Powder electrostatics particle cohesion

Powder or particle variables o particle size o size distribution o shape o surface texture o cohesivity o surface coating o particle interaction o wear or attrition characteristic o propensity to electrostatic charge o hardness o stiffness o strength o fracture toughness... 

Mazumder, M. K., E-SPART Analyzer Its Performance and Application to Powder and Particle Technology Processes, KONA, No. 11, 1993, pp. 105-118. Mazumder, M. K., R. E. Ware, T. Yokoyama, B. J. Rubin, and D. Kamp, Measurement of Particle Size and Electrostatic Charge Distributions on Toners Using E-SPART Analyzer, IEEE Trans. Ind. Appl., 27, 1991, pp. 611-619. McLean, K. J., Cohesion of Precipitated Dust Layer in Electrostatic Precipitators, J. Air Pol. Contrl. Assoc., 27, 11, Nov. 1977, pp. 1100-1103. Misev, T. A., Powder Coatings, Chemistry and Technology, Wiley, NY, 1991, p. 292. 

Such small particles usually are generated by air-jet micronization and less frequently by controlled precipitation or spray drying. As bulk powder, they usually tend to be very cohesive and exhibit poor flow and insufficient dispersion because of large interparticle forces such as van der Waals and electrostatic forces (Zeng et al. 2001 Podczeck 1998 Hickey et al. 1994). The control of sufficient powder flow and deaggregation (dispersion) is thus of utmost importance to ensure efficient therapy with a dry-powder aerosol. Two different formulation approaches are used currently in marketed DPI preparations to fulfill the requirements. Most often, coarse particles of a pharmacologically inactive excipient, usually a-lactose monohydrate, are added that act as a carrier and provide sufficient powder flow to the mixture. Other carbohydrates, amino acids, and phospholipids have been suggested frequently (Crowder et al. 2001). 

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. 

Some materials tend to form granules when sieved. Coating the particles in order to reduce cohesiveness can often reduce this effect. Powders may, for example, be shaken in a container with 1% fatty acid (stearic acid is often used) or fumed silica. Alternatively, the powder may be sieved wet. The addition of 0.1% sub-sieve carbon black has been found useful (although rather messy) for eliminating electrostatic charge. 

Good flow properties are a prerequisite for the successful manufacture of both tablets and powder-filled hard gelatin capsules. It is a property of all powders to resist the differential movement between particles when subjected to external stresses. This resistance is due to the cohesive forces between particles. Three principal types of interparticle force have been identified (Harnby et al. 1985) forces due to electrostatic charging, van der Waals forces and forces due to moisture. 

Cohesiveness and adhesion Cohesiveness of a powder is the ability of individual particles to stick together. This attraction may be caused by the presence of moisture, electrostatic charges or the fineness of the particles within a powder. Fine powders generally have a high surface area, which is usually associated with an increase in the surface energies within the powder and thus enhancement of the binding forces between particles (Stanley Wood et al. 1990). 

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