Manipulating the properties of powdered materials can help make ceramic processing easier and faster while supporting improvements in product quality. One of the variables often controlled is particle size.

However, this may be complicated by the tendency of powders to agglomerate and cake, which can increase particle size away from the design intent and ultimately compromise production. Powder rheometry can be used to determine the conditions that will help prevent powder caking.

Particle Size and Powder Flow

Modern ceramic powders undergo careful processing to ensure that factors such as purity and particle size do not adversely affect production. Powders often require mixing with water or additives to make them suitable for different forming methods such as extrusion, slip casting, or injection molding.

When forming a green body from the resulting ceramic mixture, the sizes of any pores within it relate directly to the size of the ceramic particles present. Larger particles pack inefficiently and can result in pores that weaken the finished product, while powders with smaller particles (or a mix of particle sizes where the fine particles fill the voids between larger particles) can be used to control pore formation.

Compared with smaller particles that provide a high surface area, the presence of larger particles can also increase the time and temperature required for the product to achieve full density during firing or sintering. Manipulating particle size (and, potentially, particle size distribution) in ceramic manufacturing processes is therefore important for achieving greater process control.

Once the appropriate particle size specification is identified, the focus shifts to achieving and maintaining the most advantageous powder properties during manufacturing. Here, caking and agglomeration can be a complicating factor, since these processes increase particle size, thereby reducing the beneficial impact of early processing steps or of a carefully selected feed material.

Characterizing Powders

A widespread problem in many areas of powder processing, agglomeration occurs through mechanical or chemical interactions between particles during storage. Many of the raw materials used in the ceramic industry exist in powdered forms, with caking compromising both process and end use performance.

The particles that make up a ceramic powder can range from the nanometer to the micrometer scale, so an initial granulation step is often required to make them uniform in size. This step presents an opportunity to improve powder flow and reduce caking through the careful blending of powders and any added constituents. However, making the most of this opportunity in order to develop strategies that reduce the risk of caking over the longer term requires the means to characterize the powders involved and to understand their behavior under specific conditions.

Caking is primarily an issue related to powder storage. Humidity, temperature and consolidation are all factors that may promote caking, which can occur through a number of different mechanisms. Temperature changes that lead to condensation in a keg, container or tanker, for example, can result in particles dissolving, enabling the formation of agglomerate bridges through chemical bonding. Alternatively, particles forced together by consolidation may eventually simply mechanically aggregate. Whatever the mechanism, the end result is the same: changes in the way the powders flow, which leads to problems with storage, supply and potentially compromised product quality.

Powder Rheometers

The reliable analysis of powders and their behavior has logistical and economical benefits in many instances, for both processing and product quality. However, powder behavior is complex. Many physical, environmental and chemical factors affect powders, making it difficult to reproducibly measure properties and predict behavior. With a number of traditional tests, accuracy, reproducibility and/or process relevance are not always guaranteed.

Powder rheometers* offer multi-faceted powder characterization that delivers an array of powder properties designed to support process optimization studies. Such systems allow for repeatable and robust measurements that sensitively differentiate between physically and chemically similar samples. Dynamic powder measurement, one of the methodologies used by a powder rheometer, has been shown to be an especially reliable method of analyzing powders when investigating the impact of factors such as aeration, compaction and attrition.

Dynamic measurement involves recording the axial and rotational forces acting on a helical blade as it traverses through a sample (see Figure 1). Basic flowability energy (BFE) is the key dynamic baseline measure of a powder and is defined as "the energy required to rotate a blade down through a sample at a controlled rotational and vertical velocity." Powder rheometers use well-defined and automated test methodologies that make BFE measurements highly reproducible for many different types of powders.

Experimental Caking Data

Experiments based on measuring changes in the BFE of a powder under different conditions provide information about the likely severity of caking in the variety of environmental conditions encountered during storage. The study described here, which examines the effect of consolidation on caking activity, illustrates how successful this experimental approach can be. Similar protocols can be used to study the impact of elements such as humidity or temperature, for example, or to inform decisions taken during the formulation stage or when setting granulation targets.

In the experiment described, duplicate sets of a powder blend were stored for periods of up to 10 days. One sample set was stored without consolidation, while the other was subjected to compaction with a consolidating stress of 9 kPa. BFE was measured as a function of storage time for each set of samples (see Figure 2).

For both the consolidated and the unconsolidated samples, BFE increased only marginally during the first four days of the experiment. However, after five-and-a-half days, the BFE of the consolidated samples showed that the powder was twice as resistant to flow as it was when first loaded into the storage vessel-a state that the unconsolidated samples did not reach until day eight.

The BFE continued to rise for both sample sets with no sign of leveling off. The implication for this blend is that shorter storage times under low stress conditions are advantageous, and consequently should be the target within the process environment.

Mass Flow vs. Funnel Flow

Experimental data of this type can be used to inform storage design or selection. If the powder blend described previously is stored in a vessel that operates under mass flow conditions, for example, then reducing storage times and only partly filling the container will help alleviate problems associated with caking. With mass flow, material transitions uniformly through a container on a "first in, first out" basis, a condition achieved through the appropriate matching of the geometry of a storage vessel with the properties of a powder.

On the other hand, if this powder is stored in a hopper that exhibits funnel flow, then the results of the experiment suggest that problems are likely. Funnel flow occurs when the hopper walls are too shallow and/or the outlet is too small compared to the optimum conditions for a powder. A "funnel" forms within the center of a surrounding "stagnant" region where powder is static.

Material trapped in this stagnant region and toward the base of the container is subject to consolidating pressure, creating ideal caking conditions for susceptible powders. In this instance, steps would be needed to modify or change the hopper, preferably toward a mass flow regime.

Caking Management

Caking can quickly reduce the value of an expensive product, and it is important to develop strategies that reduce its occurrence. Understanding how powders behave in specific circumstances can play an essential role in process optimization.

The advent of powder rheometers has delivered the means to sensitively differentiate between even very similar samples and to examine how various environmental factors affect the behavior of particular powders. The resulting information aids in the management of caking and in ameliorating the impact of factors such as humidity and consolidation, encouraging the development of storage conditions that fully meet the requirements of the powder.

For more information, contact Freeman Technology Ltd. at Boulters Farm Centre, Castlemorton Common, Welland, Worcestershire, England WR13 6LE; call (44) 01684-310860; fax (44) 01684-310236; or visit www.freemantech.co.uk.

*Including the FT4 from Freeman Technology.

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