Polyvinyl Alcohol (PVA): Film-Forming Mechanism and Moisture Barrier Performance in Film Coating Technology

1. General introduction to film-coated tablets:

Tablets are solid pharmaceutical products, with a definitive shape, each containing an exact amount of one or more active ingredients, formed by compressing a mass of powder or granules with or without excipients on a tablet press. Tablets are the most common dosage form, accounting for nearly two-thirds of pharmaceuticals circulating on the market. This popularity is primarily due to the convenience of administration and their suitability in formulation research, manufacturing, transportation, and storage, as well as patient compliance.

Film coating, also known as thin-film coating, is the process of covering the surface of a tablet core with a very thin layer of excipients. The composition of this film coating layer typically includes a film-forming agent (polymer), solvent, plasticizer, opacifier, colorant, and other excipients depending on the intended use. The film coating is applied to solid dosage forms for various purposes to maintain the physical and chemical integrity of the active ingredient, including enhancing drug stability by creating a physical barrier against environmental storage conditions (light, oxygen, or moisture).

2. The role of polymers in moisture-protective film coatings:

Depending on their physicochemical properties, different polymers will create barriers with varying effectiveness against moisture. In moisture-protective film coating, the type of polymer and its usage rate are selected based on the water resistance property provided by the polymer, as well as the hygroscopic nature of the active ingredient used.

Below is a summary table of some commonly used polymers in moisture-protective film coatings along with their respective usage rates.

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Depending on their physicochemical properties, different polymers will create barriers with varying effectiveness against moisture. In moisture-protective film coating, the type of polymer and its usage rate are selected based on the water resistance property provided by the polymer, as well as the hygroscopic nature of the active ingredient used. Below is a summary table of some commonly used polymers in moisture-protective film coatings along with their respective usage rates.

For moisture-protective film coatings, the coating thickness or theoretical weight gain must be determined to ensure the function of the film layer and must be determined experimentally. The thickness of the coating remains a critical factor regarding the moisture resistance of the film. Increasing the thickness helps prolong the disintegration time and improves the tensile strength of the tablet core. However, it must be ensured that increasing the coating weight (film thickness) will not increase the disintegration or dissolution time of the coated tablet. The theoretical weight gain for moisture-protective film coatings is typically 5%.

3. PVA and the mechanism of forming a moisture-protective film:

One of the common polymers for moisture-protective film coating purposes is PVA. PVA is a water-soluble synthetic polymer with a molecular weight ranging from 40,000 to 600,000 Daltons and is a non-toxic, thermally stable polymer. The proportion of PVA typically used is between 25% and 55% of the total solid mass of the coating dispersion.

The moisture-protective mechanism of the PVA film coating is that this polymer acts as a moisture barrier involving water absorption, followed by water retention via hydrogen bonding, thereby preventing the further penetration of water into the tablet core. This phenomenon is due to the higher degree of crystallinity of PVA compared to HPMC, which hinders the diffusion of water molecules through the film. Concurrently, the coating utilizing PVA also exhibits increased film adhesion compared to coatings utilizing cellulose derivatives. Conversely, with cellulose derivatives such as HPMC or HPC, moisture can penetrate rapidly and migrate deeper into the tablet, thereby increasing the risk of degradation of moisture-sensitive active pharmaceutical ingredients.

4. Comparison of PVA with some other moisture-protective film-coating polymers:

• Water vapor molecules are transmitted through the film in three steps:
• Water vapor molecules are adsorbed onto the film and accumulate on its surface.
• Water vapor molecules diffuse through the pathways within the film matrix.
• Water vapor molecules are desorbed from the film surface.

The moisture adsorption-desorption behavior of films is highly comp0lex, and different types of polymer films exhibit varying rates and degrees of adsorption-desorption.

When comparing the moisture-protective film-forming capabilities among 3 common polymers/polymer mixtures: HPMC, PVA, and the combination of HPMC + HPC:

• HPMC film demonstrates higher water vapor permeability because water molecules can interact with the hydrophilic groups within the film and act as plasticizers. Consequently, moisture can migrate deeper into the tablet core.
• Formulating HPMC in combination with HPC helps improve the adhesion of the coating film to the tablet core, reducing the voids between the core and the film, providing fewer pathways for moisture to penetrate deeply and affect the inner core.
• PVAprovides superior adhesion and potential advantages in film strength as well as moisture resistance due to the higher degree of crystallinity of PVA compared to HPMC, which obstructs the diffusion of water molecules.

Thus, it can be concluded that PVA creates a film coating layer with an effective moisture-protective mechanism, suitable for protecting moisture-sensitive active ingredients such as aspirin, clavulanic acid, acetylsalicylic acid, ranitidine, vitamin C, enalapril, herbal extracts, etc. Furthermore, the PVA film layer offers good mechanical strength, high stability, and does not interfere with the release of the active ingredient.