Tablet Coating
Tablet coating is a critical process in pharmaceutical manufacturing that involves applying a layer of coating material to the surface of the tablet. This process can serve various purposes, including protecting the drug substance, masking taste, controlling drug release, and improving the tablet’s appearance.
Types of Coating
1. Sugar Coating
– Definition: A traditional method that involves multiple layers of sugar-based solution applied to tablets to improve taste, appearance, and protection.
– Advantages: Aesthetic appeal, taste masking.
– Disadvantages: Time-consuming, increases tablet size and weight.
2. Film Coating
– Definition: A thin, polymer-based coating applied to tablets to enhance appearance, protect from environmental factors, and control drug release.
– Advantages: Faster and more efficient than sugar coating, minimal weight increase, can control drug release.
– Disadvantages: Requires more sophisticated equipment.
3. Enteric Coating
– Definition: A type of coating that prevents tablet dissolution in the acidic environment of the stomach, allowing it to dissolve in the more neutral pH of the intestine.
– Purpose: Protects the drug from stomach acid, delays release until the tablet reaches the intestine.
– Advantages: Protects acid-sensitive drugs, prevents stomach irritation.
4. Controlled Release Coating
– Definition: Coating designed to release the active ingredient of the tablet over an extended period, providing a prolonged therapeutic effect.
– Purpose: Controls the rate of drug release over time.
– Advantages: Improves patient compliance, maintains steady drug levels in the bloodstream.
5. Functional Coating
– Definition: Coating that provides additional functionality, such as extended release, taste masking, or enhanced stability.
– Purpose: Can include protective, aesthetic, and functional properties such as extended release, taste masking, and stability enhancement.
Coating Materials
1. Polymers
– Definition: Long-chain molecules used to form the structural framework of the coating.
– Examples: Hydroxypropyl Methylcellulose (HPMC), Ethyl Cellulose, Cellulose Acetate Phthalate (CAP).
– Purpose: Provides the structural framework of the coating.
2. Plasticizers
– Definition: Additives that increase the flexibility and reduce the brittleness of the coating.
– Examples: Polyethylene glycol (PEG), triethyl citrate.
– Purpose: Improve flexibility and reduce brittleness of the film.
3. Colorants
– Definition: Substances used to impart color to the coating for aesthetic and identification purposes.
– Examples: Titanium dioxide, iron oxides.
– Purpose: Enhance aesthetic appeal and facilitate product identification.
4. Opacifiers
– Definition: Agents that increase the opacity of the coating to protect light-sensitive ingredients.
– Example: Titanium dioxide.
– Purpose: Increase opacity to protect light-sensitive drugs.
5. Solvents
– Definition: Liquids used to dissolve coating materials for application.
– Examples: Water, ethanol, isopropanol.
– Purpose: Dissolve the coating materials and facilitate application.
Formulation of Coating Composition
A typical coating formulation includes a polymer, plasticizer, colorant, opacifier, and solvent. The specific formulation depends on the desired coating properties, such as mechanical strength, flexibility, and release characteristics.
1. Polymer: Provides the structural framework of the coating.
2. Plasticizer: Improves film flexibility and adhesion.
3. Colorant: Adds color for identification and aesthetic appeal.
4. Opacifier: Increases opacity to protect light-sensitive components.
5. Solvent: Dissolves the other ingredients for application to the tablet.
Methods of Coating
1. Pan Coating
– Definition: A method where tablets are placed in a rotating pan, and the coating solution is sprayed onto them while they are mixed.
– Types: Conventional pan coating, perforated pan coating.
– Advantages: Suitable for large batches, relatively simple equipment.
2. Fluidized Bed Coating (Wurster Coating)
– Definition: A method where tablets are suspended in an air stream, and the coating solution is sprayed from below.
– Advantages: Uniform coating, efficient drying, suitable for both film and enteric coatings.
3. Compression Coating
– Definition: A method where a dry coating material is compressed around the tablet core.
– Advantages: Suitable for moisture-sensitive drugs, no need for solvents.
Equipment Employed in Coating
1. Coating Pans
– Definition: Machines used to mix and apply the coating solution to tablets.
– Conventional Pan: A simple rotating pan.
– Perforated Pan: A pan with perforations to allow air flow for drying.
2. Fluidized Bed Coaters
– Definition: Equipment that suspends tablets in an air stream and sprays the coating solution from below.
– Wurster Coater: Sprays the coating solution from the bottom.
3. Spray Guns and Nozzles
– Definition: Devices used to atomize the coating solution for uniform application.
– Purpose: To atomize the coating solution for uniform application.
4. Drying Systems
– Definition: Equipment used to remove the solvent and dry the coating.
– Purpose: To remove the solvent and dry the coating.
Defects in Coating
1. Blistering
– Definition: The formation of blisters or bubbles on the surface of the coated tablet.
– Cause: Rapid evaporation of solvent causing film to separate from the tablet surface.
– Solution: Optimize drying conditions.
2. Chipping
– Definition: The breaking off of small pieces from the edges of coated tablets.
– Cause: Insufficient adhesion of the coating, brittle film.
– Solution: Adjust plasticizer concentration.
3. Cracking
– Definition: The formation of cracks on the surface of the coated tablet.
– Cause: Over-dried tablets, high compression force during coating.
– Solution: Optimize drying process and compression force.
4. Color Variation
– Definition: Uneven distribution of color on the surface of coated tablets.
– Cause: Uneven mixing of colorant, poor coating application.
– Solution: Ensure uniform mixing, optimize spraying process.
5. Orange Peel Effect
– Definition: A rough, bumpy surface texture resembling an orange peel.
– Cause: High viscosity of coating solution, improper spraying technique.
– Solution: Adjust viscosity, optimize spray conditions.
6. Twinning
– Definition: The sticking together of two tablets during the coating process.
– Cause: Tablets sticking together during coating.
– Solution: Adjust pan speed, use anti-twinning agents.
7. Picking and Sticking
– Definition: The adherence of coating material to the equipment or the sticking of tablets to each other.
– Cause: Inadequate drying, overly adhesive coating solution.
– Solution: Optimize drying process, adjust formulation.
Conclusion
Tablet coating is a multifaceted process that enhances the functionality, stability, and appearance of tablets. Understanding the types of coatings, materials used, formulation of coating compositions, and the various methods and equipment employed is crucial for successful tablet manufacturing. By addressing common coating defects and optimizing the coating process, manufacturers can produce high-quality coated tablets that meet stringent pharmaceutical standards.
