Lubricants

Lubricants work by reducing friction by interposing an intermediate layer between the tablet constituents and the die wall during compression and ejection and also between particle during compression.
Solid lubricants, act by two mechanism :
  1. Boundary mechanism, results from the adherence of the polar portions of molecules with long carbon chains to the metal surfaces to the die wall. Example : Magnesium stearate.
  2. Hydrodynamic mechanism i.e. fluid lubrication where two moving surfaces are separated by a finite and continuous layer of fluid lubricant.
Solid lubricants are more effective and more frequently used, because adherence of solid lubricants to the die wall is more than that of fluid lubricants

Since primarily lubricants are required to act at the tooling or material interface, lubricants should be incorporated in the final mixing step, after granulation is complete. When hydrophobic lubricants are added to a granulation, they form a coat around the individual particles (granules), which may cause an increase in the disintegration time and a decrease in the drug dissolution rate. Presence of lubricants may results in a less cohesive and mechanically weaker tablet because it may interfere with the particle – particle bonding (Lessen tensile strength).

Surface area is important parameter for deciding lubricant efficiency. Lubricants with high surface area are more sensitive to changes in mixing time than lubricant with low surface area. Therefore lubricant mixing time should be kept minimum. Mixing time in 1 batch production scale is about 3 minutes.

Tooling used to compress the tablet is important for deciding type and level of lubricant used. Additional lubricant is often added to the tablet formulations that are to be compressed with curved face punches.
The amount of lubricant increases as the particle size of the granulation decreases but its concentration should not exceed to 1% for producing maximum flow rate.

Lack of adequate lubrication produces binding which can results in tablet machine strain and can lead to damage of lower punch heads, lower cam track, die seats and the tooling itself. And it may also yield tablets with scratched edges and are often fractured at the top edges. With excessive binding the tablet may be cracked and fragmented by ejection.


Classification of Lubricants
Lubricant are classified (based on their water solubility) into two groups :

1. water insoluble
2. water soluble

Selection of lubricant is depends partly on :
  • mode of administration
  • type of tablet
  • disintegration and dissolution properties
  • physicochemical properties of granules or powder
  • cost 

Water Insoluble Lubricants

Water insoluble lubricants are most effective and used at reduced concentration than water soluble lubricants. Water insoluble lubricants effectiveness is related to :
  • surface area
  • extent of particle size reduction
  • Time
  • procedure of addition
  • length of mixing

Insoluble LubricantsConcentrationComments
Stearates(Magnesium Stearate, Calcium Stearate, Sodium stearate)0.25-1 Reduce tablet strength and prolong disintegration
Talc1-2 Insoluble but not hydrophobic, moderately effective.
Sterotex0.25-1-
Waxes1-5-
Stearowet1-5-
Glyceryl behapate (Compritol 888)1-5 Both lubricant and binder
Liquid paraffinUp to 5 Dispersion problem, inferior to stearates

Stearic acid and its calcium and magnesium salts are very effective lubricant. Magnesium stearate is very popular as a lubricant and is preferred to calcium stearate. Both of these compounds are alkaline innature and as such should not be used with acidic drug.

Water Soluble Lubricants

Water Soluble Lubricants are used when a tablet is completely soluble or when unique disintegration and dissolution characteristics are required. Tablet containing soluble lubricant shows higher dissolution rate than tablet with insoluble lubricants. Physical mixture of this lubricant are sodium lauryl sulfate or magnesium lauryl sulfate with stearates can lead to the best compromise in terms of lubricity, tablet strength and disintegration.

List Of Soluble Lubricants


Water Soluble LubricantsConcentration range (%w/w)
Boric acid1
Sodium benzoate5
Sodium oleate5
Sodium acetate5
Sodium lauryl sulfate (SLS)1-5
Magnesium lauryl sulfate (MLS)1-2

Antiadherents 

Some material have strong adhesive properties towards the metal of punches and dies or the tablet formulation containing excessive moisture which has tendency to result in picking and sticking problem. Therefore, antiadherents or anti-sticking agents prevent adhesion of the tablet surface to the die walls and the punches. Talc, magnesium stearate and corn starch have excellent antiadherent properties. Vegan had suggested that silicon oil can be used as antiadherent.

List Of Antiadherents

AntiadherentConcentration Range (%w/w)Comments
Talc1-5 Lubricant with excellent antiadherents properties
Corn starch3-10 Lubricant with excellent antiadherents properties
Colloidal silica0.1-0.5 Does not give satisfactory results due to small surface area. Cab-O-Sil® and Syloid®
DL-Leucine3-10 Water soluble lubricant; excellent antiadherents properties
Sodium Lauryl Sulfate less than 1 Antiadherents with water soluble lubricant
Stearatesless than 1 Antiadherents with water insoluble lubricant

Glidants 

Glidants are added to the formulation to improve the flow properties of the material which is to be fed into the die cavity and aid in particle rearrangement within the die during the early stages of compression. If the flow properties are extremely poor then glidants are ineffective and consideration of force free mechanisms may be necessary. The effect of glidants on the flow of the granules depends on the shape and size of the particle of the glidant and the the granule.

The commonly used glidants are talcum, starch, colloidal silica silicates, stearates calcium phosphate, etc. Starch is a popular glidant because it has additional value of disintegrant. Concentration of starch is common up to 10%, but should be limited otherwise it will worsen the flow of material. Talc is a glidant which is superior to starch; its concentration should be limited because it has retardant effect on dissolution-disintegration profile. Silaceous material like colloidal silica i.e. syloid, pyrogenic silica (0.25%), hydrated sodium silioaluminate (0.75%) are also successfully used to induce flow. Recently, silica type glidants are becoming popular due to their small particle size. Magnesium oxide and other magnesium salts are generally added as auxiliaries to silica type glidants where granules have hygroscopic inclinations. The magnesium compounds mop up the excess moisture keeping the granules dry and free flowing

Glidants act by interposing their particles between those of material and lower the overall interparticulate friction of the system by virtue of their reduced adhesive tendencies. Similar to lubricants, they are required at the surface of feed particles and they should be in fine state of division and appropriately incorporated in the mixture. As a general rule hydrophilic materials act better on hydrophilic granules and lipophilicones on the lipophilic granules. The glidants in a particular formulation ensure increasing flow of granules up to a certain optimum concentration. If the concentration of the glidant is taken beyond this, a drag action may come intooperation bringing down the rate of flow

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