Carr [2-4] developed several tests in an attempt to determine the flowability and compressibility of bulk solids. His methods give indices that have had mixed reviews over the last 40 years. Many companies have devised their own internal methods and have published these for others to use — but they’re often subjective and depend on the operator’s scrutiny. Examples are observation of flow though different orifice sizes, lumping and compression as well as frangibility and friability tests.
Here’s a highspotting of the pros and cons of the most common qualitative tests for evaluating clumping potential:
• Carr Flowability Index (ASTM-D6393)  — is quick and easy to do (simple equipment) and used extensively, gives a relative indication of how much a powder will compact and the strength of cohesive material, but can’t provide quantitative results and isn’t useful for time and temperature effects on solids;
• BASF lumping and compression tests (BASF Bulletin TPU 0402)  — uses small amount of material (10–15 g) to get qualitative results, but provides limited ability to study the effect of storage time and temperature on flow;
• Flow though an orifice or down a surface — offers qualitative results on how big an orifice needs to be for 50% of solids to flow out of a container (commercial instruments are available (Figure 1); however, many companies have constructed their own devices) or the necessary angle of slide (specific to wall material), but can’t give the effect of storage time on flow; and
• Frangibility (sometimes called friability) tests — indicate relative strength of agglomerates, can be used to estimate crushing strength or how easily a clump can be broken, which may eliminate the need to solve a clumping problem, but use non-standard equipment (large balls are placed on the upper screen of a sieve stack and vibrated; amount of solids that pass to the pan is compared to the amount that goes to the pan without the balls).
In general, these tests will give an indication of the potential for clumping to occur upon storage when starting particulate solids are cohesive or flow poorly, i.e., Carr flowability index of less than 50 (see Table 1). However, initial good flowability doesn’t predict lack of clumping upon storage. So, when facing a problem, it’s important to look at the 10 most common sources of agglomeration to ascertain the likely culprit(s). In that respect the following techniques are useful in determining potential for phase change and can identify chemical components that are the cause of the problem:
• Raman spectroscopy — can determine polymorphs;
• Differential Scanning Calorimetry (DSC) — can identify changes in structure and presence of unstable chemical forms or polymorphs;
• Scanning Electron Microscopy — can see formation of bridges (this can be combined with a probe to look at specific elements);
• Transmission electron microscopy — can detect bridge composition and impurities (but has limited application unless sample is very strong because it must be thin); and
• Atomic force microscopy— can observe real-time agglomeration (this technique is evolving rapidly and eventually may be able to see atomic-level changes).