Lubricant Types, Properties, Uses, Examples, Selection

This article covers lubricant types, uses, mechanisms of action, selection, limitations, quality tests, properties, and examples. Lubricants are additives that, when placed between two rubbing surfaces, reduce friction that arises at the interface^[1]. Lubricants may be Personal lubricants, or “Lube” or Pharmaceutical lubricants or Machinery lubricants. Here, we focus on pharmaceutical lubricants. Lubricants are crucial pharmaceutical excipients like binders, disintegrants, glidants and diluents etc. Also, lubricants known as Lubricating Agents. Glidant vs Lubricants

What Are Lubricants?

According to USP, Lubricants typically are used to lessen the frictional forces between particles and between particles and metal-contact surfaces of manufacturing equipment such as tablet punches and dies used in the manufacture of solid dosage forms. Before compaction, liquid lubricants may be absorbed by the granule matrix. Liquid lubricants can also be utilized on manufacturing equipment to lessen metal-metal friction ^[2].

In other words, Lubricating Agents are non-toxic, pharmacologically inert substances added to the formulation to reduce friction between the tablet’s surface and the die wall cavity in which the tablet was formed and to reduce wear and tear of dies and punches.

Lubricant Types

According to several features including their functional mechanism, physical form, solubility and chemical nature, lubricating agents can be categorized. Lubricant types are given below:

Lubricant types according to the functional mechanism

As per USP, Lubricants are three types:

a) Boundary lubricants: Boundary lubricants are long-chain fatty acid salts, such as magnesium stearate, or fatty acid esters, such as sodium stearyl fumarate, that have polar heads and fatty acid tails.

b) Fluid film lubricants: Fluid film lubricants are solid fats that melt under pressure such as hydrogenated vegetable oil, type 1, glycerides (glyceryl behenate and distearate), or fatty acids (stearic acid).

c) Liquid lubricants: Liquid lubricants are liquid constituents that are released from granules under pressure.

Lubricant Types based on solubility

There are two types of lubricants based on solubility

a) Water soluble (Hydrophilic) Lubricants: Generally, they are poor lubricants and they have no glidant or anti-adherent properties. For example, Boric acid, Sodium oleate, Sodium Benzoate, Sodium Lauryl Sulphate, Sodium Acetate,  Sodium Chloride and Magnesium Lauryl Sulphate etc.

b) Water insoluble (Hydrophobic or Lipophilic) Lubricants: Most widely used lubricants are water-insoluble. Usually, they are good lubricants and many of them have both glidant and anti-adherent properties. In addition, they are effective at relatively low concentrations. For example, Calcium stearate, Magnesium stearate, Sterotex, Talc, and Sterowet etc.

Lubricant Types as per chemical nature ^[4]

a) Metallic Salts of Fatty Acids: For example, zinc stearate, magnesium stearate, and calcium stearate are the metallic salts of stearic acids.

b) Fatty Acids: For example, Myristic Acid, Palmitic Acid, Stearic acid

c) Fatty Acid Esters: For example, Glyceride Esters (Glyceryl Monostearate, Glyceryl Tribehenate, and Glyceryl Dibehenate) And Sugar Esters (Sorbitan Monostearate and Sucrose Monopalmitate).

d) Inorganic Materials: such as talc (a Hydrated Magnesium Silicate)

e) Polymers: such as PEG 4000

Lubricant types based on their uses

Lubricants may be several types such as:

a) Personal lubricants, or “Lube”: specially used for intercourse.

b) Pharmaceutical lubricants: These are pharmaceutical excipients for drug product formulation.

c) Machinery lubricants: used for machinery, engines, and various mechanical systems.

Mechanisms of Lubricating Agents

1. Boundary lubricants work by adhering to solid surfaces (granules and machine parts) and minimizing friction between the particle-particle or the particle–metal. The orientation of the adherent lubricant particles is influenced by the properties of the substrate surface. The boundary lubricant particles should consist of tiny, plate-like crystals or stacks of plate-like crystals for best performance.

2. Fluid film lubricants firstly melt under pressure and thus produce a thin fluid film around particles and on the surface of punches and dies in tableting presses, which aids to reduce friction. Fluid film lubricants resolidify after the pressure is released.

3. Liquid lubricants are released from the granules under pressure and produce a fluid film. When the pressure is released, they do not resolidify but rather are gradually reabsorbed or redistributed through the tablet matrix.

Uses of Lubricants

1. Lubricating Agents often are used to reduce the frictional forces between particles and between particles and metal-contact surfaces of manufacturing tools such as tablet punches and dies used in the manufacture of solid dosage forms. Before compaction, liquid lubricants may be absorbed by the granule matrix.
2. Lubricating agents may prevent granules from adhering to the tooling and adhesion of the tablet to the die wall.
3. In addition, by lowering inter-particulate friction, the lubricant may enhance granule flow characteristics. Thus indirectly helps to prevent tablet weight variation.
4. Metal-metal friction can also be reduced on manufacturing equipment by using liquid lubricants.
5. Lubricating agents are used to ensure smooth compression. During compression, lubricating agents eradicate sticking and picking.
6. Accelerate the filling processes for dry powder or sachets.
7. Lower operational costs as a result of a smooth process.
8. Increase machinery effectiveness.

