- THE MAGAZINE
When working with dry and wet powders in the ceramic industry, powder flowability is an important parameter. The ability to quantify the flowability of a ceramic powder enables the manufacturer to accurately predict the amount of water, dispersant, binder or solvent that needs to be added to create a slip. If powders are die pressed using automatic feed systems and die filling, the ability to quantify powder flowability can eliminate stoppages and increase product quality by ensuring that the powder fills the mold completely.
However, measuring and quantifying the flowability of powders is complicated, as the inherent changeability of powders makes their behavior difficult to predict. Additionally, the hard, abrasive powders used in the ceramic industry—along with the polymer lubricants that are often susceptible to moisture absorption—present their own challenges.
Where liquids and solids are concerned, only one or two variables have a significant impact on the key parameters. In sharp contrast, however, powders can be affected by numerous physical, chemical and environmental variables. As a result, many traditional tests, such as the cone angle test or funnel test, give unreliable and inaccurate results and cannot quantify powder flowability.
Recently, a new powder rheometer* was developed specifically to overcome these challenges. The instrument gives repeatable, accurate results and is able to distinguish between very similar samples. It also provides information about aeration, compaction and flow rate indices, as well as secondary characteristics such as attrition (the effect of wear and tear), segregation, moisture adsorption, electrostatic charge and its effect on flowability, bulk density dependence on attrition, and wall effect studies (i.e., how different materials affect flow performance).
How Does the Instrument Work?The powder rheometer works on a patented helical blade principle, in which the blade displaces powder as it moves along a helical path through the sample. Depending on the direction and speed of movement, a broad range of flow patterns and rates can be achieved. The axial and rotational forces acting on the blade are measured, and these data form the basis of the flowability assessment. Force measurements are converted into energy to determine the total energy consumed or work done during the traverse.
Materials can be moved either gently or aggressively. The gentle mode is used for sample conditioning, whereby powder is carefully disturbed to loosen particles, expel excess air and allow the sample to be laid down as a uniformly packed powder bed. Test cycles employ a more aggressive displacement mode, producing high levels of compaction and inter-particulate shearing.
The instrument measures all of the relevant forces needed to give a complete and accurate measurement of powder flowability. Sensitive and reproducible data are obtained by measuring both axial forces (as the blade moves vertically through the sample) and rotational forces (or torque) to determine flow energies. Most of the variables affecting powder flowability can be independently assessed using this system (see Table 1).
Many of these parameters are especially important when filling a mold with a powder. Pouring a powder aerates it, and the aeration ratio is a measure of how fluid the powder will become. When the mold is almost full, the powder flow rate is reduced to a negligible level. At this point, the flow rate index, a measure of the sensitivity of the powder to the rate of flow, becomes important. When the mold is almost full, the powder becomes consolidated, and the compaction index gives valuable information about whether compaction is induced by pressure or vibration (or both). At this point, the powder’s ability to release the air it entrained during pouring is important. The new powder rheometer can measure the de-aeration characteristics of any powder, ensuring a high level of quality in the finished products.
Putting the Instrument into PracticeBecause powders are a combination of solids and air, one of the most important parameters in measuring powder flowability is the packing condition. This is because the energy needed to establish flow varies greatly, from an aerated state at one extreme to a fully consolidated state at the other. Typically, the required energy can vary by around a 100 times, but in extreme cases up to a 5000-fold increase may be needed. This can make filling a mold very difficult. Such sensitivity to aeration means that unless test samples have an identical packing condition, the results of repeat tests cannot be compared.
Pre-conditioning of powders before measurement therefore becomes an essential step because it ensures that each powder sample is tested from the same starting point and under the same conditions. This means that direct comparisons can be made between results from different samples and enables the creation of information databases, like those available for other materials, such as solids and liquids.
An example of the kind of data available from a powder rheometer can be seen in the comparison of two samples of aluminum oxide. One sample was treated with an additive to try to improve its flowability, while the other sample was untreated.
Tests show that while the additive improved the powder’s flowability, it also increased the bulk density of the powder once it was in the mold. This is because the additive reduced the surface friction coefficient of the individual particles and allowed them to pack closer together, giving the treated powder a higher bulk density than the untreated powder. This higher packing density affected the basic rheology of the powder and the compaction index—a measure of how compaction affects powder flowability.
This intriguing fact was discovered because the new powder rheometer was used to determine the treated and untreated powders’ basic flowability energy (BFE), defined as the amount of energy needed to move a conditioned powder along a predetermined flow path using a patented helical blade principle. It was found that the untreated material had a relatively low BFE and aerated a little. On the other hand, the treated material was readily aerated to such an extent that it fluidized. Surprisingly, however, the treated powder had a higher BFE than the untreated powder. This means that more energy was needed to make the particles shear past each other and indicated that the treated powder was more densely packed than the untreated powder.
