Lubricants are the essential part of any tablet, capsule, or powder formulation. Their main function is to reduce the friction between the metal parts of machines such as tablet, capsule, or powder filler and the blend. Many theories proposed their mode of action. The theory that has greater experimental evidence is that lubricants get themselves deposited on the metallic surfaces that come in direct contact with the powder blend during tableting or capsule filling operation and reduce the friction between the metallic parts and the blend. Type of materials used as lubricants include fatty acids, metallic salts of fatty acids, fatty acid esters, and inorganic materials.
Types of Lubricants
Stearic acid is a straight-chain saturated monobasic acid with 18 carbon chain. It is manufactured from hydrolysis of animal fat or from hydrogenation of cottonseed or vegetable oil. Commercially available stearic acid is present as a mixture of stearic acid with palmitic and myristic acid. Depending on the ratio of stearic to palmitic acid, it can vary from macrocrystalline to microcrystalline structure. It is present in different polymorphic forms (forms A, B, and C), with form C being most stable.
Most of the commercially available magnesium stearates are crystalline. This includes anhydrate, dihydrate, and trihydrate forms. Anhydrous form is known to convert hydrates in the presence of high amount of moisture. However, commercially available magnesium stearates are generally a mixture of forms. Amorphous magnesium stearate is also commercially available. The amorphous magnesium stearate absorbs high amount of moisture and shows different sorption characteristics.
Role of lubricant
- True Lubricant Role: To decrease friction at the interface between a tablet’s surface and the die wall during ejection and reduce wear on punches & dies.
- Anti-adherent Role: Prevent sticking to punch faces or in the case of encapsulation, lubricants prevent sticking to machine dosators, tamping pins, etc
- Glidant Role: Enhance product flow by reducing interparticulate friction.
- Low Shear Strength- Want the lubricant to hear during blending, not the granulator other excipient in the formulation.
- Able to form a”durable layer” over the surface covered.
- Chemically inert
- Unaffected by process variables
- Prevent the adhesion of tablet.
- Reduce interparticulate friction.
- Improve the rate of flow of tablet granulation.
- High viscosity index eg. Vegetable oil.
- High resistance of detoriation in storage
- Highly stable eg. Temperature radiation.
- Some lubricants is the production of oil spots.eg steric acid salts and derivatives.
- Talc samples are found to contain trace quantities of iron.
- Poor self healling property.
- Poor heat dissipation eg. Polymers.
- Higher coefficient frication.