There is demand within the powder metallurgy industry for powders smaller than 1µm. These powders can now be produced at 0.2µm and it is expected within a few years that it will be possible to produce powders as small as 0.1 µm. Tap density measurements (in which the powder has been tapped to settle contents in a container under specified conditions) are often used to control process parameters when working with fine powders. However, it can be difficult to use this type of measurement because very fine powder will adhere to the surfaces of weighing dishes and laboratory glassware.

According to Tom Villett, Subcommittee B09.02 member and manager of the analytical resources group for Umicore Canada, measuring tap density by constant mass with fine “sticky” powders yields unreliable results for several reasons. Some powder is lost during the transfer from the weighing dish to the graduated cylinder. In addition, some of the powder may adhere to the cylinder wall and not be included in the measured volume. The relatively high aspect ratio of the graduated cylinders increases particle to cylinder wall friction, inhibiting the migration of the particles to their final packed position.

Because these fine powders are not “free-flowing” they do not pack evenly, often resulting in an uneven top surface of the packed column. This uneven packed surface means that visual interpretation of the volume is required.

Villet says that the constant volume measuring method that is described in WK13023 provides a way to do tap density measurements on very fine powders. “When measuring by constant volume these problems are reduced or eliminated,” says Villet. “The low aspect ratio of the density cup reduces friction. The mass of the sample is weighed after tapping with no loss of powder. Because the tapped height of the column is constant, there is no visual interpretation necessary.”