Film coating is a widely used process in the pharmaceutical, food, and chemical industries to apply a thin polymer-based layer to the surface of tablets, granules, or particles. The coating is primarily used to protect the core material, control the release of the active ingredient, improve aesthetics, or make the product easier to swallow. Below is a detailed explanation of the film coating process:


1. Purpose of Film Coating

  • Protection: The coating protects the tablet from moisture, light, or air, which can degrade the active ingredient.
  • Taste masking: Some drugs have a bitter taste, so the coating hides the unpleasant flavor.
  • Ease of swallowing: A smoother surface makes tablets easier to swallow.
  • Controlled release: Coatings can be designed to control when and where the active ingredient is released in the body (e.g., enteric coatings dissolve only in the intestines).
  • Aesthetic and branding: Coatings can give tablets a polished, colorful, and branded appearance.

2. Key Components of Film Coating

  • Polymer: Forms the base of the film coating, chosen based on whether the coating needs to be immediate-release, delayed-release, or sustained-release.
    • Immediate-release polymers: Hydroxypropyl methylcellulose (HPMC), hydroxypropyl cellulose (HPC).
    • Delayed-release (enteric) polymers: Eudragit, cellulose acetate phthalate (CAP), and hypromellose phthalate.
    • Sustained-release polymers: Ethylcellulose, certain methacrylate polymers.
  • Plasticizers: Enhance the flexibility and adhesion of the coating by making the polymer softer. Common examples include polyethylene glycol (PEG), triethyl citrate, and glycerin.
  • Colorants and pigments: Add color to improve appearance or for branding. Titanium dioxide is commonly used for white coatings, while iron oxides or natural colorants are used for other shades.
  • Solvents: Dissolve or disperse the polymers for spraying. They can be water (aqueous coatings) or organic solvents (organic coatings). Aqueous systems are preferred for safety and environmental reasons.

3. Types of Film Coating

  • Aqueous film coating: Uses water as the solvent, making it safer and more environmentally friendly. It is the most common type today.
  • Non-aqueous film coating: Uses organic solvents such as ethanol, methanol, or acetone. These can evaporate quickly but have health, safety, and environmental concerns.

4. Process Steps of Film Coating

a. Preparation of the Coating Solution

The coating solution or suspension is prepared by dissolving or dispersing the polymer, plasticizer, colorants, and other additives in the solvent (water or an organic solvent).

b. Loading of Tablets into the Coating Pan

Tablets (or particles) are placed into a specialized machine called a coating pan. This is a rotating drum that allows the tablets to tumble as the coating solution is sprayed onto them.

c. Spraying the Coating Solution

The coating solution is sprayed onto the tablets using atomizing nozzles. Atomization breaks the solution into fine droplets, ensuring a uniform coating layer. The process is typically done under controlled conditions of temperature, humidity, and airflow.

d. Drying

As the coating is applied, the solvent evaporates, leaving behind a thin polymer film. The drying process is crucial and typically occurs simultaneously with spraying. Hot air is blown into the coating pan to ensure the solvent evaporates quickly without affecting the core tablet properties.

e. Layer Formation

Multiple layers of coating may be applied depending on the desired thickness, functionality (e.g., delayed or extended release), and tablet size.

5. Equipment Used in Film Coating

  • Perforated Coating Pan: The most common equipment for film coating. It consists of a perforated drum that rotates, and the coating solution is sprayed while hot air is introduced to aid solvent evaporation.
  • Fluidized Bed Coater: Used for coating smaller particles or pellets. The particles are suspended in an airstream, and the coating solution is sprayed on them.
  • Spray Nozzle: Delivers the coating solution in fine droplets to ensure even coverage.

6. Critical Process Parameters

  • Spray Rate: The rate at which the coating solution is sprayed must be balanced with the drying capacity. Too fast can cause sticking of tablets; too slow may lead to uneven coating.
  • Air Temperature and Flow: These parameters are critical to drying the solvent and preventing over-drying or under-drying. High temperatures can cause defects like rough coatings, while low temperatures may lead to longer process times.
  • Pan Speed: The speed of rotation of the coating pan affects tablet tumbling, which influences how uniformly the coating is applied.
  • Humidity: High humidity can slow down the drying process, causing sticking of tablets, while too low humidity may lead to defects like surface cracking.

7. Common Defects in Film Coating

  • Picking and sticking: Occurs when tablets stick together or to the pan due to an overly wet or tacky coating.
  • Roughness: Caused by improper drying, leading to a non-smooth surface.
  • Cracking: Happens if the coating is too brittle due to incorrect formulation or drying.
  • Color variation: Caused by uneven spraying or inconsistent mixing of colorants.

8. Applications of Film Coating

  • Pharmaceuticals: For tablets to improve stability, control release, or mask taste.
  • Food supplements: Used to enhance the appearance and mask unpleasant tastes in vitamins and minerals.
  • Agriculture: Insecticides or fertilizers can be film-coated to control the release of active ingredients over time.

The film coating process is critical to ensuring that tablets and other products meet quality, efficacy, and safety standards, providing both functional and aesthetic improvements.