Powders can be defined as a particle system of solid particles which contain little or no liquids. These particles are created from solids that are ground, crushed or undergo disintegration. One of the characteristics of powders is that they can flow when the container they're held in is shaken or tilted.
The behavioral aspects of powders are related to those of its constituent particles. Granular powders have larger particles and flow more freely, while fine powders tend to lump or clump together during flow.
What Is Cohesion?
Cohesion is an indicator of the mechanical behavior of a powder. It can be defined as the flow behavior of powders, the maximum resistance against shear or shear strength when no normal stress is applied to the plane of shear.
The flow of powder granules is very different from that of liquids and much more difficult. It is also variable and unpredictable. These aspects are a result of the cohesive and adhesive properties of powders, which are essentially surface properties.
Powder cohesion occurs between two like surfaces, such as similar component particles in a bulk solid. This is different from adhesion which occurs between two different objects/particles such as a powder and a container wall.
Factors affecting Cohesion
The amount of cohesion and adhesion depend on particle size, roughness/smoothness, shape, surface free energy etc. Cohesion varies with particle size and also changes in relative humidity. Electrostatic forces due to frictional charging can also affect cohesion.
The angle of repose is a simple measure of powder flow. The particles begin to slide when the angle of inclination is large enough to overcome frictional forces and they come to rest when the angle is below that required to combat cohesion/adhesion. It is this balancing of forces that causes powders to form a heap when poured on to a horizontal surface.
This is referred to as the angle of repose or the maximum angle at which powders can remain without sliding. When the angle of repose is exceeded, the particles tend to “freeze” into a dynamic angle of repose. This is the angle of the surface of a flowing powder relative to the horizontal.
Compressibility
Powder compressibility is a measure of the relative volume-change of a solid as a response to a pressure change. Powders have the ability to decrease in volume when subjected to pressure. Solids are compressible and can experience bulk density changes according to the amount of pressure or mean stress applied or removed. It's important to know the product's compressibility because this affects hopper/feeder design parameters.
Powder handling equipments like powder rheometers provide different data regarding flowability of powders. This ensures that flowability problems are detected early, analyzed and corrected before they cause problems in manufacturing processes.
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