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SC Publishing. Tablets & C

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tablet coating

Tablet coating basics Michael D. Tousey

After making a good tablet, you must often coat it. The coatingcan have several functions. It can strengthen the tablet, controlits release, improve its taste, color it, make it easier to handle andpackage, and protect it from moisture. This article reviews the basics of tablet coating and describes common tablet coatingdefects.

There are many ways to coat tablets. Sugar coatingwas one of the earliest methods, and the process is stillwidely used in the confectionery industry. Wurster coat-ing is another means. It employs a cylindrical chamberin which tablets are suspended by air and a coating solu-tion is introduced into the air stream. Fluid-bed coatingis a similar process. Dry coating is the technique of mak-ing a tablet within a tablet. But the principle means ofapplying a coating to pharmaceutical and nutraceutical

tablets is called film coating, and it is the focus of thisarticle.

Coating solutionsFilm coatings are a mixture of solids and liquids. For

many years, the liquid component of coatings was avolatile solvent, such as alcohol or other quick-dryingsubstances like methylene chloride. While solvent-basedcoatings performed well in many respects, they presentedproblems in handling, operator safety, recovery, and odor.They could even make the finished tablets smell like sol-vent, which is not a desirable side effect. Solvent-basedcoatings are still used in some applications, but water-based, 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 beforeit can jeopardize the integrity of the tablet.

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of the liquid component. It might be helpful for you tothink of film-coating tablets as spray-painting a bunch ofgolf balls. You can envision that it’s best to spray themlightly and evenly so that successive light coatings locktogether. That’s how tablet coating works.

Once the base coating is applied, you can increase therate of solution addition and the pan speed proportion-ately. Typically, it takes about 20 minutes before you canincrease the spray rate and pan speed significantly. Softtablets and tablets that are very porous may require aninitial spray rate that is slower than the average of 100milliliters per minute per gun. Be sure to monitor spray-ing to see whether the spray pattern changes. If it does,there is likely a buildup of solids on the gun tips. You cancorrect this only by cleaning the tips, which means stop-ping the spray and the pan. The images on page 20 showtablet coating spray nozzles being cleaned.

The film coating solution dries on the tablet surfacebecause there is a constant supply of hot air entering thedrum and passing through the drum’s perforations intothe bed of tablets. Over time, the film builds layer afterlayer of solids. How long it takes to form the final filmvaries from dozens of minutes to a few hours. It dependson tablet quality, the coating solution type (solvent-basedcoatings dry faster), the percentage of solids in the coat-ing, and the rate of coating addition. Other importantfactors include the air volume, air temperature, and theair pressure within the coating cabinet. After you’ve fin-ished applying the solution and drying it, the tablets mustcool.

For coatings to adhere properly, the tablets mustremain at a specific temperature, the solution must beapplied at a consistent rate, and the motion of the tabletsmust be active yet tranquil. Disrupt any of these condi-tions, and you will often produce a defective tablet. Forreproducible results, you have to eliminate or minimizeevery possible variable. That begins with tablet quality.

Tablet qualityMy description of tablet coating presumes you are

coating high-quality tablets that are tough enough totumble as they’re coated and dried. If tablet quality isconsistent, the coating process is much easier.Consistency is typically not a problem for pharmaceuticalmanufacturers. It’s more of an issue for makers of vita-mins, herbals, and other dietary supplements, becausethey use many natural ingredients that vary in moisturecontent, bulk density, granule structure, flow characteris-tics, and compressibility. So naturally—pardon the pun—the quality of their tablets tends to vary. You can’t coat abad or marginal tablet and expect a good tablet whenyou’re done.

First, the tablets must be consistent in porosity andhardness. They must also be free of dust. Furthermore,they must not break apart during the preheat cycle at thestart of the coating process or during the first few min-utes of exposure to the atomized solution.

