Powder mechanics cohesion

Sintering is the consohdation, under the effect of temperature, of a powdery agglomerate, a non-cohesive granular material (often called compact, even though its porosity is typically 40% and therefore its compactness is only 60%), with the particles of the starting powder welding with one another to ereate a mechanically cohesive solid, generally a polyciystal. 

Tomas J Product design of cohesive powders—mechanical properties, compression and flow behavior, Chem Eng Technnol 27 605-618, 2004. 

Group C powders are of small particle size, cohesive by nature and hence, difficult to fluidize. Either the fluidized bed lifts as a solid plug of material or forms stable channels of air flow, which allows the fluidizing gas to escape. The latter phenomenon is referred to simply as channelling . It may be possible to fluidize such powders by mechanical agitation (e.g., stirring, vibration). 

The mechanical properties of a material play an important role in powder flow and compaction by influencing particle-particle interaction and cohesion, that is to say, by influencing the true area of contact between particles. For example, Hertz [26] demonstrated that both the size and shape of the zone of contact followed simply from the elastic properties of a material. Clearly then, the true area of contact is affected by elastic properties. From the laws of elasticity, one can predict the area of contact between two elastic bodies. More recent work has demonstrated, however, that additional factors must be taken... 

Zhou, Q., Armstrong, B., Larson, I., Stewart, P. J., Morton, D. A. V. Improving powder flow properties of a cohesive lactose monohydrate powder by intensive mechanical dry coating. J. Pharm. Sci., 99, 2010, 969-981. 

Binders increase cohesion between the particles of fuel and oxidizer, facilitating better consolidation and resulting in better mechanical properties of the end products. In addition, an equally important function is to coat and protect metallic or non-metallic fuels (such as metal or non-metal powders) which may otherwise react with moisture and oxygen. They also modify the burning rate and hence the performance, and at the same time reduce sensitivity to impact or friction. A comprehensive list of binders, natural to synthetic resins used for pyrotechnic formulations follows. 

Solids of group C are very fine-grained, cohesive powders (e.g. flour, fines from cyclones, and electrostatic filters) that virtually cannot be fluidized without fluidization aids. The adhesion forces between particles are stronger than the forces that the fluid can exert on the particles. Gas flow through the bed forms channels extending from the grid to the top of the bed, and the pressure drop across the bed is lower than the value from cq 1. Fluidization properties can be improved by the use of mechanical equipment (agitators, vibrators) or flowability additives such as Aerosil. 

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