Evonik Degussa improves shear testing in powders liable to caking

Instrumentation from Stable Micro Systems overcomes lack of repeatability and objectivity of manual testing

Evonik Degussa, a global supplier of specialty chemicals, has developed a unique powder testing procedure using instrumentation from Stable Micro Systems, a world leader in powder flow assessment.

The test is conducted with a newly-designed powder cartridge

The test, which is conducted with a newly designed powder cartridge, allows Evonik Degussa 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’s powder assessment took place with a manual ’caking’ test that 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 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 needed to develop the testing procedure further to allow accurate and repeatable testing of products that 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’s applications experts have therefore developed an adapter that 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 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.

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