Issue link: https://www.e-digitaleditions.com/i/666166
16 April 2016 Tablets & Capsules There are many coating types and application meth- ods. This article focuses on the most common method, film coating, where a mist of solution (suspension) coa- lesces and adheres as a thin film on the surface of tablets. When done right, it's a very predictable and reliable sci- entific process. Why coat? Film coatings offer several benefits. A primary one is protection against erosion, chipping, and breakage. With a film coating, tablets are stronger and tougher, so they can withstand the routine buffeting of transport and packaging. Coatings also extend shelf-life, mask bitter tastes and off-putting odors, and add flavor and color. Functional coatings go even further. They can deliver active ingredients to accelerate the therapeutic effect, control the release of the active ingredient, and provide a barrier against moisture. Functional coatings that are pH- dependent, known as enteric coatings, allow tablets to pass through the highly acidic environment of the stom- ach unaltered and then disintegrate in the near-neutral intestines. Different surface, different hardness To minimize the art and maximize the science of film coating, reduce the variables. One of the biggest vari- ables is tablet hardness, specifically the surface hardness. No batch of tablets is more difficult to coat than the one with a wide range of hardnesses, because the coating will not interact with each tablet the same way. Even when all the tablets are within the specified "acceptable range," some will have a softer surface that erodes more easily. The tablets with harder surfaces may not allow the coating to adhere as well, which could lead to defects such as peeling. Identifying the variation in tablet surface hardness is difficult, however, because there is no instrument to measure it. (Traditional hardness testers actually quantify breaking strength, which tells us little to nothing about surface hardness.) In fact, we should say surface hardnesses, because the hardness of the top, bottom, and sides of a single tablet differ. We know tablet surface hardnesses are different because of how tablets are compressed and ejected. When a tablet is formed, the top is softer because air pushed upward gets trapped within the upper cup of the tooling; fines migrate to the vent area (gap) between the die wall and the upper punch tip. Because of the hardness differences, a coating may not adhere equally to all areas. Understand the substrate As discussed above, the top cap of a tablet is often softer than the lower cap because of how it's made. In addition, because the top bears a logo or other debossing, it's more prone to erosion than the tablet's bottom cap. As for the side (side band) of the tablet, the larger it is, the greater the friction and heat when the tablet is ejected from the die. To minimize friction, most tablets include lubricants, also known as mold-release agents. The lubricant creates a glossy surface on the tablet and it tends to get glossier as tablet hardness increases. Hard, glossy surfaces are not ideal for film coating. Incidentally, the larger the side band, the more difficult it is to apply an even coating all the way around the tablet. The tablet edges, formed by the land area of the punch tip, also warrant comment. They don't accept coatings as well as other areas. In fact, after finishing a batch, it's common to find tablets that aren't coated along the edge. Plus, that's where the granulation can extrude into the gap between the punch tips and die wall. Known as flash, wing, or crown, this excess material varies in size and shape according to the condition of the punches and dies, how they've been installed, the clearances, and their wear properties. To prevent the flash from causing defects, remove it using a deduster or during the preheat cycle before spraying begins. The tablet's constituents also affect the coating. Ideally, when a coating suspension is sprayed on tablets, the carrier (usually water, but sometimes a solvent) dries rapidly, leav- ing the solid portion to lock together into a "shell." But if some portion of the tablet's surface resists coating impinge- ment, variability increases. In terms of absorption proper- ties, some ingredients are like cotton t-shirts and others like raincoats. Because particles of each type can reach the A capped tablet. This reject should never enter the coating drum. Sometimes the heat of the coating process will cause acceptable tablets to cap. The erosion visible beneath the coating occurred during the preheat cycle, which should have been stopped. Tablets with a soft surface are nearly impossible to coat without creating defects.