Carr index

The Carr index (also: Carr's index or Carr's Compressibility Index) is an indication of the compressibility of a powder. It is named after the pharmacologist Charles Jelleff Carr (1910–2005). It measures the relative significance of interparticle interactions.

The Carr index is calculated by the formula $C=100{\frac {V_{T}-V_{B}}{V_{T}}}$, where $V_{B}$ is the volume that a given mass of power would occupy if let settled freely, and $V_{T}$ is the volume of the same mass of powder would occupy after "tapping down". It can also be expressed as $C=100\times (1-{\frac {\rho _{B}}{\rho _{T}}})$, where $\rho _{B}$ is the freely settled bulk density of the powder, and $\rho _{T}$ is the tapped bulk density of the powder.

The Carr index is frequently used in pharmaceutics as an indication of the flowability of a powder. In a free-flowing powder, the bulk density and tapped density would be close in value, therefore, the Carr index would be small. On the other hand, in a poor-flowing powder where there are greater interparticle interactions, the difference between the bulk and tapped density observed would be greater, therefore, the Carr index would be bigger. A Carr index greater than 25 is considered to be an indication of poor flowability, and below 15, of good flowability.

Another way to measure the flow of a powder is the Hausner ratio, which can be expressed as $H=\rho _{T}/\rho _{B}$.

Both the Hausner ratio and the Carr index are sometimes criticized, despite their relationships to flowability being established empirically, as not having a strong theoretical basis. Use of these measures persists, however, because the equipment required to perform the analysis is relatively cheap and the technique is easy to learn.

General Bibliography

Mark Gibson (2001). Pharmaceutical Preformulation and Formulation: A Practical Guide from Candidate Drug Selection to Commercial Dosage Form. Boca Raton: CRC Press. ISBN 1-57491-120-1.