Issue link: https://www.e-digitaleditions.com/i/1101841
Tablets & Capsules April 2019 17 ensure that the coating solution wetted the tablet surface enough to provide adequate film adhesion. These challenges became less important as coating sup- pliers introduced new types of polymers into their coating formulations and the sprayable solids content gradually increased to 15 to 18 percent w/w. Ongoing innovations have further increased the sprayable solids content, ini- tially to 20 to 25 percent w/w and, more recently, to 30 to 35 percent w/w. These developments have drastically reduced coating process times and facilitated the success- ful introduction of continuous coating processes. The data shown in Figure 1 illustrate how increased sprayable solids content can reduce processing time, in this case using a pilot-scale (40-kilogram batch size) O'Hara LabCoat IIX coater. The figure compares the per- formance of a standard HPMC-based coating formulation Still, early aqueous coating formulations were limited both in scope and in technical performance because they primarily used the same cellulosic polymers that had formed the backbone of the earlier organic-solvent-based formulations. However, innovations in formulation design during the early 1980s, supported first by the introduction of fully-formulated coatings and later by the expanded focus on non-cellulosic polymers (such as vinyl polymers), have enabled the industry to overcome the shortcomings of early aqueous formulations. Innovation in aqueous film-coating technology has continued, and aqueous coatings are available today that can produce high-quality film-coated products when sprayed at up to 35 percent solids (w/w) while significantly reducing pro- cessing cost and time. This article will show how modern coating formula- tions are successfully addressing industry trends, including: • High-solids coatings that can drastically reduce processing times and facilitate the use of continuous coating processes. • Specialized taste-masking coatings designed to meet the challenges imposed by pediatric dosage form development. Improving process economics and meeting the challenges of continuous coating processes As previously mentioned, early aqueous film coating formulations had many shortcomings, not least of which was the fact that the attainable sprayable solids content was limited to approximately 10 percent w/w. This low solids content, coupled with the reduced volatility of water compared to organic solvents, often resulted in extended processing times and increased stability issues for moisture-sensitive products. Another technical issue was related to the higher surface tension of water com- pared to organic solvents, which made it more difficult to Figure 2 Examples of tablets coated using an O'Hara LabCoat IIX coater fitted with a 30-inch pan a. Standard HPMC coating (Aquarius Prime, 12.5 percent solids) c. Ultra-high solids coating (Aquarius Genesis, 35 percent solids) b. Intermediate solids coating (Aquarius Preferred HSP, 20 percent solids) Figure 1 Impact of sprayable coating solids on coating process time using an O'Hara LabCoat IIX coater fitted with a 30-inch pan (40-kilogram charge) Standard HPMC Aquarius HSP Aquarius Genesis 140 120 100 80 60 40 20 0 40 30 20 10 0 Time (minutes) Solids (%)