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).
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.
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.
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).
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.
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.
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.