tablet coating

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, make it easier to handle and
package, and protect it from moisture. This article reviews the
basics of tablet coating and describes common tablet coating
defects.
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, and it is the focus of this
article.
Coating solutions
Film coatings are a mixture of solids and liquids. For
many years, the liquid component of coatings was a
volatile solvent, such as alcohol or other quick-drying
substances like methylene chloride. While solvent-based
coatings performed well in many respects, they presented
problems in handling, operator safety, recovery, and odor.
They could even make the finished tablets smell like solvent,
which is not a desirable side effect. Solvent-based
coatings are still used in some applications, but waterbased,
or aqueous, coatings have largely replaced them.
As a result, coating has become much more challenging,
because water-based coatings are much less forgiving.
You must apply the coating and remove the water before
it can jeopardize the integrity of the tablet.
of the liquid component. It might be helpful for you to
think of film-coating tablets as spray-painting a bunch of
golf balls. You can envision that it’s best to spray them
lightly and evenly so that successive light coatings lock
together. That’s how tablet coating works.
Once the base coating is applied, you can increase the
rate of solution addition and the pan speed proportionately.
Typically, it takes about 20 minutes before you can
increase the spray rate and pan speed significantly. Soft
tablets and tablets that are very porous may require an
initial spray rate that is slower than the average of 100
milliliters per minute per gun. Be sure to monitor spraying
to see whether the spray pattern changes. If it does,
there is likely a buildup of solids on the gun tips. You can
correct this only by cleaning the tips, which means stopping
the spray and the pan. The images on page 20 show
tablet coating spray nozzles being cleaned.
The film coating solution dries on the tablet surface
because there is a constant supply of hot air entering the
drum and passing through the drum’s perforations into
the bed of tablets. Over time, the film builds layer after
layer of solids. How long it takes to form the final film
varies from dozens of minutes to a few hours. It depends
on tablet quality, the coating solution type (solvent-based
coatings dry faster), the percentage of solids in the coating,
and the rate of coating addition. Other important
factors include the air volume, air temperature, and the
air pressure within the coating cabinet. After you’ve finished
applying the solution and drying it, the tablets must
cool.
For coatings to adhere properly, the tablets must
remain at a specific temperature, the solution must be
applied at a consistent rate, and the motion of the tablets
must be active yet tranquil. Disrupt any of these conditions,
and you will often produce a defective tablet. For
reproducible results, you have to eliminate or minimize
every possible variable. That begins with tablet quality.
Tablet quality
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.
Coating equipment
A modern tablet coating system combines several
components: a coating pan, a spraying system, an airhandling
unit, a dust collector, and the controls. The
coating pan is actually a perforated drum that rotates
within a cabinet. See Figure 1. The cabinet enables you
to control airflow, air temperature, air pressure, and the
coating application. The spraying system consists of several
spray guns mounted on a manifold, a solution pump,
a supply tank and mixer, and an air supply. The pump
delivers the coating solution to the guns, where it combines
with atomizing air to create a fine mist that is
directed at the bed of tablets in the coating pan. The air
handling unit heats and filters the air used to dry the
coating on the tablets. Depending on your circumstances,
it may include a humidifier or dehumidifier. The dust collector
extracts air from the coating pan and keeps a
slightly negative pressure within the cabinet. The controls
enable you to orchestrate the operation of all the
components to achieve the desired results.
Coating in action
Once you load a batch of tablets into the coating pan,
you need to preheat the tablets and allow time for dust
and tablet flash to exit the pan. Once the temperature of
the outlet air reaches 42° to 46°C, usually within 15 minutes,
spraying can begin.
The spray guns create a fine mist of coating solution
that dries just after it contacts the tablet. As the water
evaporates, it leaves the solids behind to form a thin film
on the tablet. The key to tablet coating is to get the surface
slightly wet and immediately dry. Your objective is
to apply the coating in many short, fast exposures, not in
long, slow exposures.
I was taught the three D’s of tablet coating: dose, distribute,
and dry. Dose is the exposure to the solution.
Distribute is the fast motion of the tablets rubbing against
one another to transfer the solution. Dry is the removal
Figure 1
A simple diagram of a tablet coating system
Coating pan
Inlet Tablet bed
Coating
Exhaust
Heated air-vapor
mixture in
Air-vapor
mixture out
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.
Control. Many problems occur in coating when you
can’t control every important parameter, such as temperature,
pan pressure, spray rates, and atomization pressure.
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. See the sidebar on defects on
page 22 and the accompanying photos.
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.
Bridging. 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.
Capping. 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.
Erosion. This can be the result of
soft tablets, an over-wetted tablet surface,
inadequate drying, or lack of
tablet surface strength.
Twinning. 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.
Chipping. This is
the result of high
pan speed, a friable
tablet core, or a
coating solution that
lacks a good plasticizer.
Mottled color. 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.
Orange peel. 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.
T&C
This photo shows multiple defects. The initial
problem was erosion of the tablet edge due to a
soft or friable tablet or because the pan was
turning too fast or both. Peeling and breakage
also appear here.
I attribute the peeling in this photo to excessive
moisture within the tablet, which prevented the
coating from adhering. However, the tablet
coating also pulled the granulation out of the
tablet, a picking defect. That is usually caused
by over-wetting the tablet or by a tablet that is
too soft.
Just one broken tablet can distribute particles
to all the other tablets and mar their appearance.
These tablets likely broke because they
had poor hardness.
This photo shows a very porous tablet that
prevented the coating from adhering to the
surface. These tablets should have been
coated longer, and the atomization pressure
should have been reduced to decrease the
slight orange peel, or textured, surface.
Coating defects
Tablet quality. 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.
Waiting period. 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.
Batch size. 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.
Solution preparation. 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.
Gun geometry. 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.
Gun nozzles. 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.
Pan loading. 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.
Cleaning. 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.