Flow properties of pharmaceutical particles University of Sulaimani College of Pharmacy 5th Stage Practical Industrial Pharmacy 49 Flow properties of pharmaceutical particles Lab (4) Shahen S. Mohammed MSc pharmaceutics

Powder flowability definition A simple definition of powder flowability is the ability of a powder to flow. The same powder may flow well in one hopper but poorly in another. Likewise, a given hopper may handle one powder well but cause another powder to hang up. Therefore, a more accurate definition of powder flowability is the ability of powder to flow in a desired manner in a specific piece of equipment. With this definition, the term free flowing, so commonly used, becomes meaningless unless the specific equipment handling the material is specified.

Flow properties The specific bulk characteristics and properties of a powder that affect flow and that can in principle be measured are known as flow properties. Examples of flow properties include density (compressibility), cohesive strength, and wall friction. These flow properties refer to the behavior of the bulk material and arise from the collective forces acting on individual particles (e.g., van der Waals, electrostatic, surface tension, interlocking, and friction).

Problems of material flows Several problems can develop as material flows through the equipment. If the powder has cohesive strength, an arch or rathole may form. An arch is a stable obstruction that forms within the hopper section (i.e., the converging portion of the bin) usually near the bin outlet. Such an arch supports the rest of the bin's contents, preventing discharge of the remaining powder. A rathole is a stable pipe or vertical cavity that empties above the bin outlet.

Flow patterns Two flow patterns can develop: funnel flow and mass flow (see Figure below).

In funnel flow, an active flow channel forms above the outlet with non-flowing powder at the periphery. This is a first in-last out flow sequence. As the level of powder decreases, layers of non-flowing powder may or may not slide into the flowing channel, often resulting in the formation of a stable rathole. In addition, funnel flow can increase the extent to which segregation affects the discharging powder.

In mass flow, all of the powder is in motion whenever any is withdrawn, powder from the center as well as from the periphery moves toward the outlet. Mass flow provides a first in-first out flow sequence, eliminates stagnant powder, provides a steady discharge with a consistent bulk density, and yields a flow that is uniform and well controlled. Mass flow also reduces the extent to which some types of segregation affect the powder.

Factors affecting powder flow Factors affecting powder flow are related to those affecting particle adhesion and cohesion. These include; particle size shape Density porosity surface texture.

Powder flow is reduced as particle size decreases, due to the greater adhesion forces relative to gravitational forces. Generally, particles greater than 250 μm are free-flowing, particles smaller than 100 μm have reduced flow, and particles smaller than 10 μm are very cohesive and have poor flow under gravity, except as large agglomerates. The flow properties of such powders may be improved by removing the “fines” or adsorbing them onto added larger particles.

Particle shape influences powder flow due to the contact area between particles. Optimal flow properties are provided by spherical particles, which have minimal interparticulate contact. Irregular-shaped particles, such as flakes or dendrites, have high surface-to-volume ratios and poorer flow properties.

Particle density affects powder flow through its influence on the relative contributions of gravity and surface forces. Denser particles generally have better flow properties. Surface roughness affects powder flow due to its influence on particle adhesion. Rough particles have greater tendency for mechanical interlocking than particles with smooth surfaces. Moisture adsorption on the particle surface may improve powder flow by the combined effects of pore-filling, increased particle density, and lubrication to a certain level.

Angle of repose Angle of repose: Is the angle between the horizontal and the plane of contact between two bodies when the upper body is just about to slide over the lower; also known as angle of friction.

The angle is a measure of the cohesiveness of the powder, as it represents the point at which the interparticle attraction exceeds the gravitational pull on a particle. A free-flowing powder will form a cone with shallow sides, and hence a low angle of repose, while a cohesive powder will form a cone with steeper sides.

Measuring powder flowability A. Indirect method Calculation of angle of repose: if powder been allowed to pass through a funnel to a horizontal plane it will form a cone like shape that has a constant angle that called angle of repose which can be calculated by using the following equation.

B. Direct method Use of powder flow meter: flow rate is the time require for certain amount of powder to be passed through an orifice.

Procedure: Compare the flowability of substances like Flow sodium bicarbonate, sand. Use all of mentioned methods to test the flow.