Components Of Tablet Coating Suspension|Formulation of tablet coating

Tablet coating is a process in which a thin layer of polymer is deposited over a core tablet and the weight gain of the film may vary from 1-5 % for film coating to 10-12% for enteric coating. Sugarcoating is also a type of coating but in sugar coating instead of a polymer layer, we use a sucrose solution to form a layer of sugar over the core tablets. All of us are very well familiar with the basic types of tablet coating which are as follows,

• Sugarcoating
• Film coating
• Enteric coating
  

Sugarcoating

• Sugarcoating is a type of tablet coating in which a layer of sucrose or sugar is deposited over the core tablet.
• Sugarcoating is a multistage process and tablet weight may be increased up to 50-80% after coating.

Read more about sugar coating at the following link.

Sugar coating process and stages.

Film coating

• Film coating is an immediate release coating.
• Film coating is a single-stage coating process.
• Film coating is a type of tablet coating in which a polymer is used to form a layer over the tablet.
• In film coating concentration of polymer and other solid contents  used is 1-5% so the weight of the tablet is not increased as much as in the case of sugar coating.
• Film coating is done to protect the product from temperature, humidity etc and to mask the taste.

Read more about film coating in following,

Basics Of Tablet Coating.

Enteric coating

• Enteric coating is a type of coating which is used to protect the drugs which are degraded by the stomach acid.
• In this type of coating, a layer of polymer is deposited over the core tablet but the solubility of this polymer is pH-dependent so we use polymers that do not dissolve in an acidic environment  but in the small intestine.

           

Coating Suspension

To deposit the layer of polymer over the tablet core we prepare a suspension or dispersion of polymers and other excipients.

In this article, we will discuss different components of tablet coating which are as follow,

1. Polymer/Film former
2. Solvent
3. Plasticizer
4. Colourants
5. Opaquants
6. Miscellaneous
   

1.0 Polymer/Film former

• Film former or polymer is the main component of the coating suspension.
• Polymer is the ingredient that makes a film over a tablet so it is of great importance.
• Polymer is divided into following classes
• Cellulose Derivatives
• Vinyl Polymer
• Glycol Polymer
• Acrylic derivatives

Cellulose Derivatives

In cellulose derivatives, we have different derivatives of cellulose which are used as a film former.

Examples

• HPMC
• HPC
• EC
• CAP 

Vinyl Polymer

Vinyl polymers are also used as film former and a common example is PVP or polyvinyl pyrolidone.

Glycol Polymer

Glycol polymers are also used as film former and a common example is PEG or Polyethylene glycol.

Acrylic derivatives

• Acrylic derivatives are another important group of polymers used as film former.
• An example is eudragit L and Eudragit S.
• Eudragit L and Eudragit S are used to produce a film over tablets and films are resistant to the acidic pH of the stomach.
• Eudragit L and Eudragit S are soluble in the small intestine.
• Eudragit L and Eudragit S are available in the following forms
  • Powder Form
  • Concentrate in organic or aqueous Solvent

Properties of Polymer

A good polymer must have the following main properties,

• The polymer should easily be dissolved or suspended in the desired solvent.
• Polymer used should be resistant to high temperature and humidity.
• Polymer used during coating should not fill the logo or monograms mean it should not result in bridging.
• It should be colourless & tasteless.
• It should be cost-effective.
• It should be resistant to crack.
• It should give an elegant appearance to the product.
• Polymer used should produce a suspension of suitable viscosity.
• Polymer Should be compatible with the excipients used in core tablets.
• It should be compatible with the other ingredients used in the emulation.

Types Of Coating Based On Polymer

Based on the polymer used in coating suspension, the tablet coating is divided into two types

• Non-functional coating
• Functional coating
  

Non-functional coating

• The type of coating in which the polymer used does not affect the release of API or active pharmaceutical ingredient is known as non functional coating.
• Examples of polymer used for non-functional coating are 
• • HPMC
  • HPC

Functional Coating

• The type of coating in which the polymer used affects the release of API is known as a functional coating.
• In simple words, Polymer is used to alter or modify the release of drugs to either make the release of API  sustained or extended-release.
• Example of polymer used for functional coating are 
  • Eudragit 
  • CAP

2.0 Solvent

• The solvent is also the most important component of coating suspension.
• Solvents used for coating suspension  act as a carrier for transferring solid contents from suspension to the tablet surface.
• Solvent containing solid contents is sprayed by atomization on the tablet surface and  the solvent is evaporated by the heat and solid content deposited on the surface of the core tablets.
  

