Application of HPMC in film coating of tablets

In recent years, our company's research team has conducted trial production of Hydroxypropyl Methylcellulose (HPMC) as a film-coating material for tablets, and several products have already been launched in the market. This article introduces the application of HPMC in tablet film coating.

I. Properties of Hydroxypropyl Methylcellulose (HPMC)

Characteristics:

Appearance: White or off-white fibrous or granular powder, odorless.

Solubility: Swells in water to form a clear or slightly turbid colloidal solution. It remains stable in most electrolytes but is insoluble in anhydrous ethanol, ether, or chloroform.

Technical Data:

pH: 5–9

Viscosity: <50 centipoise

Loss on drying: <5%

Residue on ignition: <1.5%

Hydroxypropyl content: 3–12%

Methoxyl content: 19–30%

Heavy metals (as Pb): <0.002%

Arsenic (as As): <0.0002%

Water-insoluble substances: ≤0.5%

Total bacterial count: <1000 CFU/g

Total mold count: <100 CFU/g

Escherichia coli: Absent

Features:

1.Non-toxic and free of side effects.

2.Soluble in water and water-organic solvent mixtures.

3.Forms a tough, elastic film that is colorless, odorless, and transparent in solution.

4.Stable properties, with a longer storage life than sugar-coated tablets. The solution can mix with various colorants and is resistant to temperature and moisture.

5.Gastric and intestinal fluids can penetrate the film pores, allowing drug dissolution.

6.HPMC swells in water and exhibits higher activity under slightly alkaline conditions, providing excellent isolation for tablet and granule coating and extended storage life.

II. Basic Production Process of HPMC Film Coating

For batches production of tablet film coating, a 1000 mm diameter coating pan is used, with an imported high-efficiency coating pan being more suitable. A standard coating pan can process 50 kg per batch.

HPMC material is soaked in water for 24 hours to prepare a 2–4% (w/v) aqueous solution (based on dry weight). After complete dissolution, the solution is filtered through a 12-mesh sieve.

The compressed solution (air pressure: 4 kg/cm²) is sprayed through a 2 mm nozzle onto the moving tablets in a mist form. The spraying volume is 30,000 mL, and the coating pan rotates at 25 rpm. The rotation speed can be adjusted based on tablet properties and hot air volume until spraying is complete, forming the film.

III. Basic Formulation and Operation of Film Coating

Through multi-variety, large-scale production practices, the process of using HPMC for film-coated tablets has become increasingly mature.

Common Formulation:

Includes 2–4% HPMC solution, talc, iron oxide powder, cocoa powder, and additives such as castor oil, propylene glycol, Tween-80, and diethyl phthalate. Additives provide lubrication, acceleration, gloss, plasticization, and solubilization.

Solution Preparation:

Add the required amount of iron oxide, cocoa powder, or talc to the HPMC solution, mix evenly, and grind through an 80-mesh sieve. Add colorants and stir until uniform.

Process Operation:

Weigh the tablet cores and place them in the coating pan. Preheat with hot air for about 10 minutes, then rotate the pan to heat the cores to 50°C.

Turn on the air compressor (4 kg/cm²) and spray the solution onto the moving tablets in a mist form while continuously blowing hot air for rapid drying until a uniform and glossy film is formed. Cool and store in a silica gel-controlled environment before packaging.

Double-Layer Film Coating:

For highly hygroscopic drugs, a double-layer coating can be applied. First, a protective layer is formed on the tablet core and dried at high temperature, followed by the outer HPMC layer.

The protective layer can use a 2% acrylic resin II aqueous-alcoholic solution. Preheat the tablet cores for 10 minutes, then spray the resin solution to form the protective layer. The second layer is the HPMC coating.

Accelerated tests show that double-layer film-coated tablets have better moisture resistance than those coated with HPMC alone. Drug diffusion tests indicate slower dissolution rates for double-layer film-coated tablets, but this does not affect their usability.

For example, when "21" Golden Vitamin double-layer film-coated tablets were soaked in 1000 mL of water (25°C) for 1.5 hours at room temperature, the drug was almost completely dissolved, leaving empty shells floating in the water. In contrast, tablets coated with HPMC alone dissolved completely in 45 minutes, leaving empty shells.

Summary

Based on our production experience with HPMC film coating, we have the following insights:

Film-coating materials consist of a water-insoluble polymer and a dispersed water-soluble pore-forming substance. When the coating swells in gastrointestinal fluids, the water-soluble substance dissolves, allowing the drug to diffuse through the porous coating. Thus, film-coated tablets have controlled-release functionality.

Tablets coated with HPMC exhibit comparable appearance, hardness, disintegration, and gloss to similar products from Germany.

HPMC can be applied using water or water-ethanol mixed solvents, reducing the sticking issues common in sugar coating.

For tablets with different properties, a double-layer coating or mixed-film process can improve film performance.