Film coating formulations usually contain the following components
1. Polymer
2. Plasticizer
3. Colorants / Opacifiers
4. Solvent / Vehicle
5. Flavours and sweeteners
6. Surfactants
7. Antioxidants
8. Antimicrobials/Preservatives


Polymers
A film former capable of producing smooth thin films reproducible under the prescribed coating conditions. Amongst the vast majority of the polymers used in film coating are cellulose derivatives or acrylic polymers and copolymers.


Polymers for film coating:

1. Immediate Release Coating Polymers

a. Cellulose derivates :
The most widely used of cellulosic polymers is HPMC (Hydroxypropyl Methyl Cellulose). It is readily soluble in aqueous media, forms film with good mechanical properties (strength, flexibility and adhesion to the tablet core). It also easy application of the coat. Other examples: MC (Methyl Cellulose) & HPC (Hydroxypropyl Cellulose)

b. Vinyl derivates :
The most widely used vinyl polymer derivate is PVP. It has a limited use in film coating because of its inherent tackiness. A copolymer of PVP and vinyl acetate forms better films.

2. Modified Release Coating Polymers
a. Extended Release Coating Polymers
They are dissolved in organic solvent or dispersed in aqueous medium. Cellulose derivates also often used. Cellulose derivates are highly substituted cellulosic ether, thus rendering the polymer water-insoluble, example : Ethyl Cellulose (EC).

b. Enteric Coating Polymers
1. Methacrylic Acid
The presence of carboxylic acid groups renders this class to be insoluble in water at low pH (stomach) but gradually becomes soluble as the pH rises towards neutrality (upper part of the small intestine). Example : Eudragit
2. Phthalate esters
Example : Cellulose acetate phthalate(CAP).


Plasticizers
Affords flexibility and elasticity to the coat and thus provide durability.
Plasticizers are simply relatively low molecular weight materials which have the capacity to alter the physical properties of the polymer to render it more useful in performing its function as a film-coating material.

It is generally considered to be mechanism of plasticizer molecules to interpose themselves between individual polymer strands thus breaking down polymer-polymer interactions. Thus polymer is converted in to more pliable materials. Plastisizers are classify in three groups. Polyos type contain glycerol, propylene glycol, PEG( Polyethylene glycol ). Organic esters contain phthalate esters, dibutyl sebacete, citrate esters, triacetin. Oils/glycerides contain castor oil, acetylated, monoglycerides, fractionated coconut oil.

Solvents/Vehicles
The key function of a solvent system is to dissolve or disperse the polymers and other additives. Volatile organic solvents may be used to Allow good spreadability of the coat components over the tablet and allowing rapid evaporation, but they are expensive and show environmental hazards and solvent residue in the formulation must be investigated (certain limit). Aqueous vehicles are safer, but they show slower evaporation and may affect drug stability.

All major manufactures of polymers for coating give basic physicochemical data on their polymers. These data are usually helpful to a formulator.

The major classes of solvents being used are
a. Water
b. Chlorinated hydrocarbons


Colourants / opacquants
Provides an elegant appearance. Ex.: Iron, oxide, pigment, Titanium dioxide and Aluminum lakes.
Identification of the product by the manufacturer and therefore act as an aid for existing GMP procedures.
- Reinforcement of brand imaging and reduction in product counterfeiting.
- Identification of the product by patients by using colourants.

Colorants for film coating are having, in more or less amount, property of opacifier. So they would give protection to active ingredients in presence of light. Colorants are mainly classified in to three part. Sunset yellow, tartrazine, erythrosine are examples of Organic dyes and their lakes. Iron oxide yellow, red and black, titanium dioxide, talc are the examples of Inorganic colours. Anthrocyanins, ribofloavine and carmine are the examples of natural colours.


Flavors and sweeteners are added to mask unpleasant odours or to develop the desired taste. For example, aspartame, various fruit spirits (organic solvent), water soluble pineapple flavour (aqueous solvent) etc.


Surfactants are supplementary to solubilize immiscible or insoluble ingredients in the coating. For example, Spans,Tweens etc.


Antioxidants are incorporated to stabilize a dye system to oxidation and colour change. For example oximes, phenols etc.


Antimicrobials/Preservatives are added to put off microbial growth in the coating composition. Some aqueous cellulosic coating solutions are mainly prone to microbial growth, and long-lasting storage of the coating composition should be avoided. For example alkylisothiazloinone, carbamates, benzothiazoles etc.

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