Tablet Coating
In previous articles, we have seen various excipients used in the tablets, types of tablets and evaluation parameters.
Click here to view article on excipients used in the formulation of tablets.
Click here to view article on evaluation parameters for the tablets.
Click here to view article on types of tablets.
In case of formulation of tablets, tablet coating is an important point. Purpose of tablet coating and the excipients and their role is important for the GPAT exam.
Click here to view article on excipients used in the formulation of tablets.
Click here to view article on evaluation parameters for the tablets.
Click here to view article on types of tablets.
In case of formulation of tablets, tablet coating is an important point. Purpose of tablet coating and the excipients and their role is important for the GPAT exam.
- Tablets are either film coated or sugar coated.
- In case of film coating, non-enteric coating or enteric coating excipients are used for the various purposes.
PURPOSE OF TABLET COATING
- To mask the taste, color or odor of the drug.
- To provide physical and chemical protection.
- To control the release of drug from tablet.
- To protect the drug from acidic environment of stomach using acid-resistant coating material.
A. Sugar coating
B. Film coating
I) Coating using non-enteric film formers
II) Coating using enteric film formers.
A. Sugar coating
Sugar coating is mainly carried out for the bitter tasting drugs. Those drugs need to be formulated in such a way that tablet will not release the slightest amount of drug in the oral cavity.
For this purpose small tablet is coated with sugar syrup and its bulk is increased to round up edges.
Sugar coating increases the thickness of tablet by 50-52%.
Following steps are involved in the sugar coating:
- Sealing: Sealing is done with water-proof polymers/materials. This step is important to avoid leakage of bitter tasting drug into outer sugar coating of the tablet. It is also important to protect the drug from moisture absorption and subsequent softening of the tablet. Examples : Shellac, Zein ( alcohol soluble protein derivative derived from corn)
- Sub-coating: It is carried out to build up the tablet size and to round up edges. Sub-coating increases the weight of tablet by 52%. This step consists of applying alternate layers of adhesive/sticky binder (gelatin/acacia) and sub-coating powders (talc/calcium carbonate) and then drying. This process is carried out for 3-5 times or untill desired thickness is achieved.
- Syrup coating: Syrup coating involves filling of surface imperfection and final coloring of tablet using sugar syrup. Multiple coats of syrup are given until tablet becomes smooth, then in final coat color is added in sugar syrup.
- Polishing: Polishing is done using powder wax (Carnauba wax or bees wax) or warm solutions of these waxes in volatile organic solvents are sprayed. Polishing causes smooth appearance of tablet.
B. Film Coating
Depending on the expected release profile of drug (rapid release, sustained release), protection of drug from acidic environment (enteric coating) and location of drug release (pH dependant), the material used for coating varies.
After film coating weight of the tablet increases by 2-6%.
I) Coating using non-enteric film formers.
These are mostly the cellulose derivatives.
HPMC (Hydroxy Propyl Methyl Cellulose): It is the most commonly used film former.
Depending on viscosity, different grades of HPMC are available among which lower grades are used for coating.
This material is used in combination with other film formers. When used alone it fills the uneven surfaces of tablets.
Ethyl cellulose:
This material is completely insoluble in water and gastrointestinal fluid. Its dissolution is pH independent.
For this purpose ethyl cellulose is not used alone for coating. It is used mainly in the delayed release/ sustained release dosage form along with water soluble material.
Povidone:
It is 1-vinyl 2-pyrrolidinone.
It also acts as binder and causes uniform dispersion of colorants in coating solution.
Depending on its viscosity there are 4 grades of povidone, viz., K15, K30, K45 & K60.
HPC (Hydroxy propyl cellulose):
It is soluble in water below 40°C and insoluble above temperature 45°C.
It produces extremely tacky film.
NaCMC (Sodium carboxymethyl cellulose):
It easily get dispersed in water and produces colloidal solution. It is insoluble in most of the organic solvents.
PEG (Polyethylene glycol):
Low molecular weight PEG (200-600): These are used as plasticizers for coating solutions and are liquid at room temperature.
PEG (MW 900-8000): These are waxy white solids at room temperature. These are used in combination with other polymers to modify the film properties.
Acrylate polymers:
Now-a-days these are commonly used for coating.
• Eudragit E - It is a cationic polymer. It is the only Eudragit material that is soluble in gastric fluid upto pH 5.
• Eudragit RL & RS - These show pH independent solubility and hence used in delayed release formulations.
II) Coating using enteric film formers
Enteric coating is done for several reasons like-
To protect the acid-labile drugs from decomposition in gastric fluid, e.g. antibiotics & enzymes.
To prevent the gastric discomfort, e.g. sodium salicylate.
To deliver the drugs intended for local action in intestine, e.g. drugs used in Irritable Bowel Syndrome.
The pH of stomach content/ gastric acid varies from 1.5 to 4.
Pylorus is the last part of stomach. Its pH is nearly 5. So, an ideal enteric polymer should become permeable or dissolve at or above pH 5.
At or above pH 5, polymers like CAP, HPMCP, polyvinyl acetate phthalate starts to loose their integrity due to ionization of carboxylic group on chain and subsequent hydration. Further, intestinal fluid contains esterase enzyme which causes breakdown of ester linkages in polymer chains. This process dissolves the polymer or improve its permeability causing drug release in the intestine.
