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112Pharmaceutical TabletCoating112.1 History .......................................................................... 112-1

112.2 Reasons for Coating Tablets........................................ 112-1112.3 Types of Coating.......................................................... 112-2

Bibliography .............................................................................. 112-3

112.1 History

The coating of solid pharmaceutical dosage forms began in the ninth century B.C., with the Egyptians.

At that time the primary solid dosage form was the pill, a hand-shaped spherical mass containing drug,

sugar, and other diluents. A variety of materials were used to coat pills, such as talc, gelatin, and sugar.Gold and silver were also used. Many of these coatings proved to be impervious to chemical attack in

the digestive tract; as a result, the pill never released its active ingredient and was thus ineffective.

The candy-making industry was the first to develop and enhance the art of coating. It is most likely

that the pharmaceutical industry adopted sugar coating technology for its own use. The first sugar-coated

pills produced in the United States came out of Philadelphia in 1856. Coatings resistant to enteric or

gastric fluids were developed in the 1880s. In 1953 the first compression-coated tablet was introduced,

and in 1954 the first film-coated tablet was marketed.

112.2 Reasons for Coating TabletsThere are many reasons for coating tablets; some aesthetic, some functional. One important reason is to

enhance drug stability; that is, to protect the drug from oxygen, moisture, and light, the three key causes

of drug degradation. Coating can also be used to separate reactive components in a tablet formula.

Another important reason for tablet coating is identification. Tablet coatings may take on a variety of

colors. A coated tablet may also be imprinted with a symbol or word. In the case of the film-coated tablet,

the tablet core may be embossed with a symbol or word that remains visible after the coating process.

The definitive identification of a coated tablet has saved patients and health care professionals alike.

Additionally, coating is used to uniquely identify a branded product.

Tablet coating is done for aesthetic reasons as well. Often the appearance of the tablet core is mottled

or otherwise unattractive. Coating masks this. Many times, too, the drug itself has a bitter taste. Coating

masks this as well.

Tablet coating can also be used to control the duration and site of drug release. Overall, tablet coating,

through an additional step in the manufacturing process, is often vital.

Joseph L. JohnsonAqualon Company

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The Sugar-Coated Tablet The Film-Coated Tablet

Compression Coating

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112-2 Coatings Technology Handbook, Third Edition

112.3 Types of Coating

There are two main types of tablet coating done today: sugar coating and film coating; film coating is

the more popular. Coated tablets fall into three main subcategories depending on how the drug is released:

immediate release, enteric release, and sustained release:

Immediate-release coating systems, as the name implies, allow immediate release of the drug com-

pound to the body.

Enteric coatings are soluble only at a pH greater than 5 or 6. Thus, the drug is not released in the

stomach but in the small intestine. Enteric coatings are by far the most unreliable because of the

wide and unpredictable variance in gastric pH profiles. Gastric pH varies considerably based on

stomach content, age of the patient, and disease state.

Sustained-release coatings permit drug to dissolve slowly over a period of time. This helps to reduce

dosing intervals and improves therapeutic reliability.

Film coating can be carried out using either an organic solvent system, such as ethanol or methylenechloride, or by using water as a solvent. The solvent film coating systems are fast disappearing because of

cost, environmental, and safety concerns. Most film coating carried out today is done with aqueous systems.

112.3.1 The Sugar-Coated Tablet

The sugar-coated tablet is the most elegant solid dosage form produced today. Its glossy appearance,

slippery feel, and sweet taste are unmatched by any other coated tablet. The sugar-coated tablet is also

the most difficult and time-consuming to produce. The tablet consists of a core upon which layer after

layer of coating material is slowly and carefully built up. In some cases this is done by hand and in other

cases automatically. In any event, there is still an art to sugar coating.To successfully accept a sugar coating, the tablet cores must be robust. They are subjected to wetting

and rolling in a coating pan with 50 kg or more of other cores. Generally the coating pan is spherical

and has a solid exterior surface. Temperature-controlled air is introduced and removed from the pan via

external ducts. The following procedure is used for the manual sugar coating of tablets.

The first step is to slightly waterproof the tablets by applying a coat of pharmaceutical-grade shellac.

This prevents the cores from dissolving prematurely in the presence of the other coating liquids

that are to be applied.

The second step is subcoating: a solution composed of acacia, gelatin, and sugar is applied to the

tablets. The wetted cores are then dusted with dicalcium phosphate or calcium sulfate and allowedto dry. This step is repeated many times until a smooth rounded tablet form has been achieved.

