specific process. Equally important, the repeatability, reproducibility and sensitivity of dynamic measurement enable the user to identify and quantify even subtle differences in powder behavior, making the technique a valuable tool for detailed flow additive studies.