Selection of lubricants for a formulation

There are several factors to take into account while choosing a Lubricating Agent, including:

1. Compatibility: After a drug-excipients compatibility study, a lubricant should be selected if it is compatible with the active ingredient and other excipients in the formulation or with the tableting equipment and manufacturing process. For example, Mg Stearate is incompatible with Strong Acids, Iron Salts, Aspirin, Alkalis, Few vitamins. So in that case use other lubricants such as Sodium Lauryl Sulphate and magnesium Lauryl Sulphate.
2. Stability: A lubricant should be used if it has no effect on the drug product after considering the stability and bioavailability of the final drug product.
3. Concentration: Lubricants may cause tableting issues such as capping, lamination, hardness, friability, and disintegration time at greater concentrations. So, use alternate lubricants that require less to produce optimum lubrication.
4. Impact on coating: Some lubricants like lipophilic lubricants may affect the tablet coating. Thus, use alternate ones such as water-soluble lubricants.
5. If your lubricants affect the disintegration time and dissolution time, use hydrophobic lubricants for example Sodium Lauryl Sulphate, Magnesium Lauryl Sulphate.

Limitations of using lubricating agents

1. Blending time: Blending time should be optimum. Increased blending time or higher concentration of hydrophobic lubricants results in following tablet defects, prolonged disintegration time, retard dissolution, reduce compaction, low hardness, and tablet capping.
2. Lubricating agents work better when they have a larger surface area. Sieving increases the surface area of lubricants. Therefore, after sieving through a #40 mesh or higher mesh sieve, always use lubricants.
3. Do not combine lubricants with other excipients for sieving.
4. Add the lubricants and glidants after blending all other excipients.

Test for ensuring the quality of lubricants

1. Light Diffraction Measurement of Particle Size,
2. Characterization of Crystalline Solids by Microcalorimetry and Solution Calorimetry,
3. Characterization of Crystalline and Partially Crystalline Solids by X-Ray Powder Diffraction (XRPD).
4. Particle Size Distribution Estimation by Analytical Sieving,
5. Crystallinity,
6. Thermal Analysis,
7. Loss on Drying,
8. Optical Microscopy,
9. Specific Surface Area,
10. Water Determination.

What is the Mixing Time of Lubricants?

Usually, the Lubricating Agent mixing time is too short (2-5 minutes) during granulation. The lengthy mixing time of lubricant may decline the tablet’s hardness.

Lubrication Techniques

1. Manual Lubrication: suitable for small batches and lubricating into a polyethylene bag.
2. Automatic Lubrication Systems: for large-sized batches and lubricating into a blender like bin blender, drum blender, octagonal blender, or cone blender.

Properties of lubricants

Ideal Lubricating Agent should have the following characteristics:

1. More efficiency with less concentration.
2. Non-Toxic.
3. Chemically and pharmacologically inert.
4. Unaffected by manufacturing process variables.
5. Low shear strength.
6. Compatible with drugs and other excipients.
7. Increase flowability as well.
8. Reducing wear on rubbing surfaces.
9. High resistance to deterioration in storage.
10. Extremely stable.

Examples of lubricants ^[3]

Following are some examples of lubricants:

Note that, to reduce the friction of pharmaceutical machinery, engines, and various mechanical systems utilize machinery Lubricating Agents or lubricating oils.

Examples of machinery lubricants

1. Mineral oil
2. Solid lubricants: Polytetrafluoroethylene, Graphite, hexagonal boron nitride, molybdenum disulfide
3. Aqueous lubrication: PEG
4. Biolubricant: castor oil, palm oil, sunflower seed oil
5. Synthetic oils: Polyalpha-olefin (PAO), Polyalkylene glycols (PAG), Perfluoropolyether (PFPE), Alkylated naphthalenes (AN), Multiply alkylated cyclopentanes (MAC).

Additional information for you, Personal lubricant, commonly referred to as “lube,” is a liquid or gel used during intercourse to reduce friction.

In conclusion, Lubricants are the unsung heroes that keep our production running smoothly to reduce friction. From reducing friction to extending the lifespan of pharmaceutical mechanical components, their significance cannot overstate.

Keywords: Properties of Lubricants, Examples of Lubricants, List of Lubricants, Types of Lubricants, Uses of Lubricants, Classification of Lubricants, Lubricant Types, Lubricants examples.

References

1. Strickland, W.A., Jr., Higuchi, T. and Busse, L.W., The physics of tablet compression. J. Am. Pharm. Ass. 1960; 35-40

2. The United States pharmacopeia The National formulary. Rockville, Md.: United States Pharmacopeial Convention, Inc.

3. Rowe, R. C., Sheskey, P. J., Owen, S. C., & American Pharmacists Association. (2006). Handbook of pharmaceutical excipients. London: Pharmaceutical Press

4. Li, J., & Wu, Y. Lubricants in Pharmaceutical Solid Dosage Forms. Lubricants, 2(1), 21-43. https://doi.org/10.3390/lubricants2010021