As a supplier of aluminium powders and pastes, Benda-Lutz has decided to familiarize their customers and the readers of AAC Worldwide with all the parameters describing the properties of aluminium, to present methods that allow for the measurement of these parameters, and to show their impact on the activity of aluminum in the AAC production process.

In the following issues of AAC Worldwide, Benda-Lutz will present material parameters and ways of describing them, including specific surface area, bulk density, particle size distribution, dispersion in water, active aluminium content, gas evolution curve, stability in water suspension, demand for surfactant, and others. Laboratory methods used to describe these parameters and the relationships between them will be discussed, as well as their impact on the AAC growth parameters.

Description of the test method

The first parameter to be described is the so-called Water Coverage. It can be determined when dealing with non-wettable aluminium powders because they have the ability to float on the water surface. This method cannot be used to describe wettable powders and aluminum pastes. It is commonly used in applications like coatings and paints but was adapted to also describe the activity of aluminium in AAC. The parameter describes the surface area that can be covered by a specific amount of aluminium powder when dispersed on a water surface. A certain amount of aluminum powder is manually spread on the water until the aluminum flakes are arranged in one layer on the water surface. This is a process that requires the experience of the person performing the test and the ability to assess the moment when the entire sample is evenly distributed on the water surface. This is done in a so-called bathtub in which the exposed water surface area can be adjusted and adapted to the area occupied by the spread of the aluminium powder. The measurement result is given in the unit of [area per weight of the sample], e.g. in [cm²/g]. According to the Polish Standard PN-77/H-04949, results should be described with an accuracy of +/- 10 cm²/g. But in practice, according to the internal test method LHB/PA/522/BLS, +/- 100 cm²/g is applied and, according to experts’ experience, considered sufficient. As a result of the experiments and other processes, results obtained in practice range from approximately 2,000 to 15,000 cm²/g. However, the most reliable and repeatable results can be obtained for the range of 4,000 to 12,000 cm²/g. Outside this range, the measurement is subject to a higher measurement error or inconsistency of the results.

An example of the Water Coverage test is shown in Figure 1.

Scan the QR code to download a short video showing the Water Coverage measurement.

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Interpretation and relevance of the test results

This method is used to illustrate the surface area or in other words the grinding degree of aluminium particles and is applicable to describe metal powders with a flake-like structure. This measurement can be used to assess how the surface of the aluminium powder produced using Benda-Lutz technology changes, allowing the product to be controlled. According to chemical principles, the speed of reaction depends (among other aspects) on the contact surface between reagents. It is known that the reaction rate of aluminium with calcium hydroxide in an aqueous environment depends on the specific surface area of the ingredients, including the surface of the aluminium powder. Professionals working with AAC know very well, that the process is very sensitive and even small changes in variables, such as differences between batches of raw materials or ambient temperature changes can have an impact on the production process and consequently, the quality of the end product. Therefore, it is very important to describe and control aluminium powder during production not only by a single parameter, such as D50 for example. It is also crucial to control other parameters and consciously design the relationships between them to ensure the repeatability of aluminium powder activity between batches. Only in this way can aluminium be eliminated from the list of variables in the process and provide the technologist with improved production control.

The Water Coverage parameter is a very good complement to particle size distribution analysis using laser granulometry. Laser granulometry may not be reliable in assessing particles with a flake-like structure. It may not detect the difference between particles of the same diameter but different thickness. In contrast, it is possible to distinguish this difference when using the Water Coverage test. By using both methods in combination, a more complete picture of the powder’s activity can be obtained. If one relies only on laser granulometry, it may happen that powders with the same D50 can significantly differ in specific surface area, and consequently, in activity in AAC. There are other methods allowing for surface area control, but in this article the focus is only on Water Coverage.

Examples of differences in Water Coverage of ​​aluminum powders achieved with the same D50 range are provided in Table 1.

Table 1: Differences in Water Coverage of ​​aluminum powders achieved with the same D50 range.

To describe the differences in activity between powders with the same range of D50 and different Water Coverage (different surface area), gas evolution charts are shown in Figure 2.

 Summary

The Water Coverage test is a method applicable in practice only to testing aluminum powders that are not wettable by water. It is a simple test that does not require expensive measuring instruments and can also be performed by the customer on site, as a simple method of supply control. Completion of the measurements takes about 10 - 15 minutes per sample. However, it is a subjective method, and its correct implementation requires experience and an appropriate amount of time to perform it correctly. Despite its drawbacks, it allows sufficient control of the stability and repeatability of aluminum powder as a blowing agent. The Water Coverage is particularly useful in combination with other parameters, such as, for example, the grain size measured with laser granulometry.

This article is also available in Polish and Russian. Scan the QR codes to download the file in PDF format.