Properties Of Good Solvent

Solvent  used for coating suspension should have the following properties

• The used solvent should properly dissolve or suspend the solid contents.
• The selected solvent should have a rapid drying rate.
• The solvent used should be safe for the operator and the environment.
• The solvent used should be cost-effective.
• The viscosity of coating suspension should be such that the suspension  flows smoothly.

Types Of Coating Based On Solvent

Depending upon the solvent used coating is divided into following  three types

• Aqueous Coating
• Organic Coating 
• Hydro-Alcoholic Coating

Aqueous Coating 

• In Aqueous Coating the solvent used for preparation of coating suspension  is water.
• Water is economical and is safe for the environment and operators .
• In case of aqueous coating,evaporation of water requires more heat and great care to prevent product damage so more sophisticated equipment is used for aqueous coating.
• 12-15% coating dispersion is prepared for aqueous coating suspension for easy and smooth flow of coating suspension. 

Organic Coating Suspension

In  Organic Coating,organic solvents are used like 

• IPA
• Rectified Spirit

Organic solvents are costly and are also harmful to the environment and operator.

Evaporation of organic solvent requires less heat.

5-8 % coating dispersion is prepared for organic coating suspension.

Hydro-Alcoholic Coating Suspension

In hydro alcoholic coating suspension,the water and organic solvent both are used in a fixed ratio of 10:90 respectively or depending upon the formulation.

8-10% coating dispersion is prepared for Hydro-Alcoholic coating suspension.

3.0 Plasticizers

• Plasticizers are one of the important components of coating suspension.
• Plasticizers are low molecular weight materials and are used to increase the film-forming capacity of polymers.
• Polymers are long-chain molecules and plasticizers break the long-chain polymer-polymer interaction and rearrange them to enhance their film-forming capacity.
• Plasticizers decrease the glass transition temperature of polymers.
  

Practical Observation

• When we observe that the coating film is peeled from the surface of the tablet it is due to the fact that coating suspension does not have a plasticizer or the amount of plasticizer is low.

Classification of Plasticizer

The plasticizers are classified into the following three classes

Polyols

Examples are 

• Glycerine
• PEG
• Propylene Glycol

Organic Esters

Examples are 

•  Triacetin
• Dibutyl Sebacate
• Triethyl
• Acetyl Triethyl

Oils

Examples are 

•  Castor Oils
• Fractionated Coconut oil

4.0 Colourants

• Colour May be added  to the formulation depending upon the requirements.
• Colour gives an attractive and appealing look to the tablets.
• Different colours are used for different strengths of the same product to prevent mix up.
• FD&C or D&C type colours are used.
• For coating lakes and dyes are used.
• Dyes are water-soluble so when the heat is applied dye migration occurs and this results in colour variation.
• Lakes are water-insoluble so during drying they are resistant to migration and prevent the problem of colour variation.

Classification Of Colors

• Colours are classified into following classes 
• Organic dyes and their lakes
• Inorganic Colors
• Natural Colors
  

5.0 Opaquants

• Opaquants are inorganic materials and are also known as opaquants -extenders.
• Opaquants are used to mask the  colour of core tablets.
• Opaquants act as a base and are used to prepare an opaque background.
• If opaquants are not used then more colour quantity is required.
• Opaquants are less expensive so they are used so that colourants may use in less quantity.

Examples

Commonly used opaquants are

• Titanium dioxide
• Magnesium oxide
• Aluminium silicates
• Talc
• Miscellaneous

In miscellaneous we have the following categories

• Flavours
• Antioxidants
• Surfactants
• Antimicrobial

Flavours 

• Depending upon formulations flavours can be used in a coating suspension to give a specific flavour to the product.

Antioxidants

Antioxidants are added to prevent oxidation and to stabilize the dye system.

Surfactants

Surfactants may be added to the coating suspension to enhance the solubility of poorly soluble or insoluble ingredients.

Examples

• Spans
• Tweens

Antimicrobial preservatives

• Some coating suspension formulations may promote microbial growth so antimicrobial preservatives can be added to the coating formulation to enhance the stability of the coating suspension.

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