Many APIs are moisture-sensitive and thus need protection against ambient moisture. Usually, packaging materials are employed that provide a tight enclosure, but this adds costs to the already expensive manufacturing process. A more cost-effective alternative is to use well-formulated moisture protective coatings that reduce water vapour transmission rate (WVTR).
Physical characteristicsWVTR can be described using Fick's first law,2 which states that the amount of water vapour diffusing across a film barrier at a given time is a function of the following:
To prevent free diffusion of water molecules across the film membrane, water-insoluble additives, such as pigments, can be incorporated into the coating formulation.4,5
The effect of this is prolonged diffusion time and distance within the core, which helps protect against moisture. A crucial polymer characteristic for this method of moisture protection is the flexibility of the polymer film; it must allow the incorporation of large amounts of water-insoluble pigments without negatively affecting the film strength and flexibility.
Preparing the polymers
To compare the characteristics of different polymers, we first prepared 20% polymer solutions, which were then cast as films on a Coatmaster (Erichsen, Germany) The cast films were left to acclimatise at 23 °C/50% relative humidity (RH)6 for at least 48 h. Film strips with a 4 mm diameter were cut out using a manual punch, care being taken not to transfer moisture from the fingers to the film.
The cross section of the film was calculated based on the film thickness and width to calculate the strain. A self programmed Excel macro was then used to compute values obtained. The following elongation at break values were obtained for the polymers used: HPMC (3cP-grade: 3%; 6cP-grade: 11%), PVA (226%) and Kollicoat Protect (256%).6
Comparing the WVTR of different polymers
One method of determining the effectiveness of a moisture protective coating is by measuring the WVTR of the polymer films.
Moisture protection formulations based on HPMC, (incorporating stearic acid for moisture protection), PVA and Kollicoat Protect were prepared, and cast directly onto a PVC layer placed on a heatable metal plate. Typically, film coating processes are performed with an inlet air temperature of 50–80 °C resulting in product temperatures of 35–45 °C,6,7 so the cast films were dried at 45 °C.