Aqueous Coating 101

Aqueous Tablet Coating 101


I. Introduction
What makes aqueous coating so dynamic? Once a we have a compressed tablet, we often need to add a coating. The coating can serve many purposes; it may make the tablet stronger and tougher, it may improve taste, it may add just a color, and may make the tablet easier to handle and package. The coating may be a thick sugar based coating or a very thin film. Most pharmaceutical tablets are coated with a thin film coating.

II. Before we Begin: A Short History of Tablet Coating
“Panning” was the original term for the process of adding a coating to a tablet. The term panning is still a common term used in the confectionary business. Ion years past coating began basically using a rotating drum (pan) on a stand. A coating solution was added using a ladle, while the rotation of the pan distributed the solution throughout the bed of tablets. This technology was slow waiting for the coating solution to dry; and the trick was to get it to dry evenly.
With the advent of film coating a film or thin membrane, usually representing 1-3% of the total tablet weight, was sprayed on using a perforated pan. To decrease the overall process time, holes where drilled through the pan so that treated air (hot or cold) could be pulled through the pan, much like a clothes dryer, allowing the tablets to dry more quickly. With this advent of improved drying came the ability to switch the film coating solution from a solvent based solution to a water based solution, thus the beginning of what we know today as aqueous film coating.

Many Pharmaceuticals manufacturers have moved to a water based solution replacing solvents. Exposing tablets to a water based solution presents a challenge in applying and quickly removing the water so it does not disrupt the integrity of the tablet. Tablet film coating equipment has evolved to enhance this drying capability.


III. Basic Aqueous Coating 101: The Basic Coating Process
Essentially a tablet coating system is much like a fancy clothes dryer. The water based solution is sprayed in a fine mist so as to dry almost immediately as it reaches the tablets. As the water dries it leaves the solids as a thin film on each tablet. The coating system continuously supplies hot air; at the same time it pulling air through small holes in the coating drum. Over time, the film builds into a layer of coating solids which is called the film coating. This process can take as little as 30 minutes or as much as several hours, depending upon the size and shape of the tablet and the size of the coating pan.
The length of time it takes to coat a batch of tablets depends on several variables: tablet quality, coating solution type (water or solvent), coating solution percentage of solids, rate of coating solution addition, the air volume and temperature control within the equipment and maintaining the proper balance of room to pan pressure. the tablets must be tough enough to tumble while the solution is added. The solution is distributed from tablet to tablet during the tumbling and drying process. The spraying, distribution and drying all takes place place at the same time. When you add still another factor that core tablet quality maybe somewhat variable, we have a collectively a condition that requires true operator skill and understanding.

Stated differently, vitamin or nutriceutical tablets having a high percentage of “natural” ingredients are more difficult to coat than products with a high percentage of excipients. Natural ingredients may have variations in moisture content, bulk density, granule structure, flow characteristics and compressibility. Therefore the consistency of tablet quality and its surface characteristics influence process setting to be used. The more inconsistent the surface of the tablet, the more likely the final tablet quality may not meet the desired quality level. Again, for this and other reasons, the tablet coating process is dynamic. There are various phyiscal actions taking place at the same time. Nevertheless, we must begin with a good core tablet in order to finish with a well coated tablet.

IV. Coating Equipment Used Today

Tablet coating equipment combines several technologies and is commonly referred to as a coating system. The system consists of a solution preparation tank, the air handling unit, the coating pan, the spraying system, the dust collector and a set of controls.

The solution preparation system usually consists of a solution preparation tank equipped with a mixing devise.
The air handling unit (AHU) is basically a way of heating and filtering the air. Dehumidification and humidification maybe be needed depending on your location and application requirements. This system is engineered to provide a consistent stream of treated air each time one uses the coating pan.

The coating pan is a perforated drum within a cabinet, allowing for control of air flow, air temperature and solution application.

