Powder flow frictional characteristics

The weaknesses associated with the flow through an orifice and angle of repose measurements limit their application for powder flow studies and hopper designs. Consequently, several powder shear testers and methods that permit a more thorough and precisely defined assessment of powder flow characteristics were developed. Shear testers that measure the frictional characteristics of a powder bed under load yield valuable information with regard to powder flow in high-speed tablet equipment. A number of types of shear cell testers are available, but the most common types used in the pharmaceutical industry are the Jenike shear cell and the Schulze ring shear tester.61,62... 

The flow through an orifice is an excellenf mefhod fo compare materials with good flow. Pharmacy, however, most often deals with cohesive powders that do not readily flow. The flow of these materials is increased by employing force feeders, resulting in a dynamic process. Angle-of-repose measurements that do not utilize force and that are often a measurement of stagnant powder have therefore limited application. Shear testers, that measure the frictional characteristics... 

The primary function of tablet lubricants is to reduce the friction arising at the interface of tablet and die walls during compression and ejection. Lubricants also possess antiadherent (prevention of sticking to the punch and, to a lesser extent, to the die wall) and gli-dant (improvement of flow characteristics of powders or granulates) characteristics and are useful in the processing of hard gelatin capsules. 

Inasmuch as friction conditions determine the flow characteristics of a powder, coarser powder particles of spherical shape flow fastest and powder particles of identical diameter but irregular shape flow more slowly. Finer particles may start to flow, but stop after a short time. Tapping is needed in order to start the flow again. Very fine powders (fine powder particles to coarser ones may increase the apparent density, but usually decreases the flow quality. Metal powders having a thin oxide film may flow well. When the oxide film is removed and the friction between the particles therefore increases, these powders may flow poorly. 

The influence of consolidation load on the flowability of sucrose is shown in Fig. 8. For this material, the effective angle of internal friction is nearly constant yet the shear index is seen to change with state of consolidation. Apparently, for sucrose, increased consolidation results in a somewhat more free flowing although still cohesive material. As such, sucrose can be considered a complex powder [49] with perhaps somewhat better flow characteristics when consolidated (as might occur in a hopper). 

In general, this Ck)ulomb yield criterion can be used to determine what stress will be required to cause a ceramic powder to flow or deform. All that is needed are the two characteristics of the ceramic powder the angle of friction, 8, and the cohesion stress, c, for each particular void fraction. With these data, the effective yield locus can be determined, from which the force required to deform the powder to a particular void fraction (or density) can be determined. This Coulomb yield criterion, however, gives no information on how fast the deformation will take place. To determine the velocity that occurs durii flow or deformation of a dry ceramic powder, we need to solve the equation of motion. The equation of motion requires a constitutive equation for the powder. The constitutive equation gives the shear and normal states of stress in terms of the time derivative of the displacement of the material. This information is unavailable for ceramic powders, and the measurements are particularly difficult [76, p. 93]. 

Die fill characteristics depend upon material flow properties that are primarily affected by particle size and shape. Additionally, high interparticle friction can have a detrimental effect on die fill characteristics due to powder bridging and non-uniform flow characteristics. A non-uniform particle size distribution may also lead to material segregation resulting in uniformity problems. Tablet presses employ volumetric filling of the material into the die cavity. Most high-speed tablet presses are equipped with force feeders, which use rotating paddles to promote uniform die fill characteristics. For certain materials, attention must be... 

In case of use of pressure nozzles, the liquid feed is pumped through a small orifice under high pressure, typically 2-300 bars, and the liquid breaks up into droplets by friction with the atmosphere. Nozzles may produce powders with particularly narrow particle size distribution or relatively dense particles. The particle size from a given pressure nozzle is influenced by the feed rate. This is controlled by varying the feed pressure which again influences the flow capacity. Pressure nozzles are therefore less flexible than centrifugal atomizers with regard to ease of operation and control of product characteristics. 

They have demonstrated excellent performance characteristics in rigid PVC compounding, extruding, and wlding, where they serve to reduce friction and plate-out, ioq>rove powder-blend flow, regulate fusion rates, and enhance heat stability. 

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