Another important but not unproblematic group of excipients for solid dosage forms are the lubricants. Their task is to reduce the adhesive forces between punches or die and the tablet to prevent sticking or damage to the tablet when being ejected.
Magnesium Stearate: The Gold Standard
The gold standard is magnesium stearate which has a greasy texture and is a salt of stearic acid. It has excellent lubrication properties but is almost insoluble in water. That’s why it’s not so popular for effervescent tablets as it forms a haze on the water surface. Additionally, sometimes show chemical incompatibilities with APIs.
Alternatives to Magnesium Stearate
Possible alternatives are stearic acid (or other fatty acids) or sodium stearyl fumarate which is at least somewhat water-soluble. There are also other substances used as lubricants like different oils or PEG types.
The Importance of Proper Lubricant Concentration
As lubricants not only reduce the adhesive forces between the stainless steel and the tablets but also within the tablet, they should be used in possibly low concentrations. Otherwise, the bonding capacity is reduced to such an extent that the formulation will no longer form stable tablets.
Adapting Lubricants to Material Behavior
Depending on the densification behavior of the compressed material, the amount of lubricant needs to be adapted.
In general:
- Brittle materials need more lubricant because they break during compression and form new fresh surfaces, which are not covered with lubricant.
- Plastic deforming substances, on the other hand, require significantly less lubricant, because no or fewer new surfaces are created. These types of substances therefore tend to overlubricate.