Tablet is a pharmaceutical solid dosage form, comprising a mixture of active substances and excipients, commonly in powder form, pressed or compacted right into a stable. After making a good tablet, you must often coat it. The coating can have several functions. It can strengthen the tablet, control its release, improve its taste, color it, and make it easier to handle and package, and protect it from moisture.
There are many ways to coat tablets. Sugar coating was one of the earliest methods, and the process is still widely used in the confectionery industry. Wurster coating is another means. It employs a cylindrical chamber in which tablets are suspended by air and a coating solution is introduced into the air stream. Fluid-bed coating is a similar process. Dry coating is the technique of making a tablet within a tablet. But the principle means of applying a coating to pharmaceutical and nutraceutical tablets is called film coating.
My description of tablet coating presumes you are coating high-quality tablets that are tough enough to tumble as they’re coated and dried. If tablet quality is consistent, the coating process is much easier. Consistency is typically not a problem for pharmaceutical manufacturers. It’s more of an issue for makers of vitamins, herbals, and other dietary
supplements, because they use many natural ingredients that vary in moisture content, bulk density, granule structure, flow characteristics, and compressibility. So naturally—pardon the pun— the quality of their tablets tends to vary. You can’t coat a bad or marginal tablet and expect a good tablet when you’re done.
First, the tablets must be consistent in porosity and hardness. They must also be free of dust. Furthermore, they must not break apart during the preheat cycle at the start of the coating process or during the first few minutes of exposure to the atomized solution.
Consistent hardness of the tablet surface enables the coating to “lock” into the surface. If the surface is too soft, the impingement of the solution can erode the tablet. Too hard a surface will not allow the solution to impinge and adhere, and the coating will peel away. Both of these coating defects can also occur by over- or under-applying the coating
solution or by applying the coating with too much or too little force. A combination of these factors could also be at work.
Here is a list of common defects associated with coated tablets and some likely causes.
Picking and sticking.
This is when the coating removes a piece of the tablet from the core. It is caused by over-wetting the tablets, by under-drying, or by poor tablet quality.
This occurs when the coating fills in the lettering or logo on the tablet and is typically caused by improper application of the solution, poor design of the tablet embossing, high coating viscosity, high percentage of solids in the solution, or improper atomization pressure.
This is when the tablet separates in laminar fashion. The problem stems from improper tablet compression, but it may not reveal itself until you start coating. How you operate the coating system, however, can exacerbate the problem. Be careful not to over-dry the tablets in the preheating stage. That can make the tablets brittle and promote capping.
This can be the result of soft tablets, an over-wetted tablet surface, inadequate drying, or lack of tablet surface strength.
This is the term for two tablets that stick together, and it’s a common problem with capsuleshaped tablets. Assuming you don’t wish to change the tablet shape, you can solve this problem by balancing the pan speed and spray rate. Try reducing the spray rate or increasing the pan speed. In some cases, it is necessary to modify the design of the tooling by very slightly changing the radius. The change is almost impossible to see, but it prevents the twinning problem.
Peeling and frosting.
This is a defect where the coating peels away from the tablet surface in a sheet. Peeling indicates that the coating solution did not lock into the tablet surface. This could be due to a defect in the coating solution, over-wetting, or high moisture content in the tablet core.
This is the result of high pan speed, a friable tablet core, or a coating solution that lacks a good plasticizer.
This can happen when the coating solution is improperly prepared, the actual spray rate differs from the target rate, the tablet cores are cold, or the drying rate is out of spec.
This refers to a coating texture that resembles the surface of an orange. It is usually the result of high atomization pressure in combination with spray rates that are too high.
Tablet coating checklist
Since spraying, coating distribution, and drying take place at the same time, tablet coating is a dynamic, complex process that is affected by many variables. In no particular order, here are some of the parameters that you should check when evaluating your coating operation to determine the source of defective coated tablets.
Many problems occur in coating when you can’t control every important parameter, such as temperature, pan pressure, spray rates, and atomization pressure.
As discussed earlier, the tablets must have the proper porosity, surface, hardness, and moisture content. You can’t have consistent coating without consistent tablet quality.
Most tablets cannot be coated immediately after they’ve been compressed. The energy within the tablets is still fairly high. In fact, they are still warm. In addition, tablet hardness changes over 24 to 48 hours. Let the tablets rest at least that long before you coat them.
Variation in batch size changes the required pan speed, gun geometry, spray rates, and temperature. The more your batch sizes vary, the more quality issues that will arise in the coating process.
Again, consistency is the name of the game. Does your company prepare coating solutions the same way, regardless of the batch, the shift, or the operator? Track the solution temperature, mixer speed, and storage time. All are important. Oh, and is the mixing blade correctly installed? Be sure by marking it “ top” and “bottom.”
Spray gun calibration.
You should calibrate or check the calibration of the guns every time you change products. This means checking the gun’s overall condition and its filter, nozzle alignment, and needle condition.
Geometry refers to the gun-to-gun alignment, gun-to-tablet bed alignment, and distance from the gun to the end of the pan. Use a ruler to be sure the distances are consistent. Furthermore, make sure all the guns are pointed in exactly the same direction and are maintaining the same spray pattern. Make certain that the tubing and connections are tight and do not interfere with alignment, which is a common problem.
The spray gun nozzles must be kept clean and free of product buildup. Use a flashlight during coating to look into the cabinet and check the nozzles.
While loading the tablets, look for tablets that are broken, capped, chipped, or covered with black specks. Doing so will help you pinpoint the source of any defects that occur. Do the defects appear during loading, during initial pan rotation, or after preheating? A visual inspection is critical when coating tablets that are friable or that chip or break easily.
Make sure you’ve cleaned and dried each component of the spraying system before re-installing it after a product changeover.
In tablet coating, small changes in almost any parameter can lead to big differences in results. The more consistent you make operations, and the tablet, the less you must rely on the skill of the operator. Coating may be something of an art, but you’ll get better results
when you apply a little science to it.