Quantifying Humidity's Impact on Powder Processing
Dynamic test methods assess moisture to improve process understanding and development
By Jamie Clayton and Brian Armstrong, Freeman Technology
Controlling powder processing is key to tableting and solid dosage form manufacturing. However, powders can be especially challenging to work with because they are vulnerable to so many different process variables. Humidity is one such variable that can have a tremendous impact on powder flow and other behavior.
However, tools are available to monitor humidity and its impacts on powders, allowing for improved control, process understanding, design and operation. This article examines dynamic shear and bulk test methods, and shows how they can be used to measure and compare the impact of moisture on two excipients, microcrystalline cellulose and lactose. Even small changes in moisture level or atmospheric humidity can have a strong influence on powder behavior.
Sometimes the impacts are positive. Wet granulation, for example, uses moisture to agglomerate fine, difficult-to-handle powders and transform them into free-flowing granules, while improving content uniformity. Moisture can also dissipate unwanted electrostatic charge in a powder or lubricate particle-particle movement, thereby improving flow properties.
In other cases, though, moisture can result in caking and reduced flowability when the materials are being stored. During processing, it can also result in problems after crystallization and wet milling.
These problems are often solved by drying the product or by controlling the storage environment, but such measures may be energy-intensive and costly. It is therefore essential for formulators and process engineers to understand the extent to which a powder takes up moisture when exposed to humidity and, perhaps more importantly, how this can alter its processing characteristics. Such understanding supports the development of realistic and economical strategies for moisture control.
Any method used to assess moisture must be reproducible and reliable, and generate data that correlate with performance in a manufacturing envi-ronment. A multifaceted approach, examining a range of parameters, is required.
Bulk properties such as density, permeability and compressibility can provide a valuable insight into powder behavior, while shear properties are particularly useful for understanding the behavior of consolidated powder in a hopper.
Automation has enhanced the reproducibility of bulk property test methods. Recently, dynamic testing methods have further extended the reach of bulk density and shear testing. One rheometer  can measure the torque and axial force acting on a rotating blade as it moves through a sample of powder (Figure 1) to generate values of flow energy which provide valuable correlation with in-process powder behavior. This technique can be applied to powders that are consolidated, conditioned, aerated, and even fluidized, allowing users to assess the impact of ambient air, and also, to simulate the conditions in a typical processing environment.
This rheometer was used to take dynamic measurements to evaluate humidity and its impacts on powder behavior. Studies focused on two different excipients, in powder form, widely used in pharmaceutical manufacturing: microcrystalline cellulose (MCC) and lactose (Table 1).
The first step was to assess how much moisture each material adsorbed or absorbed when allowed to equilibrate in environments of varying relative humidity.
As shown in Figure 2, test results suggest that lactose takes up relatively small quantities of water (0.3-0.7%), but that MCC, at the same relative humidity, takes up an order of magnitude more water (4-9%). These results posed another question: Are there differences in the ways the two powders take up moisture? More measurements were taken to help answer this question.
Using the rheometer, basic Flowability Energy, Specific Energy and Aerated Energy  were measured for both powders following equilibration at varying levels of relative humidity. Basic Flowability Energy (BFE) is measured as the instrument blade rotates downward through a conditioned sample. This action forces the powder against the base of the test vessel, imposing moderate compressive stresses. BFE is a good indicator of how the powder will flow under “forced” conditions, such as in feeders or extruders.
Lactose exhibits a slight decrease in BFE as moisture content increases (Figure 3). This suggests that the water may be acting as a lubricant and reducing inter-particular forces.
In contrast, the behavior of MCC is more complex, with a minimum BFE value observed at approximately 6% moisture content. During testing, it was observed that the desiccated MCC adhered to the wall of the glass test vessel, suggesting that the powder may have become electrostatically charged. As a result, it is reasonable to suggest that small increases in the moisture content initially reduce BFE by grounding the particles. However, at moisture levels above 6%, the BFE increases again, which could be as a result of increased agglomeration. Perhaps most importantly, this unusual behavior occurs in the 30-60% RH range (Figure 2), which demonstrates how the flow properties of MCC can change markedly in typical plant conditions.
Figure 4 shows how Specific Energy (SE), which is measured as the blade traverses upward through an unconfined sample, varies as a function of moisture content for both materials. SE is a good indicator of how a powder flows in the absence of applied stress, for example, when poured from a vessel or when flowing into an empty die.