Powder mechanics bulk density

In general terms, compressibility refers to the variation in bulk density with respect to consolidating confined pressure acting on a powder bed. Bulk density (in terms of apparent, compact, or tap density) and normal stress have been associated in empirical logarithmic or semilogarithmic relationships, from which a constant slope value is defined as mechanical compressibility. Simultaneous decrease in a powder s loose bulk density and increase in compressibility indicate greater attractive and cohesive interactions among powders. 

The form is typically pellets, flake, or powder. Most pellets and flake are easy to feed since they have a high bulk density, their individual dimensions are sufficiently small with respect to extruder channel depth, and they do not lubricate the screw and thus impede the drag flow mechanism. Standard profile screw elements with a pitch of 1-2D are appropriate. 

Yan et al. (2001), studied how bulk density of instant nonfat milk, spray-dried coffee, and freeze-dried coffee was affected by HHP processing times, particle size, and water activity. The experimental curves for each powder in Figure 10 show that the powder bulk density increased as the pressure increased but remained constant after the pressure reached a critical value of 207 MPa for spray-dried coffee and 276 MPa for freeze-dried coffee at different water activities. The final compressed densities were not significantly different. When the pressure is higher than the critical value, there are no void spaces between the agglomerates or primary particles even the primary particles are crushed, leaving no open or closed pores within. Bear in mind, it is assumed that the compression mechanisms are the same as those in the confined uniaxial compression tests. 

Onwulata et al. (1995) studied the effect of flow conditioners calcium stearate, aluminum silicate, and silica added at different concentrations on bulk density, flow and mechanical properties of lactose, sucrose and modified cornstarch, as well as milk fat encapsulated in the same materials. Each flow conditioner was effective in reducing compressibility (C2) of the powders when applied at 1% concentration. Compressibility of the remaining unencapsulated powders continued to decrease with added flow conditioner. However, in the case of encapsulated butter-oil powder, the only effective additive was silica (2%), which resulted in a 35-70% decrease in compressibility. The most notable effect was observed with butter-oil powder encapsulated in lactose where reductions in compressibility resulted and an increase in powders flowability. The stearate resulted in flow retardation of all powders studied. 

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