Evonik Degussa uses Shear Testing in Powders Liable to Caking

A unique powder testing procedure has been developed by Evonik Degussa GmbH, a global supplier of specialty chemicals, using instrumentation from Stable Micro Systems, a world leader in powder flow assessment. The test, which is conducted with a newly-designed powder cartridge, allows Evonik Degussa GmbH to gauge more accurately the effects of its AEROSIL® and SIPERNAT® silica products on the caking and flow properties of powdered products in the food, feed, pharmaceutical and chemical industries.

Previously, Evonik Degussa GmbH’s powder assessment took place with a manual "caking" test, which involved compressing a powder sample within a hollow metal cylinder using a solid metal cylinder. The sample was stored on a Teflon® sheet under defined heat, humidity and pressure conditions to create a “tablet” like body. Once the "tablet" was formed, both cylinders were removed and pressure was applied with a human finger. Its ability to withstand this force was measured subjectively using a simple scoring system (1-6).

Using Stable Micro Systems’ powder flow analyser allowed Evonik Degussa GmbH to overcome the lack of repeatability and objectivity of the manual test. Sample preparation and storage was identical, but only the solid metal cylinder was removed. This meant that caking properties of a powdered product could be tested whether or not they had formed self-supporting structures during storage. Aeration, compaction and slice compaction tests gave valuable indications of the powders’ surface friction properties, mixing kinetics, cohesiveness and flow and recovery properties.

Evonik Degussa GmbH needed to develop the testing procedure further to allow accurate and repeatable testing of products which contained silica. By definition, these powders are much more free-flowing than standard powders and granules and even compressed samples are relatively unstable and easy to disturb when being loaded onto testing instrumentation. Evonik Degussa GmbH’s applications experts have therefore developed an adapter which allows the prepared sample to be loaded onto the powder flow analyser while still mounted on the Teflon® sheet. This avoids any potential damage to the compacted structure before the test begins.

The powder flow analyser’s patented rotating blade is programmed to pass through the sample in a defined way, for a fixed number of rotations, according to the individual test requirements. As this happens, a load cell measures the sample’s resistance to flow. As it is not uncommon to find a "crust" on the surface of a compressed sample, Evonik Degussa GmbH can identify differences in force as the blade moves downwards through the powder. Similarly, it can quantify differences in the properties of samples containing varying levels of flow agents, including samples containing no silica at all.

The new test allows the identification of the most appropriate flow and anti-caking agent for a particular application and subsequently the specification of addition levels to control clumping and crusting behaviour, and to maintain free-flowing powders which are easier to handle and process.

The Powder Flow Analyser is available from Arrow Scientific.

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