The sugar coating process involves building up layers of coating material on the tablet cores by repetitively applying a coating solution or suspension and drying off the solvent. The process consists of various steps, each designed to achieve a particular function. A typical sugar-coating process encompasses six stages:
- Sealing of the tablet core
- Color coating`
1. Sealing of the tablet core (Waterproofing/ Protective coating)
Seal coating involves the application of specialized polymer-based coating (either by ladle or spray techniques) directly to the tablet core. It is an optional step but is usually required to prevent the tablet core and its contents from absorbing water, softening, and initiating disintegration during the subsequent steps of the sugar-coating process. Sealing also prevents certain types of materials (e.g. oils, acids, etc.) from migrating to the tablet surface and spoiling the appearance.
In a manual seal-coating operation, the sealant which usually consist of alcoholic solutions of resins (approximately 10–30% solids) is evenly and gently poured or sprayed over the tumbling tablet bed (preheated to 40oC). Warm air is then blown into the pan during the coating to hasten the drying and to prevent tablets from sticking together.
The quantities of material applied as a sealing coat will depend primarily on tablet and batch size. Another variable is tablet porosity because highly porous tablets will tend to soak up the first application of solution, thus preventing it from spreading uniformly across the surface of every tablet in the batch. Hence, one or more further application of resin solution may be required to ensure that the tablet cores are properly sealed.
Because most sealing coats develop a degree of tack (stickiness) at some time during the drying process, detackifiers, such as asbestos-free talc, are often used to minimize the risk of “twinning” or clumping.
Excessive use of talc may cause problems, firstly, by imparting a high degree of slip to the tablets, thus preventing them from rolling properly in the pan, and secondly by creating a surface that, at the beginning of the subsequent subcoating stage, is very difficult to wet. Such poor wetting often results in uneven subcoat buildup, particularly on the tablet edges. If there is a tendency for either of these problems to occur, one solution is to replace part or all of the talc with some other material such as terra alba, which will form a slightly rougher surface.
Common materials used as sealants include shellac, zein, hydroxypropyl methylcellulose (HPMC), cellulose acetate phthalate (CAP), or polyvinyl acetate phthalate (PVAP). While use of shellac has been universal, this natural polymer can undergo further polymerization on storage, causing the seal coat to become completely insoluble to the point where bioavailability of the active drug substance may be compromised. This problem can be minimized either by incorporating small amount of PVP into the shellac-based seal coat formulation or by using one of the other more stable polymers (such as PVAP). If the final product is to have enteric properties, this result can be achieved using one of the enteric polymers (such as PVAP or CAP) as the basis for the seal coat and ensuring that sufficient coating material is applied.
This step is regarded as the first major step in sugar-coating process. It involves the application of large quantities of sugar-coatings to the tablet core, significantly increasing the tablet weight by 50 – 100 %. Subcoating provides the rapid buildup necessary to round up the tablet edge. It also provides the foundation for smoothing and color coating with any weakness in the final sugar coat often being attributable to weaknesses in the subcoat.
There are two techniques for the subcoating application; lamination process and suspension subcoating process; each with its distinct features and advantages.
i. Lamination process
The lamination process is perhaps the older of the two techniques used, and involves application of a “glue” (in the form of an aqueous solution of a suitable gum, such as gum acacia, or even gelatin) in quite substantial quantities to the sealed tablet cores. Once this solution has been distributed uniformly throughout the tablet mass, it is followed by a liberal dusting of powder (which serves to reduce tack and facilitate tablet buildup) and drying. This process of application of gum solution, spreading, dusting, and drying is repeated until a satisfactory coating is achieved.
While this method has proved to be very effective, particularly where there is difficulty in covering edges, it is important to ensure that a careful balance is achieved between the relative amounts of gum solution and dusting powders used. The use of inadequate dusting powders increases the risk of sticking and twinning, whereas its excessive use can create tablets that have brittle coatings. While achievement of quality results with the lamination process typically requires employment of skilled operators, there is no doubt that this type of process can permit rapid buildup of the coating. On the downside, the lamination process can be messy, more difficult to use by less-skilled operators, and more difficult to automate (as both powders and liquids are involved).
