Introduction to tablet coating
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 which 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. Many solid pharmaceutical dosage mediums are produced with coatings, either on the external surface of tablets, or on materials dispensed within gelatin capsules.
Many tablets today are coated after being pressed or tabletted. Although sugar-coating was popular in the past, the process has many drawbacks. Modern tablet coatings are polymer and polysaccharide based, with plasticizers and pigments included. Tablet coatings must be stable and strong enough to survive the handling of the tablet, must not make tablets stick together during the coating process, and must follow the fine contours of embossed characters or logos on tablets. Coatings can also facilitate printing on tablets, if required. Coatings are necessary for tablets that have an unpleasant taste, and a smoother finish makes large tablets easier to swallow. Tablet coatings are also useful to extend the shelf-life of components that are sensitive to moisture or oxidation. Opaque materials like titanium dioxide can protect light sensitive active ingredients from photodegradation. Special coatings (for example with pearlescent effects) can enhance brand recognition.
If the active ingredient of a tablet is sensitive to acid, or is irritant to the stomach lining, an enteric coating can be used, which is resistant to stomach acid and dissolves in the high pH of the intestines. Enteric coatings are also used for medicines that can be negatively affected by taking a long time to reach the small intestine where they are absorbed. Coatings are often chosen to control the rate of dissolution of the drug in the gastro-intestinal tract. Some drugs will be absorbed better at different points in the digestive system. If the highest percentage of absorption of a drug takes place in the stomach, a coating that dissolves quickly and easily in acid will be selected. If the rate of absorption is best in the large intestine or colon, then a coating that is acid resistant and dissolves slowly would be used to ensure it reached that point before dispersing. The area of the gastro-intestinal tract with the best absorption for any particular drug is usually determined by clinical trials. This is the last stage in tablet formulation and it is done to protect the tablet from temperature and humidity constraints. It is also done to mask the taste, give it special characteristics, distinctions to the product, and prevent inadvertent contact with the drug substance. The most common forms of tablet coating are sugar coating and film coating.
Purpose of tablet coating
Protection of API from light, air and moisture- environment
Masking bitter or unpleasant taste
Increased mechanical stability during manufacture, packaging and shipment.
Protecting the API against the influence of digestive fluids
Ensuring controlled release of API
Avoidance of side effects
Increasing drug safety by better identification
In candy production, the core is converted to a luxury food. Since tooth decay and diabetes mellitus are nowadays a concern, coatings with low glucose content or non-diabetogenic sugar substitutes e.g. sorbitol and mannitol are preferred.
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 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 before it can jeopardize the integrity of the tablet.
General Coating defects
Many problems occur in coating when you can’t control every important parameter, such as temperature, pan pressure, spray rates, and atomization pressure.
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 capsule shaped 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
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.
A modern tablet coating system combines several components:
a coating pan,
a spraying system,
an air-handling unit,
a dust collector, and
The coating pan is actually a perforated drum that rotates within a cabinet.
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.
The Supercell Tablet Coater
Revolutionary tablet coater that accurately deposits controlled amounts of coating materials on tablets, even if they are extremely hygroscopic or friable.
Tablet coating technology in the pharmaceutical industry has remained fundamentally unchanged for the past 50 years. Until now, inconsistent and imperfect, this “standard” practice of tablet coating often delivers a non-homogenous product. Because the tablets are loaded in large rotating pans and vented for hot air drying, tablet edges can get grounded off,
intagliations can get filled in by coating material, and edges and corners may not be coated with the same thickness as the tablet faces. The inaccuracy of deposition of coating material limits the use of modified release coatings. In a laboratory setting, it is necessary to coat several kilograms of tablets at one time, making Research & Development of a tablet dosage form costly and difficult.
In addition, extremely hygroscopic tablets cannot be coated with current technology, nor can flat or other odd shapes be consistently coated. The process must be run slowly to prevent “twinning”, where two or more tablets stick together. Tablets may also be coated in a Wurster-type coating apparatus, but tablet attrition generally prevents all but the hardest tablets from being coated this way.
