Cohesive powders, definitions

Particle behavior is a function of particle size, density, surface area, and shape. These interact in a complex manner to give the total particle behavior pattern [28], The shape of a particle is probably the most difficult characteristic to be determined because there is such diversity in relation to particle shape. However, particle shape is a fundamental factor in powder characterization that will influence important properties such as bulk density, permeability, flowability, coatablility, particle packing arrangements, attrition, and cohesion [33-36], Consequently it is pertinent to the successful manipulation of pharmaceutical powders that an accurate definition of particle shape is obtained prior to powder processing. 

Conversely, Ehlermann and Schubert (1987) sustained that compressibility results from materials of different composition cannot be compared and that flowability characterization through compressibility must be made specifically for each food variety. Moreover, confined uniaxial compression is a simple compression test that provides an approximate measure of the flowability of powders. Therefore, it is not suitable for silo design but may prove to be a convenient method for process control in any food laboratory (e.g., to evaluate particle cohesion). Table II offers a range value definition for flowability classification by comparing flow function (ratio between the maximum consolidation stress and unconfined yield stress) with compressibility. 

The common definition of flowabihty is the ability of a powder to flow from a specific item of plant equipment at a desired degree of flowability. Flowability is generally quantified by a range of mobility, from free flow to non-flow which can, at times, be measured in a laboratory with specific flow property instrumentation. A bulk powder may be described as being free flowing, cohesive or non-flowing, and thus powder flowability tends to become an integral part of the description of bulk powder property measurement. Correlations between powder bulk properties in terms of both micro-scaled particulate factors as well as macro-... 

Yokoyama et al. (1982) measured the tensile strength with the swing method of the Hosokawa cohetester, for a range of particle sizes (1-90 urn) of different densities (0.928-6.01 g/cm ) over a range of moisture percentages (0.06-15.2% w/w), to evaluate the potential floodability of powders. Yokoyama et al. defined their dimensionless floodability, as opposed to Carr s definition of flowability, as the ratio of the cohesive force (C) - measured by cohetester - to the gravity force on particles m g). 

Per definition, only the tensile strength oz and no inclination depends directly on the consolidation pre-histoiy obtained from a Taylor series linearisation of the yield locus [28] near Mohr circle of cohesive stationary flow, see Eq.( 9). Now the simplest formulation of the linear yield locus dependent on radius (Jvr ( R,st and center stresses (average pressure in the powder) ctvm [Pg.78]

---