Tim Freeman’s Powder Flow Feature: Exploring the Impact of Humidity on Bulk Powder Properties

By Tim Freeman, Managing Director, Freeman Technology

In a series of editorials I’m looking at variables that can impact powder behaviour.  Having started by examining the effect of various properties of particles (please take a look at the other articles in this series – ‘Exploring the Impact of Particle Shape on Bulk Powder Properties’ and ‘Exploring the Impact of Particle Size on Bulk Powder Properties’), I’m now turning to a system variable - humidity. Moisture can significantly influence powder behaviour, especially flow properties and quantifying its effect is essential in order to develop effective control strategies for a specific application.  There can be significant cost involved in removing water from the atmosphere in a processing environment, or indeed from within the powder itself, and this must be balanced with maintaining acceptable processing performance. The challenge is to understand a powder’s appetite for moisture uptake and, more importantly, how moisture will affect its characteristics and performance.

The mechanisms of powder flow

For a powder to flow, the particles within it must move relative to one another. It is widely believed that introducing water makes powders flow less freely, and there are mechanistic reasons to support this concept. Water in a powder often forms liquid bridges between particles that would otherwise be subject to relatively low interparticulate forces. Wet granulation exploits this mechanism, but when it occurs in routine operation such bridging can inhibit the movement of the particles, with a detrimental impact on performance.

Figure 1 - Liquid bridging

Conversely, there are times when moisture improves flow behaviour. In the case of particles with a rough surface, for example, low levels of moisture can act as a lubricant, allowing the powder to flow more freely. Water can also improve the performance of electrostatically charged powder by improving its electrical conductivity. Dissipating electrostatic charge in this way can reduce the strength of interparticulate cohesive forces with a dramatic impact on flow behaviour, especially for powders with relatively small particle size.

Since moisture can induce these very different effects, it is vital to accurately measure its influence. We’ve recently conducted research in this area and have found dynamic powder testing, alongside bulk property measurement, to be particularly informative.

Measuring the impact of humidity

Dynamic powder testing involves measuring the axial and rotational forces acting on a blade as it rotates through a powder sample, to determine flow energies. Basic Flowability Energy (BFE) is the flow energy measured as the blade passes down through a powder sample of uniform, low to moderate packing density. However, one of the benefits of dynamic measurement is that it can be applied to consolidated, conditioned (as in the case of BFE), aerated, and even fluidised powders to assess how powders will behave in different processing environments.

The graphs below show how BFE, Aerated Energy and permeability change as a function of moisture content for microcrystalline cellulose (MCC) [PH200, FMC]. These data were generated using an FT4 Powder Rheometer^TM, with Aerated Energy measured with air flowing up through the sample at a low velocity (2mm/s) to minimize moisture loss.

Dynamic and permeability data sensitively describe the impact of humidity on MCC and support an understanding of the mechanisms defining flow behaviour in this system.

The results show interesting and perhaps unexpected behaviour, with both BFE and Aerated Energy passing through a minimum as moisture content increases from the initially desiccated condition.  During the study it was observed that the sample had a tendency to coat the test vessel, suggesting that it may be electrostatically charged. This points to a rationale for the observed behaviour, where at low moisture levels the water increases electrical conductivity, reducing interparticulate forces of attraction, whilst at higher moisture levels, capillary bonding occurs between particles, increasing adhesion and resulting in a higher flow energy.

Of the bulk properties measured, permeability data proved most informative with the marked trend supporting the proposed hypothesis. The steady increase of permeability with moisture content is suggestive of a system initially becoming progressively less cohesive as a result of electrostatic discharge, followed by increased agglomeration.

Moisture – good or bad? 

These results show that moisture is not always detrimental to powder flow behaviour and underline the importance of carrying out appropriate powder testing to quantify a material’s sensitivity to moisture. The range of characteristics demonstrated by the MCC, which cannot be predicted from first principles, were a result of exposing samples to relative humidities in the range 25 – 75% and so this variation in material performance could easily occur in routine industrial operation.

Of equal importance, the results show how different properties may help to identify and rationalise the mechanisms dominating behaviour. At Freeman Technology, our preferred approach is to measure shear, bulk and dynamic properties to address processing issues, but in this instance neither shear data nor bulk density changes were able to reliably identify the critical changes in powder behaviour. It was dynamic and permeability data that provided the information needed to assess the impact of humidity in a process relevant way, and can be used to support effective process design and optimization.

Author Biography

Tim Freeman, Managing Director, Freeman Technology

Tim Freeman is Managing Director of powder characterisation company Freeman Technology for whom he has worked since the late 1990s. He was instrumental in the design and continuing development of the FT4 Powder Rheometer® and the Uniaxial Powder Tester. Through his work with various professional bodies, and involvement in industry initiatives, Tim is an established contributor to wider developments in powder processing.

Tim has a degree in Mechatronics from the University of Sussex in the UK. He is a mentor on a number of project groups for the Engineering Research Center for Structured Organic Particulate Systems in the US and a frequent contributor to industry conferences in the area of powder characterisation and processing. A past Chair of the American Association of Pharmaceutical Scientists (AAPS) Process Analytical Technology Focus Group Tim is a member of the Editorial Advisory Board of Pharmaceutical Technology and features on the Industry Expert Panel in European Pharmaceutical Review magazine.  Tim is also a committee member of the Particle Technology Special Interest Group at the Institute of Chemical Engineers, Vice-Chair of the D18.24 sub-committee on the Characterisation and Handling of Powders and Bulk Solids at ASTM and a member of the United States Pharmacopeial (USP) General Chapters Physical Analysis Expert Committee (GC-PA EC).

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Introduction

Following Tim Freeman's articles on Powder Flow,

understanding powder behaviour to optimise process performance, increase productivity and improve quality.

More Articles by Tim Freeman:

Tim Freeman's Powder Flow Feature: Exploring the Impact of Particle Size on Bulk Powder Properties

Tim Freeman's Powder Flow Feature: What Makes a Powder Flow?

Tim Freeman's Powder Flow Feature: Understanding Powders

Tim Freeman's Powder Flow Feature: Exploring the Impact of Particle Shape on Bulk Powder Properties