Coating equipmentA modern tablet coating system combines several

components: a coating pan, a spraying system, an air-handling unit, a dust collector, and the controls. Thecoating pan is actually a perforated drum that rotateswithin a cabinet. See Figure 1. The cabinet enables youto control airflow, air temperature, air pressure, and thecoating application. The spraying system consists of sev-eral spray guns mounted on a manifold, a solution pump,a supply tank and mixer, and an air supply. The pumpdelivers the coating solution to the guns, where it com-bines with atomizing air to create a fine mist that isdirected at the bed of tablets in the coating pan. The airhandling unit heats and filters the air used to dry thecoating on the tablets. Depending on your circumstances,it may include a humidifier or dehumidifier. The dust col-lector extracts air from the coating pan and keeps aslightly negative pressure within the cabinet. The con-trols enable you to orchestrate the operation of all thecomponents to achieve the desired results.

Coating in actionOnce you load a batch of tablets into the coating pan,

you need to preheat the tablets and allow time for dustand tablet flash to exit the pan. Once the temperature ofthe outlet air reaches 42° to 46°C, usually within 15 min-utes, spraying can begin.

The spray guns create a fine mist of coating solutionthat dries just after it contacts the tablet. As the waterevaporates, it leaves the solids behind to form a thin filmon the tablet. The key to tablet coating is to get the sur-face slightly wet and immediately dry. Your objective isto apply the coating in many short, fast exposures, not inlong, slow exposures.

I was taught the three D’s of tablet coating: dose, dis-tribute, and dry. Dose is the exposure to the solution.Distribute is the fast motion of the tablets rubbing againstone another to transfer the solution. Dry is the removal

Figure 1

A simple diagram of a tablet coating system

Coating pan

InletTablet bed

Coating

Exhaust

Heated air-vapormixture in

Air-vapormixture out


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Tablet coating checklistSince spraying, coating distribution, and drying take

place at the same time, tablet coating is a dynamic, com-plex process that is affected by many variables. In no par-ticular order, here are some of the parameters that youshould check when evaluating your coating operation todetermine the source of defective coated tablets.

Control. Many problems occur in coating when youcan’t control every important parameter, such as tempera-ture, pan pressure, spray rates, and atomization pressure.

Consistent hardness of the tablet surface enables thecoating 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 impingeand adhere, and the coating will peel away. Both of thesecoating defects can also occur by over- or under-applyingthe coating solution or by applying the coating with toomuch or too little force. A combination of these factorscould also be at work. See the sidebar on defects onpage 22 and the accompanying photos.

Here is a list ofcommon defectsassociated withcoated tablets andsome likely causes.

Picking and stick-ing. This is when thecoating removes apiece of the tabletfrom the core. It iscaused by over-wet-ting the tablets, byunder-drying, or bypoor tablet quality.

Bridging. Thisoccurs when thecoating fills in thelettering or logo onthe tablet and is typi-cally caused byimproper applicationof the solution, poordesign of the tabletembossing, highcoating viscosity,high percentage ofsolids in the solution,or improper atomiza-tion pressure.

Capping. This iswhen the tablet separates in laminarfashion. The problem stems fromimproper tablet compression, but itmay not reveal itself until you startcoating. How you operate the coatingsystem, however, can exacerbate theproblem. Be careful not to over-drythe tablets in the preheating stage.That can make the tablets brittle andpromote capping.

Erosion. This can be the result ofsoft tablets, an over-wetted tablet sur-

face, inadequate drying, or lack oftablet surface strength.

Twinning. This is the term fortwo tablets that stick together, andit’s a common problem with capsule-shaped tablets. Assuming you don’twish to change the tablet shape, youcan solve this problem by balancingthe pan speed and spray rate. Tryreducing the spray rate or increasingthe pan speed. In some cases, it isnecessary to modify the design of

the tooling by veryslightly changingthe radius. Thechange is almostimpossible to see,but it prevents thetwinning problem.

Peeling and frost-ing. This is a defectwhere the coatingpeels away from thetablet surface in asheet. Peeling indi-cates that the coat-ing solution did notlock into the tabletsurface. This couldbe due to a defect inthe coating solution,over-wetting, orhigh moisture con-tent in the tabletcore.

