Preformulation: Bulk Powder Properties

Preformulation - Bulk Powder Properties

• Powders exhibit various packing states, ranging from loosely packed (maximum airspace, minimum contact points) to tightly packed (minimum air entrapped, maximum interparticulate contact).
• Porosity is the relative amount of air entrapped between particles, affected by particle shape and size. Smaller particles fill voids between larger ones.
• A packing arrangement consisting of 75% coarse and 25% fine particles yields the densest arrangement.
• Porosity of isotropic shapes like cubes can vary. Cubical packing has approximately 48% porosity and rhombohedral packing has approximately 26% porosity.
• Cubic shapes with 55% porosity and 0% porosity are also considered.
• Coarse and fine particles influence packing density. Increasing the proportion of fine particles increases packing density.
• Determining porosity involves placing a powder in a measuring cylinder without disturbance (loose packing). This volume is the 'poured bulk volume'. Reciprocal of this is minimum bulk density. The void volume (difference between total volume and particle volume) is used to calculate porosity (ɛ). Porosity can also be expressed as a packing fraction (k).
• Tapped density is determined using a tapping volumeter. A rotating cam in the device creates a series of taps on the powder, leading to a minimal bulk volume, termed tapped volume (Vt). Reciprocal of this is 'maximum bulk density' or 'tapped density'. This method monitors dynamic powder packing with time.
• Packing and flow properties are closely associated. A larger difference between poured and tapped densities suggests that more tapping is needed to reach the tightest packing.
• Carr's compressibility index is used to assess flow properties. It is calculated as: ((Tapped density-bulk density)/Tapped density) x 100.
• Carr's Index ranges from 5-15 (Excellent flow), 12-16 (Good flow), 18-21 (Fair to passable flow), 23-35 (Poor flow), 33-38 (Very poor flow), >40 (Extremely poor flow).
• A glidant improves flow characteristics particularly in poor flow classifications.
• Hausner ratio is determined by tapping the powder and calculating (Tapping density/Poured density). Values less than 1.25 indicate good flow. Between 1.25 and 1.5, a glidant normally improves flow. Values greater than 1.5 indicate poor flow. Glidants do not necessarily improve flows well beyond ratio of 1.5.
• Table 1 provides characteristics of bulk powder. It includes measurements of median particle size, aspect ratio, volume, and Carr’s Index. Based on this data, lactose displays good flow properties while others like corn starch or MCC 50M have poor flow properties.
• Lactose's low volume drop and relatively larger particle size contribute to its good flow.
• Microcrystalline cellulose (MCC) exhibits a considerable drop in volume and low bulk density, limiting its suitability for certain applications (e.g., capsule filling).
• Angle of repose is the maximum angle between the surface of a pile of powder and a horizontal plane, indicating flow characteristics. Lower values suggest better flow.
• Hopper design impacts the flow from the hopper. Funnel flow involves some stationary powder, potentially causing rat-holing and segregation problems. Mass flow involves all powder in motion, promoting uniformity, and requiring specific manufacturing.
• Arch formation and rat-holing relate to hopper design; strong interparticle forces create arches in funnel flow.
• Inadequate emptying in funnel flow silos might result due to insufficient cone angles for bulk substance drainage.
• Mass flow enables the uniform movement of powder along the hopper walls.
• Critical orifice diameter is a powder characteristic related to arching/rat holing; it depends on particle size, flow, and the ratio between orifice and particle diameters (Do/Dp).
• Flow is improved with various measures including particle properties (size, shape, charge, moisture), equipment like hoppers, aeration, and force feeders and formulations like glidants.

Additional Notes

• Aeration and vacuum techniques enhance powder flow in hoppers and feeding devices.

Explore the fascinating world of bulk powder properties in the study of preformulation. This quiz covers essential concepts such as packing states, porosity, and the influence of particle size and shape on powder density. Test your knowledge on the arrangements of coarse and fine particles and their effects on packing density.