SUPERCELL™ Tablet Coater from Niro Pharma Systems effectively solves all of these problems using a small, modular design. The tablets are coated in batches ranging from 30 to 40 grams, which linearly scale up production capacities. The tablets are coated with the coating spray in the same direction as the drying gas, resulting in a more efficient process.
Coating Process Design & Control
Tablet coating takes place in a controlled atmosphere inside a perforated rotating drum. Angled baffles fitted into the drum and air flow inside the drum provide means of mixing the tablet bed. As a result, the tablets are lifted and turned from the sides into the centre of the drum, exposing each tablet surface to an even amount of deposited/sprayed coating.
The liquid spray coating is then dried onto the tablets by heated air drawn through the tablet bed from an inlet fan. The air flow is regulated for temperature and volume to provide controlled drying and extracting rates, and at the same time, maintaining the drum pressure slightly negative relative to the room in order to provide a completely isolated process atmosphere for the operator.
Tablet coating equipment may include:
blenders and mixers,
exhaust and heating pipes,
scales and filters.
Tablet coating process
Tablet coating processes may include sugar coating (any mixtures of purified water, cellulose derivatives, polyvinyl, gums and sugar) or film coating (purified water, cellulose derivatives).
The coating process is usually a batch driven task consisting of the following phases:
Batch identification and
Recipe selection (film or sugar coating)
Loading/Dispensing (accurate dosing of all required raw materials)
Spraying (application and rolling are carried out simultaneously)
Types of coating
Sugar coating – coating with sucrose and other sugars
Film coating – requires less materials- thin membranes
Press coating- compression coating
Hot melts- applied hot and solidify while cooling on the core. Mainly for confectionery. Fats e.g. cocoa butter, PEG, sugar/alcohols are used.
The most important are sugar coating and film coating.
Types of shapes and characteristics of cores
Cores for pharmaceutical dosages forms include:
Granules of various origins
Hard and soft gelatine capsules
Whether or not a core is suitable for coating depends on its hardness, shape, surface, size, heat sensitivity and tendency to interact with the coating material. Sugar and film coating processes place different demands on the cores to be used.
Common properties of core
All coating techniques require cores of adequate strength to withstand the rolling and frictional processes in the pan and the impact stress to which they are exposed in fluid-bed equipment. Coated cores must disintegrate rapidly enough in digestive fluids to ensure good bioavailability.
In sugar coating, the first step, sealing protects core against mechanical stress and binds small amount of fines, soft and brittle substrates can be sugar coated.
Film coatings call for harder abrasion resistant substrates that are less susceptible to impact stress because formation of a coherent film coating takes some time. The film must be
resistant to water/solvent to rule out swelling and softening of surface.
If tablets are used as cores, they must be bicomplex in shape to prevent them sticking together like coins in a roll.
Ideal tablets for sugar coating will have a pronounced convex curvature and a narrow band. The more the shape of the tablet resembles that of the finished coated product, the less material is needed for coating and the more uniform the layer thickness. Uneven/faulty cores can be used but more materials are needed to round it.
Film coating supplies coated products in which the core surfaces- notches, engravings, defects, are faithfully reproduced. Films tend to chip at sharp edges or are particularly thin in
these areas. Thus, slightly curved tablet cores are preferred for film coating.
This has major influence on build up of the first few coating layers. Sugar coatings can be applied to coarse and uneven surfaces- are relatively thick and can cover defects and irregularities. Film coating requires smooth and dust-free surfaces.
Cores from about 0.2 mm diameter upwards usually needed. Smaller diameter cores adhere. Envisaged application dictates upper limit of core size. The pharmaceutical dosage form must still be swallowable.
Process heat can have adverse effect on sensitivity of the active pharmaceutical ingredients or core excipients. When critical
temperatures are exceeded, it may result in:
- Drug decomposition
- Changes in drug release/dissolution
Interactions between core and coating
Hydrophilic excipients (disintegrants) cause the core to swell under influence of moisture. If moisture that has seeped into the core during storage migrates into the coating, crack and crevices develop and reduce stability of API. Swelling substances helps also in disintegration.
Substances migrating from the core into the coating may impair the aesthetic appeal of the coated product by mottling, discolouration of fading and blooming (i.e. substances migrate to the surface by sublimation or diffusion).