Chipping. This isthe result of highpan speed, a friabletablet core, or acoating solution thatlacks a good plasti-cizer.

Mottled color. This can happenwhen the coating solution is improp-erly prepared, the actual spray ratediffers from the target rate, thetablet cores are cold, or the dryingrate is out of spec.

Orange peel. This refers to a coat-ing texture that resembles the surfaceof an orange. It is usually the result ofhigh atomization pressure in combina-tion with spray rates that are too high.

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This photo shows multiple defects. The initialproblem was erosion of the tablet edge due to asoft or friable tablet or because the pan wasturning too fast or both. Peeling and breakagealso appear here.

I attribute the peeling in this photo to excessivemoisture within the tablet, which prevented thecoating from adhering. However, the tabletcoating also pulled the granulation out of thetablet, a picking defect. That is usually causedby over-wetting the tablet or by a tablet that istoo soft.

Just one broken tablet can distribute particlesto all the other tablets and mar their appear-ance. These tablets likely broke because theyhad poor hardness.

This photo shows a very porous tablet that prevented the coating from adhering to the surface. These tablets should have beencoated longer, and the atomization pressureshould have been reduced to decrease theslight orange peel, or textured, surface.

Coating defects


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Tablet quality. As discussed earlier, the tablets musthave the proper porosity, surface, hardness, and moisturecontent. You can’t have consistent coating without con-sistent tablet quality.

Waiting period. Most tablets cannot be coated imme-diately after they’ve been compressed. The energy withinthe tablets is still fairly high. In fact, they are still warm. Inaddition, tablet hardness changes over 24 to 48 hours. Letthe tablets rest at least that long before you coat them.

Batch size. Variation in batch size changes therequired pan speed, gun geometry, spray rates, and tem-perature. The more your batch sizes vary, the more qual-ity issues that will arise in the coating process.

Solution preparation. Again, consistency is the nameof the game. Does your company prepare coating solu-tions the same way, regardless of the batch, the shift, orthe operator? Track the solution temperature, mixerspeed, and storage time. All are important. Oh, and is themixing blade correctly installed? Be sure by marking it“top” and “bottom.”

Spray gun calibration. You should calibrate or checkthe calibration of the guns every time you change prod-ucts. This means checking the gun’s overall condition andits filter, nozzle alignment, and needle condition.

Gun geometry. Geometry refers to the gun-to-gunalignment, gun-to-tablet bed alignment, and distancefrom the gun to the end of the pan. Use a ruler to be surethe distances are consistent. Furthermore, make sure allthe guns are pointed in exactly the same direction and aremaintaining the same spray pattern. Make certain thatthe 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 duringcoating to look into the cabinet and check the nozzles.

Pan loading. While loading the tablets, look fortablets that are broken, capped, chipped, or covered withblack specks. Doing so will help you pinpoint the sourceof any defects that occur. Do the defects appear duringloading, during initial pan rotation, or after preheating? Avisual inspection is critical when coating tablets that arefriable or that chip or break easily.

Cleaning. Make sure you’ve cleaned and dried eachcomponent of the spraying system before re-installing itafter a product changeover.

In tablet coating, small changes in almost any parame-ter can lead to big differences in results. The more consis-tent you make operations, and the tablet, the less youmust rely on the skill of the operator. Coating may besomething of an art, but you’ll get better results when youapply a little science to it. T&C

Michael D. Tousey is owner and technical services director ofDI Pharma Tech, 152 Wilkerson Dr., Westminster, SC29693. Tel. 864 647 5400, fax 864 647 1155. Website:www.dipharmatech.com. Tousey has been involved in thepharmaceutical industry since 1973, including work atThomas Engineering, Shaklee, Pennwalt Stokes-Merrill, andLakso. He founded his company in 1989. Tousey is a memberof the Editorial Advisory Board of Tablets & Capsules.


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