The results of this test showed that the additive improved the flow of the powder when it was in an aerated state (e.g., when it was being poured into the mold), but actually impaired the flow once the powder became consolidated (e.g., when it started settling in the mold or when it had been in a hopper). As a result, the mold containing the additive-treated powder might not be homogeneously packed. Without the use of the powder rheometer, the packing difference between the two powders might not have been identified.
Once the BFE of a powder has been determined, the instrument can also be used to examine the effect of different variables. Table 1 shows these variables and gives a definition of each, with typical values.
Factors Affecting Flow PerformanceIt is clear that conditioning is essential to remove packing variability from test samples so that reproducible measurements may be made. During handling and processing, however, the packing condition can vary from a highly aerated, or even fluidized state, to a very consolidated condition. Understanding and controlling the conditions imposed upon a powder during processing, and how these affect its flow properties, is vitally important for trouble-free processing. To achieve these goals, manufacturers must first be able to characterize the materials in relation to all of the key factors that affect flow performance. These include consolidation, aeration and flow rate sensitivity.
ConsolidationWhen subjected to direct pressure, vibration, or even storage, most powders reduce in volume as particles move closer together and bulk density increases. To determine how this affects flowability, the powder rheometer subjects appropriately consolidated powder samples to the standard flowability test, omitting any pre-conditioning cycle and measuring the energy needed to make the consolidated powder flow.
The results are expressed in terms of the compaction index—the factor by which the BFE is increased as a result of consolidation. In the case of the untreated aluminum oxide, for example, the compaction index was more than 5.3, while the compaction index for the treated aluminum oxide was 4.1. While these values are not very different because the BFE values vary so much for the two powders, the differences in flow energy requirements for a consolidated powder are pronounced (see Figure 1).
AerationIndustrial processing and handling of powders usually involves the removal or addition of air to the bulk powder. Dilute phase pneumatic conveying, for example, fully aerates a powder, whereas storage under pressure or vibratory conditions can cause consolidation. Introducing air into a powder bed will reduce the packing density, the number of physical point contacts and the inter-particulate forces, resulting in a more free-flowing material that needs less energy to promote flow.
The addition of air to some materials can cause fluidization, making it flow readily. This can be either an advantage or a cause for concern, depending on the process. Most materials do not fluidize but will aerate readily, with air channeling through the bulk powder and escaping the surface through one or more large holes. This contrasts with a fluidized bed, where air is released uniformly over the powder surface.
In general, agitation or disturbance of the powder bed during aeration produces a packing condition requiring considerably less energy. The powder rheometer can quantify this condition in terms of an aeration ratio—the factor by which the BFE is reduced by aeration. For the untreated aluminum oxide, for example, the aeration ratio was 25, whereas for the treated aluminum oxide, the aeration ratio was 610. This means that the flowability or pourability of the treated powder was much improved once the material was aerated.
Flow Rate SensitivityApplication of the standard powder rheometer measurement technique across a range of flow rates on a conditioned powder shows that the energy requirement also varies as a function of flow rate. This relationship is described using a flow rate index—the factor by which the energy changes when the flow rate is reduced by a factor of 10.
An ideal powder would be insensitive to flow rate, having a flow rate index of 1. In some powders, the flow rate index is less than 1. Generally these are free flowing and comprise quite coarse particles. Most powders have flow rate indices greater than 1—typically 3 or 4. In the aluminum oxide example, the untreated powder had a flow rate index of 1.66, while the treated powder had a flow rate index of 1.08—a significant improvement.
Achieving Accurate ResultsTo efficiently process ceramic powders, the powder characteristics must be accurately matched with those of the plant and machinery used in processing. For example, the tests discussed in this article show that while the treated aluminum oxide will flow well once aerated, it might not discharge freely from a hopper in which it has consolidated. In this case, untreated material has a flow energy requirement that varies from 110 to 9600 mJ (a ratio of 87), while the treated material varies from 9 to 29,600 mJ (a ratio of 3290)—a clear sign that the two powders need to be processed differently.
In many cases, an apparently identical powder can flow easily through a process one day but not the next, due to a range of physical properties and interactions. Basic flowability energy, consolidation, aeration and flow rate sensitivity all affect a powder’s flow properties. Many other parameters also affect the flowability of a ceramic, and a powder rheometer can prove to be an invaluable tool in determining these values.
Traditional methods of determining powder flowability are simplistic, insensitive and give inconsistent data. The powder rheometer enables those working with powers in the ceramics industry to generate accurate, reliable and reproducible data about powder flowability—an essential parameter that can improve the quality of the end product and increase the efficiency of the production process.