The spraying system consists of a solution tank & mixer, a solution/suspension pump, a set of air assisted solution guns and air lines.
The solution is pumped into the guns and the air combines with the solution for atomization into a very fine mist.
The dust collector collects the dust during the preheat and tumbling cycles and the Controls connect all of the components creating a complete coating system.

V. The Eight Critical Process Variables in Aqueous Coating

There are eight critical process variables in aqueous film coating. A basic understanding of these variables is critical for successful tablet coating. The eight variables and a brief description of each follow:

(1) Gun Geometry: The placement of the guns inside the pan to obtain maximum exposure of the tablets to the solution spray without coating the pan surfaces.
(2) Atomizing/Pattern air: The compressed air used to convert a stream of solution into a pattern of small, flat droplets.
(3) Pan Pressure: The differential pressure between the pan and the room resulting in a slight negative pressure in the pan which in turn draws the coating solution into the bed of tablets.
(4) Pan Speed: The speed of the perforated rotating drum holding the tablets. The speed may be constant or variable throughout the length of the coating cycle.
(5) Spray Rate: Usually expressed as ml/min/gun or total quantity sprayed per minute, this is the amount of solution sprayed onto the tablets at any unit time.
(6) Inlet/Outlet Air Temperature: The temperature of the inlet air provided to the pan (a set point) and the temperature of the air after leaving the pan (a function).
(7) Total Air Volume: The quantity of air provided both to the pan and taken out of the pan.



(8) Adhesion of Particles to The Gun Surface: A non quantifiable operating parameter involving prevention of sprayed dried coating solution from adhering to the critical surfaces of the spraying gun(s).


VI. Basic Tablet Defects Seen in Aqueous Coating
There are several common defects commonly seen in the finished tablets as a result of non optimized or improper setting of the critical operating parameters. These are briefly described below.

Picking & Sticking are caused by over wetting of the tablets or by under drying.
Bridging or the filling in of lettering is caused by improper solution addition, poor tablet embossing design or high solution viscosity/percentage of solids.
Capping is often a problem that is created in tablet compression but not seen until the coating.
Erosion can be the result of soft tablets, over wetted surface, and inadequate drying and a lack of tablet surface strength.
Twinning is when to tablets stick together, a common problem with a caplet shaped tablet.
Peeling & Frosting is a defect where the coating peels away from the tablet surface in a sheet. The coating solution did not lock into the tablet surface. This can be from the coating solution, over wetting or from high moisture content within the tablet core.
Chipping can be caused by high pan speed, a friable tablet core or a poor coating solution.
Mottled Color is caused by improper solution preparation, spray rates, cold cores, and improper drying rates
Orange Peal is usually the result of high atomization pressure in combination with higher solution rates combined with tablets rubbing


VII. Putting It All Together: An Example of Coating A Typical Batch
After calibrating the spraying guns and loading a batch of tablets into the coating pan, we need to preheat and remove excess dust and tablet flash. Once the tablet bed reaches the 42°C- 46°C outlet temperature, the spraying process can begin. Soft tablets or tablets with high porosity may require an initial spray rate that is slower than the recommended average of 80-100 ml/minute/gun. Most tablets need an intial coating to protect them, and usually around the 20 minute mark the spray rate and pan speeds can be increased signiciantly. During coating it is important to monitor the spray pattern. Any changes in the pattern usually means buildup of solids on the tip of the gun and this must be corrected by stopping the spray addition and manually cleaning the guns. The operator must continuously monitor the pan/room magnehelic pressure gauge making certain that there is always a slight negative pressure. Once the spraying cycle is complete a cooling cycle is sometimes used to return the tablets to room temperature.

Automatic controls follow a menu that maintains most of the critcal paramters outlined above. But the computer cannot see everything. Gun geometry and particle build up on the spray guns will forever remain under the watchfull eye of the process operator.

IX. Summary: Aqueous film coating is a technically demanding, controllable process to seal core tablets with a clear or color coating. Understanding the eight critical operating parameters and the common defects encountered by using non optimized conditions or improper settings will greatly assist the manufacturer in achieving quality coated core tablets.