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Pharmaceutical Lubricants - Types and Mechanism of Action

Pharmaceutical Lubricants

Lubricants are agents added in small quantities to tablet powder to decrease friction at the interface between a tablet’s surface and the die wall during ejection and reduce wear on punches and dies.
Pharmaceutical Lubricants

Properties of a Good Pharmaceutical Lubricants

1. Able to form a “durable layer” over the surface covered.
2. Non-Toxic.
3. Chemically Inert.
4. Unaffected by Process Variables.
5. Posses Minimal Adverse Effects on the Finished Dosage Form.

Types of Pharmaceutical Lubricants

There are two major types of lubricant:

1. Hydrophilic

Generally poor lubricants, no glidant or anti-adherent properties. E.g. Boric acid, Sodium Lauryl Sulfate, Sodium benzoate, Sodium oleate, etc.

2. Hydrophobic

The most widely used lubricants in use today are of the hydrophobic category. Hydrophobic lubricants are generally good lubricants and are usually effective at relatively low concentrations. Many also have both anti-adherent and glidant properties. For these reasons, hydrophobic lubricants are used much more frequently than hydrophilic compounds. E.g. Stearates(Magnesium Stearate, Calcium Stearate, Sodium stearate), Talc, Waxes, Liquid paraffin, etc.

Mechanism of Action

Lubrication is considered to occur by two mechanisms.

1.Fluid lubrication:

The first is termed fluid (or hydrodynamic) lubrication because the two moving surfaces are viewed as being separated by a nite and continuous layer of fluid lubricant. A hydrocarbon such as mineral oil, although a poor lubricant, is an example of a fluid type lubricant.

2.Boundary lubrication:

The second mechanism is that of boundary lubrication results from the adherence of the polar portions of molecules with long carbon chains to the metal surfaces of the die wall. Magnesium stearate is an example of a boundary lubricant.
Boundary-type lubricants are better than fluid-type lubricants since the adherence of a boundary lubricant to the die wall is greater than that of the fluid type. This is expected since the polar end of the boundary lubricant should adhere more tenaciously to the oxide metal surface than the nonpolar fluid type.

The concentration of lubricants to be used in the formulation:

Determining the level of lubricants to use and the manner in which they are incorporated into a batch is critical. If concentrations are too low, or distribution and mixing times are inadequate, problems can arise.
Some examples are as follows:
  • Punch filming.
  • Picking.
  • Sticking.
  • Capping.
  • Binding in the die cavity.
If concentrations are too high, or distribution and mixing times are too great, potential problems include:
  • A decrease in tablet hardness.
  • Inability to compress into tablets.
  • Increase in tablet disintegration times.
  • A decrease in the rate of dissolution.

Method of Addition:

Lubricants are generally added dry at a point where the other components are in a homogeneous state. The lubricant is added and mixed for a period of only 2 to 5 minutes rather than the 10 to 30 minutes necessary for the thorough mixing of a granulation. Over mixing may lead to diminished disintegration-dissolution characteristics and loss of bonding in the tablet matrix. The lubricant has also been added to granulations as alcoholic solutions (e.g., Carbowaxes) and as suspensions and emulsions of the lubricant material, As a rule, powdered lubricants should not be added prior to wet granulation since they will then be distributed throughout the granulation particles rather than concentrated on the granule surface where they operate. Powder lubricants added prior will reduce granulating agent and binder efficiency.