Technical Newsletter
Tablets are solid pharmaceutical dosage forms with a defined shape, each containing an exact amount of one or more active ingredients. They are produced by compressing a powder or granulated blend, with or without excipients, using a tablet press. Tablets are among the most commonly used dosage forms, accounting for nearly two-thirds of all marketed pharmaceutical products. Their widespread use is primarily attributed to advantages such as ease of administration, suitability for formulation development, efficient manufacturing, convenient transportation and storage, and patient compliance.
Film coating, also known as thin-film coating, is the process of covering the surface of a tablet core with a thin layer of excipients. The film coating typically consists of a film-forming polymer, solvent, plasticizer, opacifier, colorant, and other excipients depending on the intended purpose. This coating is applied to solid dosage forms for various functions, including maintaining the physical and chemical integrity of the active ingredients. One of its key roles is to enhance the stability of the drug by forming a physical barrier that protects it from environmental storage conditions such as light, oxygen, or moisture.
Polymers are the primary components responsible for the structure and function of film coatings. Forming a moisture-protective film layer around solid tablet cores is a common approach to protect hygroscopic active pharmaceutical ingredients from exposure to ambient moisture. Moisture-barrier polymer coatings offer several advantages, including fast processing, automation potential, good mechanical film properties, minimal increase in tablet size or weight, and formulation flexibility for various coating purposes. Some commonly used polymers for moisture-protective film coatings are listed in the table below.
Polymer type | Common polymers |
Water soluble polymer | HPMC (Hydroxypropyl methylcellulose hoặc Hypromellose) |
HPC (Hydroxypropyl Cellulose) | |
PVP (Polyvinyl Pyrrolidone), | |
PVA (Polyvinyl Alcohol) | |
PVA–PEG copolymer | |
Water insoluble polymer | Polyvinyl acetate |
Ammonio methacrylate | |
EC (Ethyl cellulose) | |
Polymer soluble at intestinal pH | Shellac |
Methacrylic acid copolymer |
Depending on their physicochemical properties, different polymers provide varying levels of effectiveness in moisture barrier performance. In moisture-protective film coatings, the selection of polymer type and its concentration is based on the water resistance characteristics of the polymer as well as the hygroscopic nature of the active pharmaceutical ingredient. The table below summarizes several commonly used polymers in moisture-barrier film coatings along with their typical usage concentrations.
Polymer | Percentage in coating suspension/ solution | Solvent |
HPMC | 2 – 20% | Ethanol/Water |
HPC | ~ 5% | Ethanol/Water |
PVP | 0,5 – 5% | Ethanol/Water |
PVA | 20 - 55% | Water |
For moisture-protective film coatings, it is essential to determine the appropriate film thickness or theoretical weight gain to ensure the functionality of the coating layer, which must be established through experimental evaluation. Film thickness remains a critical factor in the moisture barrier performance of the coating. Increasing the film thickness can prolong disintegration time and improve the tensile strength of the tablet core. However, it is important to ensure that the increased coating weight (film thickness) does not adversely affect the tablet’s disintegration or dissolution time. A typical weight gain for moisture-protective film coatings is around 5%.
One of the commonly used polymers for moisture-protective film coatings is PVA. PVA is a synthetic, water-soluble polymer with a molecular weight ranging from 40,000 to 600,000 Daltons. It is a non-toxic, thermally stable polymer. The typical concentration of PVA used in film coatings ranges from 25% to 55% of the total solid content of the coating solution.
The moisture-barrier mechanism of PVA film coatings involves the polymer acting as a moisture barrier through a process of water absorption, followed by water retention via hydrogen bonding. This prevents the further ingress of moisture into the tablet core. This phenomenon occurs because the crystallinity of PVA is higher than that of HPMC, which helps to hinder the diffusion of water molecules through the film. Additionally, PVA-based coatings show increased adhesion compared to coatings made with cellulose derivatives. In contrast, cellulose derivatives like HPMC or HPC allow moisture to penetrate quickly and move deeper into the tablet, increasing the risk of degradation of moisture-sensitive active ingredients.
Water vapor molecules are transmitted through the film membrane in three steps:
The adsorption-desorption behavior of moisture in the membrane is very complex, and different polymer membranes exhibit varying rates and levels of adsorption-desorption.
When comparing the moisture-barrier film formation capabilities of three common polymers/polymer mixtures: HPMC, PVA, and the combination of HPMC + HPC:
HPMC membranes exhibit higher water vapor permeability because water molecules can interact with the hydrophilic groups within the membrane, acting as plasticizers. This allows moisture to penetrate deeper into the tablet core.
When combining HPMC with HPC, it improves the adhesion of the coating to the tablet core, reducing gaps between the core and the film, thereby limiting moisture diffusion into the tablet.
PVA provides better adhesion and offers potential advantages in film strength and moisture resistance due to its higher crystallinity compared to HPMC, which hinders the diffusion of water molecules.
Thus, PVA forms a highly effective moisture-protective film, making it suitable for protecting moisture-sensitive active ingredients such as aspirin, clavulanic acid, acetylsalicylic acid, ranitidine, vitamin C, enalapril, herbal extracts, etc. Furthermore, PVA-based film coatings offer good mechanical strength, high stability, and do not affect the release of active ingredients.