May be visualized as the traditional method of coating tablets. It involves the successive
application of sucrose based solutions to tablets cores in suitable coating equipment.
Stages involved in the production of sugar-coated tablets: is a multi-stage process.
Sealing of the tablet cores
Sugar coating is an aqueous process during which the tablet cores are thoroughly wetted by syrup application. A tablet sealant in therefore applied to protect the tablet core during this initial susceptible period from the action of water.
Tablet sealants are generally water-insoluble polymers or film formers applied from an
organic solution. E.g. shellac (best combined with PVP), cellulose acetate phthalate, polyvinyl acetate phthalate, acrylate polymers.
NB: Over application of sealants can lead to disintegration problems.
Sugar-coated tablets have a completely smooth profile with no visible edges remaining from the original tablet core. The sealed tablet core must be built up to gain the desired profile. Tablet cores for sugar-coating should have a small edge so as to not to make the rounding process more difficult than it need be.
Two methods are used for sub-coating
Application of a gum/sucrose solution followed by dusting with powder and then drying. This could be repeated to achieve desired shape.
Application of a suspension (high solid content) of dry powder in the gum/sucrose solution followed by drying. Repeat to achieve correct shape.
The solution is sucrose based plus gums like acacia, gelatine or starch derivative which aid filler powder adhesion e.g. CaCO3 or talc.
Sub-coating gives rough tablet surface. The surface is made perfectly smooth by successive application of dilute syrup. Dry after each application. Combination of syrup and acacia gum can also be used.
Colour coating and flavours
Nearly all sugar coated tablets are coloured. Approved colours are used and are of two
- Water soluble dyes, and
- Water insoluble pigments.
Water-soluble dyes used in traditional sugar-coating but great skill and patience required and process is prone to coating faults- poor and uneven coverage and batch variation in colour.
Modern sugar-coating- pigments such as aluminium lakes or insoluble iron oxides are used.
Are easier to use and permit comparatively fast colour coating times compared with soluble dye coating. Because of their covering power, pigments neutralize the influence of the core on the colour and prevent the core from showing through.
After colour coating comes polishing. The tablets receive one or two applications of a wax dissolved in organic solvent- beeswax or carnauba wax used. These serve to enhance reflections and intensify the colour.
Indented monograms on sugar-coated tablets not feasible. Identification accomplished by printing. Special edible printing inks are used.
Typically, tablets are sugar-coated by a panning technique. Traditional coating pan used. The coating syrup is applied by:
- Musing a ladle
- Automatic control
- Accela Cota- Manesty Machines,
- Liverpool England Hi-Coater- Freund Company,
- Japan Driacoater- Driam Metallprodukt GmbH German Pelegrini- Zanasi Nigtis,
- Italia IDA- Dumoulin, France
Ideal characteristics of sugar-coated tablets
- Tablets must comply with compendia requirement for the finished product
- Should be perfectly smooth rounded contour with even colour coverage- high gloss
- Any printing should be distinct with no smudging or broken points.
Problems encountered in sugar coating
- Cores crumble or erode during sub-coating- too soft.
- Tablet edges poorly coated and damaged- too much abrasion.
- Coated tablets stick to the pan.
- Sugar syrup remains soft and sticky on drying- sugar syrup inverted during
manufacture or storage- Splits into glucose and fructose.
- Particles on top of the sugar coating- uneven/more powder.
- Broken areas in the coating- fast drying.
- Grooves around coated tablets- too fast drying.
- Mottling- colour migration, colour coating on irregular substrate.
- Marbling at the edges- colour coating started on a poorly rounded core.
- Colour changes from batch to batch.
- Coated tablets are rough and uneven- too fast drying.
- Poor, non-uniform gloss- coated tablets not smooth.
- Coated tablets do not shine- coated tablets too warm & dry for the polishing process.
- Oversized coated tablets & wide weight distribution.
- Precipitation and flocculation of the coating solution.
More modern compared with sugar coating. Involves deposition, usually by spray-method, of a thin film of polymer surrounding the tablet core. The coating liquid/solution is sprayed on to a rotated, mixed tablet bed. The drying conditions permit the removal of the solvent so as to
leave a thin deposition of coating material around each tablet core.
