An assumption of homogeneous caking behavior is therefore inherently unsound; crusting is commonplace. This creates a need for powder testing that detects and quantifies heterogeneity, that differentiates crusting from generalized caking. Dynamic measurements of flow energy with a powder rheometer can do this with precision and sensitivity. Katrina Brockbank explains how, and more importantly, the value of the resulting information.
Why do powder cake or crust and how can I tell if its going to be a problem?
When powders are stationary, notably during storage, the strength of particle-particle interactions can increase, inhibiting particle mobility. This is caking and it proceeds via multiple sequential and parallel mechanisms, with moisture often playing a pivotal role. A common mechanism for crusting is moisture ingress at an exposed surface with caked material forming a barrier that inhibits water penetration beyond a certain depth, preserving the flowability of the powder bulk beneath.
Caking and crusting may be preventable, for instance, by controlling storage conditions, and they may be reversible, but they aren’t predictable. Appropriate testing is the only way to access the insight needed to put in place a strategy to limit deterioration and preserve powder flowability. Such testing needs to determine not only whether caking is likely to be an issue under the conditions of use/storage, but also whether it is homogeneous, or limited to crusting, and crucially, how easily the flow characteristics of the original powder can be regained.
What insights do flow energy measurements provide?
Values of flow energy are generated from measurements of the axial and rotational forces acting on the blade of a powder rheometer as it rotates down through the powder sample. Example traces clearly illustrate the insights that this approach provides.
Here we see data gathered over six sequential days for a powder stored at a relative humidity (RH) of 53%. As early as Day 2 there is a distinct peak in energy in the upper portion of the powder bed that becomes progressively more pronounced. The sample is crusting and over time the energy required to move the blade through that crust increases indicating that the crust is getting stronger. However, the peak is not advancing through the sample over time. Below 35 mm powder flowability remains unchanged throughout the test period.
This example shows how flow energy measurements not only detect caking but clearly differentiate crusting, in the form of a deviation in flow energy values from those of the bulk powder. Crust depth can be read directly from these traces since it is the point at which flow energy values revert to the baseline, between 30 and 35 mm in this case.
However, it is also possible to generate other useful information. Peak position indicates where the crust is strongest and integrating the area under the curve provides a measure of overall crust strength. Furthermore, by repeating tests following storage under, for example, lower humidity, elevated temperature or following agitation, it is possible to determine the extent to which caking is reversible and the flow properties of the powder are recoverable.
In short, flow energy measurements can provide a sound basis for the development of robust anti-caking strategy.
How can I use dynamic powder testing results to control caking and preserve powder value?
Dynamic powder testing makes it possible to characterize the caking behavior of a specific powder and investigate its sensitivity to relevant conditions such as the diurnal temperature swings associated with storage under uncontrolled conditions in a specific geography or a reduction in RH associated with upgraded storage conditions. The performance of difference anti-caking agents can also be assessed. There are multiple options when it comes to reducing caking, but all involve cost. Testing can help to determine whether they are required, and which will be most cost-effective.
The traces above are for the same material as those presented earlier. Tests were carried out in a strictly analgous way but here the material has been stored at an RH of 75% and there is a clear difference in crusting behaviour. Under these conditions the peak advances through the powder bed at a rate that suggests that within a couple of weeks (at most) all of the powder will be adversely affected. Furthermore, in the wake of the advancing crust the powder does not regain its flow properties.
Together, these datasets provide a simple illustration of how flow energy measurements can answer questions associated with managing caking. They show that:
- This material crusts rather than caking homogeneously.
- The crust gets stronger over time with no evidence of this effect leveling out – longer storage periods are going to be more problematic.
- Lowering humidity is beneficial – it prevents the crust from advancing through the powder, thereby protecting some of the powder bulk – but it doesn’t eliminate the problem completely.
Other tests that could be usefully applied to learn more include agitating the caked material and re-measuring flow energy, to see whether values revert to those of the original powder, and testing under even lower RH to see whether crusting can be limited to a minimal crust.
Learn more about the mechanisms of caking and the investigations we’ve already carried out by reading: The measurement and quantification of caking in excipients and food products with emphasis on the non-homogeneous interaction with ambient moisture.
Freeman Technology specialises in systems for measuring the flow properties of powders and has over a decade of experience in powder flow and powder characterisation. The company invests significantly in R&D and applications development, and provides detailed know-how alongside its universal powder tester, the FT4 Powder Rheometer. Expert teams ...