The third step is the grossing coat. The cores are wetted with a sugar solution and dusted with titanium

dioxide powder. This creates a very white base coat on which color may be applied.

The fourth step is the color coat. In this instance an insoluble opaque color solid is suspended in sugar

syrup and applied to the tablet. No dusting of the cores takes place. The tablets are simply air dried.

The fifth step is the shutdown coat. In this step diluted sugar syrup is applied to the tablet and allowed

to dry. This produces a very smooth finish in preparation for the last step.

The last step is polishing of the tablets. The tablets are placed in a canvas-lined drum. Beeswax or

carnauba wax is dissolved in methylene chloride, and the solution is applied to the tablets, whichare tumbled until the solvent evaporates and tablets achieve a very high shine.

In all, 40 or more separate layers are applied during the manual sugar coating process. The process

takes between five and eight 8-hour shifts to complete.

Automated sugar coating is generally faster. For example, the various syrups used in the coating process

have the dusting powders suspended in them. The syrups are applied by spray. This process can be

automated to reduce the number of operators required. Perforated coating pans, which greatly enhance

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air throughput, are used almost exclusively. With greater air throughput, water evaporates more quickly,

thus speeding the process. Using automated techniques, tablets can be sugar coated in about 16 hours.

112.3.2 The Film-Coated Tablet

The film-coated tablet consists of a core around which a thin, colored polymer film is deposited. Thus,a film-coated tablet gains about 3% of total tablet weight upon coating. The sugar-coated tablet undergoes

a 100% weight gain. Overall, film coating is a much faster procedure, and much less prone to error.

The basic film coating formula consists of a film former, a pigment dispersion, a plasticizer, and a

solvent. A variety of polymeric film formers can be used to coat tablets. By selecting the solubility

properties of the polymer, one can produce an immediate-release, an enteric-release, or a sustained-

release tablet.

The most popular immediate-release film formers are the water-soluble cellulose ether polymers. The

two most common are hydroxypropylcellulose (HPC) and hydroxypropylmethycellulose (HPMC). The

low viscosity grades of these polymers are employed in the coating formula to maximize polymer solidsconcentration. Both these polymers are water soluble.

Water-insoluble film formers can also be used to prepare immediate-release coatings. These products

fall into two categories: cellulose ethers and acrylate derivatives. The most common cellulose ether is

ethylcellulose. This material is commercially available in two forms: as pure polymer and as an aqueous

dispersion. The pure polymer is generally dissolved in an organic solvent; the dispersion is delivered out

of an aqueous media. In both cases, a certain amount of water-soluble component (up to 50% of the

total polymer solids) is included in the coating formula, to provide immediate drug release.

The ethylcellulose and acrylate compounds are also used to formulate sustained-release products.

Again, a water-soluble component is included in the coating formula. However, the level is very low:

usually about 3% of total polymer solids. When the coated dosage form is exposed to water, the water-soluble component dissolves. This leaves a porous film surface through which drug diffuses.

The third class of coatings, the enterics, resist the attack of gastric fluids. As a result, drug is released

only in the small intestine. Enteric coatings are prepared by using a polymer with pH-dependent solubility

properties. Cellulose esters, substituted with phthalate groups, are the primary polymers used in this

application, especially cellulose acetate phthalate. Polyvinyl acetate phthalate is also used. Acrylate deriv-

atives are also capable of providing enteric release.

112.3.3 Compression Coating

Compression coating is a technique wherein a large tablet either completely or partially surrounds a

smaller tablet. Essentially, a small tablet is compressed first and is then surrounded by powder, which

undergoes compression. This type of coating technique requires the use of special tableting machinery

and it is used to produce sustained-release tablets.

Bibliography

Florence, A. T., Ed., Critical Reports on Applied Chemistry, Vol. 6, Materials Used in Pharmaceutical

Formulation. London: Blackwell Scientific Publications, 1984.

Lachman, L., H. A. Leiberman, and J. L. Kanig, Eds., The Theory and Practice of Industrial Pharmacy,Philadelphia: Lea & Febiger, 1st ed., 1970; 2nd ed., 1976; 3rd ed., 1986.

Osol, Arthur, Ed., Remingtons Pharmaceutical Sciences. Easton, PA: Mack Publishing Company, 14th ed.,

1970; 15th ed., 1975; 16th ed., 1980; 17th ed., 1985.

2006 by Taylor & Francis Group, LLC