Flow - Analyses of powders

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Chapter: Pharmaceutical Drugs and Dosage: Powders and granules

Flowability of a powder refers to its rate of passage, mass per unit time, through an aperture of given dimensions.

Flow - Analyses of powders

Flowability of a powder refers to its rate of passage, mass per unit time, through an aperture of given dimensions. Flowability can be assessed and expressed as either the lowest pore opening of a funnel through which a powder can flow well or the rate of flow of a powder through a defined pore opening of a funnel.

Importance of flowability of powders

Flowability of a powder is critical to most pharmaceutical unit operations. For example, adequate flow is important for ensuring

·           Mixing and blend homogeneity during blending of two or more powders.

·           Adequate control of dosage form weight variation during tablet and capsule filling unit operations.

·           Uniformity of roller compaction of the powder.

·           Transfer of powders between different unit operations through bins.

Factors influencing flow of powders

Powder flow is mainly influenced by particle shape, size, density, and size distribution. For example,

·           High aspect ratio and irregularity of particle shape can hinder smooth flow of particles. Needle-shaped crystals have poorer flow compared to sphere-shaped particles.

·           Powder blend with a large proportion of fines can lead to flow issues arising due to higher tendency for consolidation of powder blend. High proportion of fines can lead to the localized consolidation of powder bed, leading to stagnation, in a system requiring mass flow of the powder, such as a hopper.

·           For a given particle density, particle size is the primary determinant of gravitational and inertial force on the particles. Therefore, a powder bed consisting of very fine particles, even though they may possess a narrow size distribution, tends to have flow problems compared to a similar powder bed of coarse particles.

In addition, surface characteristics of powders such as electrostatic charge and surface roughness can increase interparticle cohesiveness, resulting in flow problems.

Quantitation of powder flow

Methods for the quantitation of powder flow are designed to simulate the large-scale manufacturing conditions. A typical flow test consists of passing a predetermined mass of powder through a small hopper, or funnel, with an aperture of known diameter and quantifying the time it takes for the powder to pass through the aperture with or without any agitation of the powder bed in the hopper. Powder flow is typically expressed in weight/time units, for example, g/s. Several commercially available equipments, for example, Erweka powder flow tester (Erweka GmbH, Germany), use this principle. A limitation of these techniques is the need for strict adherence to the experimental protocol for all the powder samples whose flow needs to be compared.

A more reliable, although indirect, technique that enables powder flow comparison irrespective of the sample size or testing equipment is the mea-surement of angle of repose. The angle of repose is the angle of the slope of a cone of powder, from the horizontal base, when the powder is made to fall on a horizontal surface in a uniform stream and allowed to settle undis-turbed. The ease of particles sliding over each other is a complex cumulative function of particle size, density, particle shape, and PSD. Lower angle of repose is indicative of ease of particle sliding across each other and inter-preted to indicate better flow characteristics of the powder.

Manipulation of flow properties of powders

Changing the particle size and shape can change flow properties of a powder or powder mixture. A coarse powder with low PSD and aspect ratio tends to flow better. Flow problems attributable to the consolida-tion characteristics of the powder, for example, high TD, can be altered by changing powder density.

In addition, flow problems that arise from electrostatic charging or cohesive nature of the particles often require surface modification of the particles. For example, the use of lubricant, such as magnesium stearate, can alter the surface characteristics of the powder by forming a hydro-phobic layer on particle surface.

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