The previous article identified the material properties that affect the blending of solids. A definition and brief description of material properties was provided. This article discusses in detail the effect of flowability on blending and blender selection.
Properties Affecting Blending
The material properties affecting blending are as follows:
- Angle of Repose
- Bulk Density
- Particle Size, Distribution
- Particle Shape
- Material Composition
- Surface Characteristics
- Moisture or liquid content of solids
- Density, Viscosity, Surface tension of liquid added
- Temperature Limitations of Ingredients
Flowability is the ease with which a bulk material flows under the influence of gravity only.
Since blending involves the flow of materials, blending mechanisms are affected by the same properties that affect flow. These properties include mechanical interlocking, surface attraction, plastic welding (from high pressures between small contact areas), electrostatic attraction, cohesive strength, wall friction, moisture and temperature conditions . Flowability of bulk solids also depends upon factors such as particle size and size distribution, particle shape, and bulk density, all of which affect blending.
Since flow behavior is multidimensional and depends on many powder characteristics, it is difficult to quantify flowability using a single flow index. Moreover, flowability is just not an inherent material property. It is a result of the combination of physical properties of materials and the equipment which contains, processes the material; each of which must be given equal consideration .
Quantifying Flowability using Angle of Repose
An easily quantifiable parameter most commonly used to measure flowability is the angle of repose. The angle of repose of a bulk material is the angle formed between the horizontal and sloping surface of a piled material, which has been allowed to form naturally without any conditioning. The coefficient of friction of a powder is the tangent of the angle of repose and is the measure for its flowability.
The reasons for preference of angle of repose are:
- It gives a reproducible numerical value for a given material.
- It is a relative measure of friction and cohesiveness of powder particles.
- It indirectly quantifies other physical properties such as size, shape, and porosity all of which directly affect the ability of a material to flow.
General Guidelines for Determining Flowability of Powders
The following are some of the generalizations that can be made to determine the flowability of powder materials.
- Higher angle of repose means poor powder flow.
- Lower angle of repose indicates good flowability.
- Bulk solids with an angle of repose between approximately 25 degrees and 35 degrees are generally considered free flowing.
- Particles of size less than 75 microns generally have a higher angle of repose and demonstrate poor flowability.
- Particles of size greater than 250 microns have a low angle of repose and flow with ease.
- Flowability of densely packed powder (high bulk density) is less than that of loosely packed powders (low bulk density).
- Particle shape affects inter-particle powder friction and thereby flow properties of the powder.
- Cohesive powders generally have an angle of repose of greater than 60 degrees and demonstrate poor flowability.
- Non-cohesive particles have an angle of repose less than 25 degrees and are generally considered as free flowing.
- Agglomeration of particles due to “van der Walls" forces and electrostatic forces inhibit powder flow.
Flowability and Blender Selection
Selecting the right blender is often considered an art rather than a science. A good blender design should ensure the following:
- There are no stagnant regions in the blender.
- The blender must promote different flow velocities in different sections of the blender.
- The blender operation must not segregate, or demix, mixture ingredients.
Powder flow properties can simplify blender selection by allowing the prediction of the behavior of materials in different types of blenders. Knowledge of basic material flow properties and segregation tendencies provides guidance in selecting the right blender for the application.
Shear, convection and diffusion are the three primary mechanisms of blending of solids. The extent to which each of these mechanisms occurs depends on the flow properties of the powder to be blended and the type of the blender used.
Diffusion blending is characterized by small scale random motion of solid particles. Blender movements increase the mobility of the individual particles and thus promote diffusive blending. Tumbler blenders like the double cone blenders and v-blenders function by diffusion mixing. With the diffusion mechanism, particles migrate through a dilated or expanded bed of powder. The ability of the bed to dilate and the mobility of powder particles depend on the cohesive strength of the powder. Powders with lower cohesive strength dilate more readily. Shorter blend times are achieved if the major component of the blend is relatively free flowing. A tumbler blender works best with ingredients that have similar angles of repose and only enough cohesiveness to prevent sifting.
Convection blending is characterized by large scale random motion of solid particles. In convection blending groups of particles are rapidly moved from one position to another due to the action of a rotating agitator. Likewise, in shear blending mechanism, the splitting of the material beds takes place by the high intensity shearing action of chopper blades. The material agglomerates are disintegrated by overcoming cohesion. As a result cohesive powder may blend faster in convective blenders like Ribbon Blenders and Plow Mixers because of chaotic patterns created by the blender action.
If the different components of the blend do not adhere to one another, free flowing materials may segregate easily during post blender handling. Common particle segregation mechanisms include sifting, angle of repose, fluidization (air entrainment), and dusting (particle entrainment). A good blend can be achieved if the minor component is somewhat cohesive or has a tendency to adhere to the major component of the blend. This is referred to as adhesive blend. Likewise, a good blend can be obtained and maintained if the blend as a whole is slightly cohesive compared with a blend that is free flowing. Therefore, blending action must be compared with actions that result in segregation.
The flow of material during a blending operation is a complex mechanism. Until now, there are no well defined principles that can adequately describe the blending behavior. As a result, blender performance for new materials or formulations can not be accurately predicted. The best approach for the selection of a blender most suitable for a specific material is by conducting trials on the different types of blenders. Besides enabling proper blender selection, the trials should be conducted to decide the position of material charging, sequence in which materials should be added, the speed of operation of the blender, and other related parameters. The objective of the blender is to produce a homogenous blend in the shortest time and avoid post-blending segregation.
- Gustavo V. Barbosa- Canovas, Enrique Ortega-Rivas, Pablo Juliano, Food Powders: Physical properties, processing, and functionality
- James K. Prescott and Roger A. Barnum, “On Powder Flowability," Pharmaceutical Technology, October 2009
- Maynard, Eric, “Blender selection and avoidance of post-blender segregation: proper blender selection can improve the mixing of bulk solids. Be sure to consider post-blending segregation problems," Solids Processing, May 2008
- Russell J. Lantz, Jr., Joseph B. Schwartz, “Mixing," Pharmaceutical dosage forms – tablets
- Jayesh Tekchandaney, “Solid Blending Mechanisms and Blend Structures"