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Pharmaceutical Tablet Coating
112.1 History .......................................................................... 112-1 112.2 Reasons for Coating Tablets........................................ 112-1 112.3 Types of Coating.......................................................... 112-2
Joseph L. Johnson
Aqualon Company
The Sugar-Coated Tablet The Film-Coated Tablet Compression Coating
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 rst 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 rst sugar-coated pills produced in the United States came out of Philadelphia in 1856. Coatings resistant to enteric or gastric uids were developed in the 1880s. In 1953 the rst compression-coated tablet was introduced, and in 1954 the rst lm-coated tablet was marketed.
112.2 Reasons for Coating Tablets
There 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 identication. 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 lm-coated tablet, the tablet core may be embossed with a symbol or word that remains visible after the coating process. The denitive identication 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.
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2006 by Taylor & Francis Group, LLC
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Coatings Technology Handbook, Third Edition
112.3 Types of Coating
There are two main types of tablet coating done today: sugar coating and lm coating; lm 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 compound 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 proles. 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 methylene chloride, or by using water as a solvent. The solvent lm coating systems are fast disappearing because of cost, environmental, and safety concerns. Most lm 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 difcult 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 rst 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 allowed to 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 fth 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 nish 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, which are 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 ve 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
2006 by Taylor & Francis Group, LLC
Pharmaceutical Tablet Coating
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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 lm-coated tablet consists of a core around which a thin, colored polymer lm is deposited. Thus, a lm-coated tablet gains about 3% of total tablet weight upon coating. The sugar-coated tablet undergoes a 100% weight gain. Overall, lm coating is a much faster procedure, and much less prone to error. The basic lm coating formula consists of a lm former, a pigment dispersion, a plasticizer, and a solvent. A variety of polymeric lm 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 sustainedrelease tablet. The most popular immediate-release lm 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 solids concentration. Both these polymers are water soluble. Water-insoluble lm 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 watersoluble component dissolves. This leaves a porous lm surface through which drug diffuses. The third class of coatings, the enterics, resist the attack of gastric uids. 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 derivatives 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 rst 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 Scientic 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
