Moisture is the silent antagonist of tablet stability, efficacy and credibility. For APIs that degrade, hydrolyze, or physically transform in its presence, the film coating on tablets is the primary line of defense.

Moisture barrier performance in film-coated tablets is one of the most technically demanding and least standardized areas of solid dosage form development. In this blog, we have explained the nuances of moisture barrier in film coated tablets. 

Why is moisture a threat to formulation? 

Water molecules interact with pharmaceutical APIs, especially hygroscopic and hydrolysis-prone compounds. It triggers hydrolytic degradation, reduces potency, and alters dissolution profiles. Let us understand a few of these reactions. 

• Hydrolysis: Water molecules in contact, attack susceptible chemical bonds such as esters, amides, lactams, and glycosidic linkages.This results in bond cleavage, which eventually degrades the tablet, reduces assay, and in many cases produces toxic impurities.

• Solid-state phase transformation: Many APIs exist in metastable polymorphic or amorphous forms that are bioavailability optimal but thermodynamically unstable. Water acts as a plasticizer. It reduces the glass transition temperature of amorphous matrices and accelerates recrystallization to lower-solubility polymorphs. The film coated tablet that passed dissolution testing during development may fail it three months into a stability study because moisture transformed its solid-state form.

• Physical tablet deterioration: Moisture absorbed by hygroscopic excipients softens tablet cores, compromises hardness and friability. In severe cases it also triggers capping, lamination, or complete physical disintegration. 

What Is a Moisture Barrier Film?

A moisture barrier film is a polymeric layer applied onto the tablet surface to reduce moisture vapor transmission rate (MVTR)i.e. the rate at which moisture permeates into the tablet core.

At the first point of contact with moisture, the tablet coating minimizes water adsorption at its surface. Then dense, semi-crystalline polymer chains impede diffusion and slow transit time. By the time water reaches the inner film surface, the flux is so reduced that moisture accumulation in the core is negligible.

Why not just apply more film coating?

If a thicker film means more path length for water to travel, why not simply increase coat weight? Film thickness does improve barrier performance, but only linearly, and only if the film is defect-free. 

For adequate moisture barrier quality, the film coating on tablets should exhibit several qualities such as adequate thickness and density, low permeability to water vapor, and high uniformity both in thickness and density. It is also important for the coating film to be stable during the dosage shelf life without any time-dependency or moisture stress-induced relaxation. 

Let us understand the science behind these tablet coating material:  

The Science Behind Barrier Performance

A film coating that genuinely functions as a moisture barrier is a precision-engineered coating. Every component selection and process parameter influences water vapor transmission rate (WVTR). Hence, each of them must be selected with precision. 

• Polymer science: Polymers used in moisture barrier tablet coating systems fall into three broad categories:

a) Water-soluble polymers such HPMC, PVA, HEC strike a balance between manufacturability and performance.

b) Water-insoluble polymers such as Ethyl cellulose, cellulose acetate are preferred when moisture protection is critical and release modification is acceptable.

c) Enteric / functional polymers such as methacrylate copolymers offer moisture protection along with enteric functionality.

• Additives: Additives significantly affect film properties such as flexibility, adhesion to the tablet core, and resistance to coating defects. 
• Process parameters: Coating process conditions such as spray rate, inlet air temperature, pan speed, and atomization pressure influence film formation at the microstructural level. 

Novo-Mix Moisture-Protective Coatings

Novo Excipients offers NOVOMIX MR, a trusted ready-mix film coating system designed to protect APIs from moisture while ensuring consistent performance. Novomix MR range is a well optimised coating formulation with the best combination of hydrophobic polymer and plasticizer. 

It is available in three specialized formulations to suit various product and process requirements:

• NOVOMIX MR (Organic & Hydroalcoholic based): For formulations requiring organic and hydroalcoholic solvents 
• NOVOMIX MR (AQ) (Aqueous-based): A enviroment friendly water-based high solids moisture barrier coating formulation 
• A highly customised Novomix MR coating formulation can be developed based on the customer’s specific project needs.

Conclusion: 

For moisture-sensitive APIs in film-coated tablets, the coating is where formulation success or failure is ultimately decided. An effective moisture barrier is not about increasing coating weight or switching polymers in isolation. It demands a well-balanced approach that considers both active and process needs. Think Moisture Barrier. Think NOVOMIX MR. Contact Novo Excipients today to learn more