Powders cohesivity

The angle of repose of a powder blend, effective angle of internal friction (EAIF) from shear cell measurements, and the mean time to avalanche (MTA) in powder cohesivity tests are useful for assessing the flow of a tableting mixture at various scales (15 18). 

There are three types of lubricants employed in solid dosage form manufacture. The first class of lubricant is the glidant. The flow properties of a powder can be enhanced by the inclusion of a glidant. These are added to overcome powder cohesiveness. The two other classes of lubricant are antiadherent excipients, which reduce the friction between the tablet punch faces and tablet punches, and die wall lubricant excipients, which reduce the friction between the tablet surface and the die wall during and after compaction to enable easy ejection of the tablet. The level of a lubricant required in a tablet is formulation dependent and can be optimized using an instrumented tableting machine. 

Wrinkled Surface Particles. In contrast to the AIR particles and Pulmospheres which are hollow and porous, solid protein particles with wrinkled surfaces have recently been prepared in our laboratory. These wrinkled particles gave a significant improvement in FPF over spherical particles of bovine serum albu-min.f The reduction of the surface contacts among the wrinkle particles (Fig. 2) is believed to reduce powder cohesion. Dispersion of the wrinkle particles was shown to be less dependent on the inhaler choice and airflow compared with the smooth spherical particles of the same aerodynamic size. 

Extensions to Mass Discharge Relations Johanson (Tram. Soc. Min. Eng., March 1965) extended the Beverloo relations to include the effect of powder cohesion, with mass discharge rate given by... 

Increase powder flowability. Powder feed rates improve with decreasing powder cohesive strength, increasing flow gaps, and increasing bulk permeability. See Solids Handling Bulk Solids Flow Characterization. ... 

The flow properties of a powder can be enhanced by the inclusion of a glidant. These are added to overcome powder cohesiveness by interposing between particles to reduce surface rugosity, thus preventing interlocking of particles and lowering the interparticulate friction. Commonly used glidants are listed in Table 11.7. 

For a single component powder cohesion normally implies that the bulk powder has a developed macro-structure which inhibits flow, and when flow does occur it will be of an erratic or stick-slip type. On a smaller scale the same powder could well develop micro-structures of small agglomerates, grits or... 

Shear studies of powder bed can also be carried out to evaluate the cohesiveness of powders. There are many variations of shear cells for evaluation of powder cohesiveness. 

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