Introduction

Humidity is one of the most influential environmental factors affecting powder behavior. During storage, transportation, or processing, powders are often exposed to varying levels of relative humidity. Depending on the material and environmental conditions, powders may absorb or release moisture, leading to changes in interparticle interactions.

These changes can significantly affect powder flowability, packing behavior, and process performance. Increased humidity may promote the formation of capillary bridges between particles, while also influencing electrostatic interactions and Van der Waals forces. As a result, powder cohesion can increase or decrease over time.

Understanding how humidity affects a powder is therefore essential for industries such as pharmaceuticals, food processing, chemicals, construction materials, and battery manufacturing. 

Why Measure Humidity Effects on Powders?

Powders rarely remain unchanged when exposed to humid environments. As moisture interacts with particles, several phenomena may occur:

• Formation of capillary bridges
• Changes in electrostatic charging
• Modification of Van der Waals interactions
• Particle surface alteration
• Increased agglomeration or cohesion

These effects can directly influence packing dynamics, flowability, powder handling, and product quality. Monitoring the evolution of powder properties after controlled humidity exposure helps predict storage stability and processing performance.

Conditioning Powders at Controlled Humidity

To evaluate the effect of humidity, powders must first be conditioned under controlled environmental conditions. This conditioning can be performed:

• In static conditions, such as a sealed humidity-controlled chamber.
• In dynamic conditions, such as a reactor with airflow and powder agitation.

Static conditioning is particularly useful for simulating long-term storage, while dynamic conditioning can reproduce exposure to humid atmospheres during powder handling or transport.

The conditioning process should be performed for different exposure times to monitor how the powder evolves as moisture progressively affects particle interactions.

Measuring Packing Behavior with GranuPack

Before conditioning, a reference measurement is performed using the GranuPack to establish the baseline packing curve of the material. The powder is then exposed to a fixed relative humidity for increasing conditioning times. After each conditioning period, a fresh GranuPack measurement is performed under identical operating conditions. Comparing the resulting packing curves reveals how humidity influences particle mobility and densification behavior.

Understanding the Packing Curve Evolution

The GranuPack measures the evolution of bulk density during successive taps. Changes in particle interactions caused by humidity directly affect the way the powder reorganizes and densifies during the test.

Figure 1 presents a typical example obtained for lactose powder stored at 75% relative humidity.

As the conditioning time increases, the packing curves progressively shift toward lower bulk densities. This behavior suggests that the powder becomes more cohesive and less able to reorganize efficiently during densification.

Figure 1: Decrease in bulk density of lactose powder due to storage at 75% RH for different times.

The Role of Conditioning Time

Humidity effects are often not instantaneous. In many cases, powders require sufficient exposure time before moisture-induced interactions become significant.

For the lactose example shown in Figure 1, little difference is observed between the initial measurement and the sample conditioned for one hour. However, after longer conditioning periods, clear changes in packing behavior become apparent. This illustrates an important practical point: The conditioning time should be investigated first before evaluating the influence of relative humidity itself.

Once a sufficiently long conditioning period has been identified, the effect of different humidity levels can then be studied more effectively.

Interpreting Changes in Bulk Density

The evolution of bulk density provides valuable information about changes in powder cohesion. A decrease in bulk density is often associated with:

• Increased capillary bridge formation
• Stronger particle-particle cohesion
• Reduced particle mobility
• Less efficient packing during tapping

Conversely, some materials may exhibit different behaviors depending on their chemistry and moisture affinity.
For this reason, the complete packing curve should always be analyzed rather than focusing on a single density value.

Applications of the Method

The protocol can be used to investigate:

• Storage stability
• Shelf-life performance
• Packaging requirements
• Environmental sensitivity
• Relative humidity effects
• Moisture uptake kinetics

It can also support the development of powder formulations that are less sensitive to environmental variations.

Industrial Applications

Humidity evaluation is particularly valuable for:

• Pharmaceutical powders
• Food ingredients
• Dairy products
• Construction materials
• Battery powders
• Additive manufacturing feedstocks
• Specialty chemicals

The method provides a simple and reproducible approach for quantifying the influence of storage conditions on powder behavior.

Conclusion

The GranuPack offers a practical way to evaluate the effect of humidity on powder behavior through changes in packing dynamics.

By monitoring the evolution of packing curves after controlled humidity exposure, users can identify changes in particle interactions and cohesion. A decrease in bulk density often indicates increased interparticle attraction caused by capillary bridges or modifications of electrostatic and Van der Waals forces.

Combined with controlled conditioning experiments, this protocol provides valuable insight into the impact of humidity and storage time on powder performance.

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