Coating solution comprises:
Fundamental ingredient, most common are derivatives of cellulose e.g. HPMC, MC, EC.
Acrylate-based polymers are also used in plain film coatings. Others are available that can give gastric insoluble films or controlled release properties. Old polymers used include: shellac or CAP in combination with PEG.
A recent development involves introduction of fine particle suspensions of polymers e.g. ethyl cellulose as a single coating medium.
Usually polymers are dissolved in organic solvents. Modern techniques use water because of problems with organic solvents.
Added to modify the physical properties. Decrease film brittleness. E.g. PEG, propylene glycol, glycerol and its esters, and phthalate esters. Plasticizers enhance mobility of polymers chains- lower Tg, increase their flexibility, facilitate their distribution on the substrate and
improve film formation. Are of two types: External and internal plasticizers.
Plasticizers increase flexibility by pushing chain segments further apart or altering the average chain conformation through molecular effects.
Similar to sugar-coated tablets.
Vast majority of film coated tablets are produced by a spray process. Equipment could be as in sugar coating with slight modification- spraying of solution of polymers.
- Glatt AG Switzerland and Germany
These are fluidized bed equipment.
Ideal characteristics of film coated tablets
Even coverage of film and colour
No abrasion of tablet edges or crowns
Logos and break lines should be distinct and not filled in
Tablets must comply with finished product specifications and any relevant compendial requirements
Problems of film coating
Cracks in the film or along edges- too much internal stress: differences in thermal expansion of film and core.
Chipping-films do not adhere properly
Blistering- drying/spraying performed at high speed.
Bridging of break lines
Picking-formation of holes by non-molecularly dispersed substances.
particles- particles broken off from the core
Roughness extreme condition of dull surfaces
Orange peel effect- very pronounced unevenness of the film- too rapid drying
Twinning-cores stick permanently together like coins in a roll.
Differs from others. Involves the compaction of granular material around an already performed core using compressing equipment similar to that used for the core itself e.g. Manesty Drycota.
Granular coating contains a high proportion of sugar like conventional sugar coating.
This is a functional coating technique used to protect tablet core from disintegration in the acid environment of the stomach for the following reasons:
- Prevention of acid attack on API unstable at low pH.
- To protect the stomach from irritant effects of certain drugs.
- To facilitate absorption of a drug preferentially absorbed distally to the stomach.
Polymers used in enteric coating
Polyvinyl acetate phthalate
They exhibit a differential pH solubility profile. Are almost insoluble in aqueous media at low pH but dissolve at higher pH.
E.g. pH 5.2 for CAP. This is possible with both sugar and film coatings.
Enteric sugar coating
Sealing coat is modified to comprise one of the enteric polymers in sufficient quantity to pass the enteric test for disintegration.
Enteric film coating
Enteric polymers listed above can be used for film coating. Use sufficient weight to obtain enteric effects- about 2 to 3 times that required for a simple film coating.
Controlled release coatings
Functional coating. Achieved by film coating technique for the purpose of conferring a
controlled release of API. Polymers include:
- Modified acrylates
- Water-insoluble celluloses e.g. ethyl cellulose
Standards for coated tablets
For coated tablets requirements for uniformity of weight and diameter has been deleted from BP.
Disintegration test specifies a maximum time of 60 min in water but permits the test to be repeated in 0.1 M HCl should the tablet fails.
If the individual tablet monograph requires a dissolution test, then the disintegration test may be justifiably omitted.
Manufacturers should test uncoated cores prior to coating using specifications for uncoated tablets.
BP treats press coated tablets as for uncoated tablets for the purpose of compendia control.
BP disintegration for enteric coated tablets directs that 6 tablets should withstand 2 hr in 0.1 M HCl without disintegrating but on replacing the fluid with pH 6.8 phosphate buffer, all six tablets should disintegrate within 1 hr.
All divided dosage forms have to be tested for content uniformity.
Resistance to digestive fluids
Solvent residues/Residual solvents
Roughness – NIR imaging
tests- tinctometer to measure tinctorial strength.