Film Coating of Tablets
Film coating has greater flexibility in allowing a range of products like tablets, granules, powder, and capsules that are to be coated.Film coating is continuously applied to a moving product (tablet), mainly by a spray atomization technique, although manual applications have also been used.
Factually, film coating was initiated to minimize the defects or limitations of the sugar coating process. Due to the many advantages film coating proved to be successful that includes;
1) In film coating of tablets very minimum weight of the tablet increased (generally 2% to 3% of tablet core weight).
2) The processing time also reduced significantly.
3) The output and process efficiency may also be increased.
4) Flexibility in formulations increased.
5) The resistance to chipping of the coating also improved.
In the early stages of film coating, the major processing advantage was the use of volatile organic solvents. Although there are a few disadvantages to using organic solvents that include:
1) Flammability of hazards
2) Toxicity of hazards
3) Environmental pollution is a big concern
4) The process is very costly.
Film coating has significant advances in equipment design and process technology. The less volatile and safer solvents like water can be easily settled by the process.
Film Coating Raw Materials
The major components of the film coating technique are mainly a polymer, colorants, plasticizers, and solvent or vehicle.The properties of polymers that contain solubility in a wide range of solvent systems enhance flexibility in the formulation, suitable mechanical strength as well as appropriate solubility in gastrointestinal fluids so that bioavailability can be achieved.
In the film coating process, the commonly used polymer is cellulose-based, hydroxypropyl methylcellulose which produced a thicker coating. Few other polymers like acrylic copolymers such as methacrylate and methyl methacrylate, and vinyl polymers such as polyvinyl alcohol, polyvinylpyrrolidone-vinyl acetate copolymer.
For the application of film coating in immediate release, the polymers are soluble in water and the solution is either used in water or organic solvents. Generally, water-soluble polymers are preferred.
The addition of plasticizer into the formulation improves the flexibility of the coating due to which the film cracking risk is reduced and also improves the adhesion of film forming the substrate (tablet).
The plasticizer shows excellent compatibility with the polymers and is retained permanently in the film. A few examples of plasticizers include propylene glycol, polyethylene glycols, glycerin, citrate esters, or phthalate esters.
Usually, the colorants are added to enhance the appearance of the products, and product identification and also improve the stability of the coated products.
The use of pigments, mainly aluminum lakes and inorganic pigments such as iron oxides and titanium dioxide are useful for coloring in film coating techniques.
However, the use of water-soluble dyes in the aqueous formulations, and the use of insoluble pigments are preferred because:
1) They are more stable.
2) They are opaque and can help to improve the stability of active pharmaceutical ingredients.
3) The permeability of the coating to moisture can also be reduced which improves the product stability.
4) They increase the solid content in the coating dispersion without increasing the viscosity and hence, act as bulking agents.
The solvents that are used in film coating include:
- Alcohol (Methanol, Ethanol, Isopropyl alcohol)
- Ketones (Acetone)
- Esters (Ethyl acetate)
- Chlorinated hydrocarbons (Dichloromethane)
- Water solvents
Modified-Release Film Coating
The film coatings are applied to the solid dosage forms (tablets) to modify the drug release (delayed-release and extended-release).Read More - Solid oral dosage Forms (Tablet or Capsules)
The delayed-release products are designed to prevent the release of drugs in the upper part of the gastrointestinal tract. The preparation of this type of dosage form is called enteric coatings.
The film coating that is required to extend the drug release over a long period (generally 6 to 24 hours) is commonly known as a controlled or sustained release film coating.
Enteric Coatings
Although the enteric coatings remain intact in the stomach they dissolved and release their active medicaments in the small intestine.The main purpose of enteric coatings is to delay the release of drugs that are inactivated by the stomach or cause irritation to the gastric mucosa.
Active pharmaceutical ingredients that are applied on the surface of enteric-coated tablets, the outermost layer of active pharmaceutical ingredients is immediately released on ingestion and the remaining drug will be released once the enteric coating has dissolved.
The oldest synthetic polymers that are used for enteric coating is cellulose acetate phthalate. The enteric coating polymers are insoluble in water (except at a pH >5).
Sustained-Release Coatings
To eliminate the need for multiple dose regimens, a sustained-release formulation was developed. It is adopted for those drugs that need to be administered in high doses with raid release.The required drug release rate from sustained-action dosage forms includes;
1) Put into the drug in a matrix.
2) Increasing the particle size of the drug.
3) Coating of dosage form that contains drug
4) Forming drug complexes with ingredients such as ion-exchange resins.
The ingredients that are suitable for preparing sustained-release coatings include:
1) The mixtures of waxes like beeswax, carnauba wax, etc. with glyceryl monostearate, palmitic acid, and stearic acid. It prepared coatings that dissolve slowly in the gastrointestinal tract.
2) The polymers like zein and shellac remain intact until the pH of gastrointestinal contents becomes less acidic.
3) Cellulose acetate acts as a semi-permeable membrane.
4) Ethylcellulose prepares a membrane around the dosage form and remains intact throughout the gastrointestinal tract.
Different methods have been used to prepare sustained-release products using film-coating techniques. Examples include:
1. Dried granules
2. Drug-loaded beads or nonpareils
3. Drug crystals
4. Drug or ion-exchange-resin complexes
5. Tablets
Film-Coating Problems
The tablets that are being coated may tend to laminate.As film coats are thin so it is not able to hide the defect. Hence, it exhibits high friability characteristics.
It is very important to identify the tablet's mechanical as well as performance-related properties before the coating process because reworking tablets may be very difficult after a coating has been applied.
Many process-related problems can occur during the application of a film coating like picking and sticking, orange peel or roughness, lack of color uniformity or mottling, cracking, and logo-bridging.
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