Many products from the pharmaceutical, food or chemical industry are sold in the form of powders.
These powders are susceptible to caking, which affects their usage and quality. In most of the cases, caking of powders is caused by absorption of moisture from the environment.
By using powder rheometers, we can study the caking rate in powders and find out the factors that contribute to caking at various stages.
Causes of Powder Caking
Environmental factors that cause caking of powders are temperature and humidity; changes in the former cause condensation of powder particles while changes in the latter causes dissolution of the powder.
Also, consolidation of powders during the testing process can cause mechanical caking of the powder. The powders need to be subjected to thorough testing in order to study the onset and progression of caking. Modern powder testing methods, such as the rheometer, are easy to operate and provide quick and useful information pertaining to powder characteristics.
The data obtained speaks about the impact of consolidation on the rate of caking, which can be useful in deciding upon the most appropriate manufacturing and storage conditions.
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Testing Methods
Rheometers are very useful in determining the dynamic properties of a sample and also enable direct measurement of the powder’s flowability.
The most important baseline characteristic is the basic flow energy (BFE); this is the amount of energy that is needed to rotate a blade that is immersed in the powder at a specific rotational and vertical speed.
Figure 1 represents the axial and rotational forces acting on a blade.
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Figure 1. Measuring BFE with a powder rheometer
The value of BFE can be measured by automated test methods and is the most important parameter that gives a good idea of the cohesive properties of the powder sample.
Caking of powders influences the value of BFE greatly for the reasons described below. The inter-particle bonding increases with caking; therefore more energy is needed to rotate the blade. if the caked sample is stiff, it will offer more resistance to movement.
Caking due to moisture absorption and increases the bulk density of the sample, which results in higher values of BFE.
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Effect of Consolidation on Caking
In order to study the effect of consolidation on caking, a few samples were subjected to compacting stress of 9kPa, while an equal number of samples were left without consolidation. The test results of all the samples with the corresponding BFE values are shown...
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This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.
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