CAP (Cellulose acetate phthalate):
This material is soluble above pH 6.
When used alone, it forms brittle and hygroscopic films. For this purpose, CAP is combined with hydrophobic material (diethyl phthalate) to prevent hygroscopicity and better enteric film Coating.
HPMCP (Hydroxy propyl methyl cellulose phthalate):
HPMC + phthalic anhydride --> HPMCP
It dissolves at pH 5-5.5.
Acrylate polymers:
Eudragit L- soluble at pH 6.
Eudragit S- soluble at pH 7.
SOLVENTS USED FOR COATING
Polymers and other additives are mixed in solvents prior to film coating.
To obtain the coating solution with proper viscosity, 2-10% of polymer is used.
Coating solution should not be too viscous (i.e. viscosity requirement is less than 300 cps).
The solution should have rapid drying rate.
Examples: Isopropanol, acetone, methanol, ethanol, methyl ethyl ketone.
When water is used as solvent, there is a problem of drug hydrolysis.
Hence, high viscosity coating solution is prepared or the drug is initially coated with non-aqueous based solvent coating.
PLASTICIZERS
Plasticizers in the range of 1-50% by weight of film former are used.
Common examples of plasticizers are PEG 200-400, castor oil and surfactants like polysorbates (Tween) & sorbitan esters (Spans).
For aqueous coating - PEG and PPG (polypropylene glycol).
For organic solvent based coating - Castor oil & Span.
COLORANTS
To achieve proper distribution of suspended colorants in the solvent, particle aizevof the dyes should be < 10 micron.
For this purpose ethyl cellulose is not used alone for coating. It is used mainly in the delayed release/ sustained release dosage form along with water soluble material.
Povidone:
It is 1-vinyl 2-pyrrolidinone.
It also acts as binder and causes uniform dispersion of colorants in coating solution.
Depending on its viscosity there are 4 grades of povidone, viz., K15, K30, K45 & K60.
HPC (Hydroxy propyl cellulose):
It is soluble in water below 40°C and insoluble above temperature 45°C.
It produces extremely tacky film.
NaCMC (Sodium carboxymethyl cellulose):
It easily get dispersed in water and produces colloidal solution. It is insoluble in most of the organic solvents.
PEG (Polyethylene glycol):
Low molecular weight PEG (200-600): These are used as plasticizers for coating solutions and are liquid at room temperature.
PEG (MW 900-8000): These are waxy white solids at room temperature. These are used in combination with other polymers to modify the film properties.
Acrylate polymers:
Now-a-days these are commonly used for coating.
• Eudragit E - It is a cationic polymer. It is the only Eudragit material that is soluble in gastric fluid upto pH 5.
• Eudragit RL & RS - These show pH independent solubility and hence used in delayed release formulations.
II) Coating using enteric film formers
Enteric coating is done for several reasons like-
To protect the acid-labile drugs from decomposition in gastric fluid, e.g. antibiotics & enzymes.
To prevent the gastric discomfort, e.g. sodium salicylate.
To deliver the drugs intended for local action in intestine, e.g. drugs used in Irritable Bowel Syndrome.
The pH of stomach content/ gastric acid varies from 1.5 to 4.
Pylorus is the last part of stomach. Its pH is nearly 5. So, an ideal enteric polymer should become permeable or dissolve at or above pH 5.
At or above pH 5, polymers like CAP, HPMCP, polyvinyl acetate phthalate starts to loose their integrity due to ionization of carboxylic group on chain and subsequent hydration. Further, intestinal fluid contains esterase enzyme which causes breakdown of ester linkages in polymer chains. This process dissolves the polymer or improve its permeability causing drug release in the intestine.
CAP (Cellulose acetate phthalate):
This material is soluble above pH 6.
When used alone, it forms brittle and hygroscopic films. For this purpose, CAP is combined with hydrophobic material (diethyl phthalate) to prevent hygroscopicity and better enteric film Coating.
HPMCP (Hydroxy propyl methyl cellulose phthalate):
HPMC + phthalic anhydride --> HPMCP
It dissolves at pH 5-5.5.
Acrylate polymers:
Eudragit L- soluble at pH 6.
Eudragit S- soluble at pH 7.
SOLVENTS USED FOR COATING
Polymers and other additives are mixed in solvents prior to film coating.
To obtain the coating solution with proper viscosity, 2-10% of polymer is used.
Coating solution should not be too viscous (i.e. viscosity requirement is less than 300 cps).
The solution should have rapid drying rate.
Examples: Isopropanol, acetone, methanol, ethanol, methyl ethyl ketone.
When water is used as solvent, there is a problem of drug hydrolysis.
Hence, high viscosity coating solution is prepared or the drug is initially coated with non-aqueous based solvent coating.
PLASTICIZERS
Plasticizers in the range of 1-50% by weight of film former are used.
Common examples of plasticizers are PEG 200-400, castor oil and surfactants like polysorbates (Tween) & sorbitan esters (Spans).
For aqueous coating - PEG and PPG (polypropylene glycol).
For organic solvent based coating - Castor oil & Span.
COLORANTS
To achieve proper distribution of suspended colorants in the solvent, particle aizevof the dyes should be < 10 micron.
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