The film-forming polymer is the most important constituent of a coating—literally, the glue that holds the coat together. Polymers usually constitute roughly 50–75% of the coat formulation and so the properties of the coating depend very heavily on the properties of the polymer.
This article briefly discusses 5 important polymer properties in film coating.
The solubility of a polymer in gastrointestinal fluids is of prime importance for coating, determining whether the polymer is appropriate for immediate-release, prolonged-release, or delayed-release applications. The polymer solubility in the coating vehicle determines whether the coating can be sprayed as a polymer solution with pigments suspended in it, or needs to be delivered as a latex or pseudolatex dispersion.
The permeability of polymer films is a key parameter for some prolonged-release products and also low permeability is vital for moisture protection and other barrier applications, although the intrinsic permeability may be reduced by additional formulation components to enhance barrier properties.
Viscosity of polymers in solution is important for coating, as it dictates whether the coating suspension can be sprayed reliably and also influences the appearance of the coated tablet. A lower viscosity coating suspension will spray more evenly and tend to give a smoother surface finish.
A rule of thumb for viscosity is that suspensions become difficult to spray reliably above 400–500 mpa s. Coating suspensions may well have viscosities around 100 mpa s or less, so it is important to ensure that the suspension is stirred continuously during coating, to prevent settling out of dispersed pigments.
Films with a high tensile strength are more resistant to cracking or splitting and generally provide more protection to the tablet than weaker films. Tensile strength and Young’s modulus increase with increasing molecular weight of polymer.
Aside from the obvious benefit of the film sticking to the tablet better, strong film adhesion means that the coat will adhere well into the debossing on a tablet and is less likely to show the phenomenon of logo bridging, where poor adhesion within the debossing causes the film to pop out and form a bridge across the logo, making it difficult or impossible to read. Poor adhesion in combination with low tensile strength can lead to split coats peeling back from the tablet surface.