Binder Solutions Used in Lamination Subcoating Process
|A, % w/w||B, % w/w|
|Gum acacia (powdered)||8.7||8.0|
Examples of binder solution used in lamination subcoating process
Dusting Powder Formulation Used in Lamination Subcoating Process
|A, % w/w||B, % w/w|
|Gum acacia (powdered)||2.0||–|
Examples of dusting powder formulations used in lamination subcoating process
ii. Suspension subcoating process
This is an alternative approach used particularly when using an automated dosing system. The process involves the application of a suspension subcoat formulation (essentially a coating formulation where the powdered materials used in the more traditional lamination process are dispersed into the gum-based solution). Employing suspension subcoating approach not only reduces the complexity of the process but also enables the less-experienced operator to achieve satisfactory results.
|Typical Suspension Subcoating Formulation|
|A, % w/w||B, % w/w|
|Gum acacia (powdered)||2.0||–|
|Gelatin (120 bloom)||–||0.01|
Examples of Suspension Subcoating Formulations
3. Smoothing or Grossing
In order to manufacture quality sugar-coated tablets, it may be necessary to smooth out the tablet surface and fill the irregularities generated during subcoating. Smoothing usually can be accomplished by applying sucrose-based solution with or without additional components such as starch and calcium carbonate. This is followed by drying until the tablets are properly rounded and smooth. Drying may last up to 20 minutes or more depending on the scale of operation. In some operations, 5 to 25 applications may be required to achieve smooth tablets that are suitable for the next stage.
Depending on the degree of smoothing required, the smoothing coating may simply consist of a 60-70% sucrose syrup, that is often cultured with titanium dioxide (an opacifier/whitening agent) to achieve the desired level of whiteness, and possibly tinted with other colorants to provide a good base for subsequent application of the color coat.
4. Color coating/ Coloring
This is one of the most important steps in the sugar-coating process as it has immediate visual impact that is associated with overall quality. It involves the multiple application of syrup solutions (60–70% sugar solids) containing the requisite coloring materials necessary to achieve the desired shade. As with film coating colors, sugar-coating colorants may be subdivided into either water-soluble dyes or water-insoluble pigments. The nature of the colorant selected often defines the type of color-coating procedure to be used.
Traditionally, water-soluble dyes have been used because they produce the most elegant sugar-coated tablets but in order to speed up the coating process, minimize color migration problems, and ensure color reproducibility from batch-to-batch, water-soluble dyes have gradually been replaced with water-insoluble pigments in pharmaceutical tablet coating. Tablet color coating with water-insoluble pigments have demonstrable advantages over water-soluble dyes, two important ones being:
- Lack of solubility in aqueous media which eliminates color migration during drying
- Superior light stability.
Although water-insoluble pigment-based color coatings are by no means foolproof, they will permit more abuse than a water-soluble dye color-coating process and are easier to use by less-skilled coating operators.
Note: The actual colorants used must comply with regulations promulgated by the national legislation of the country where the products are to be marketed.
5. Polishing/ Glossing
Sugar-coated tablets are, by nature very dull in appearance (i.e., they have a matte surface finish), and thus require a separate polishing step to give them the high degree of gloss that typifies finished sugar-coated tablets. Polishing is accomplished by applying mixtures of waxes either as powders (usually in a finely milled form) or as solutions/dispersions in various organic solvents to the coated tablets in a polishing pan.
Some polishing systems which are currently in use include:
- Organic-solvent-based solutions of waxes (beeswax, carnauba wax, candelilla wax, hard paraffin wax)
- Alcoholic slurries of waxes
- Finely powdered mixtures of dry waxes
- Pharmaceutical glazes (typically alcohol solutions of various forms of shellac, often containing additional waxes).
- Mineral oil.
While methods to achieve a desirable gloss tend to vary considerably, it is generally recommended that tablets should be trayed overnight in a suitable atmosphere (prior to polishing) to ensure that they are sufficiently dry. Excessively high moisture levels in tablets submitted for polishing will not only make achievement of a good gloss difficult but will also increase the risk of “blooming” and “sweating” of the coated tablet on standing.
It is common practice to identify all oral solid dosage forms with a product name, company name or logo, dosage strength, or other distinctive symbols. For sugar-coated tablets, such identification involves the application of special edible inks to the coated tablet surface by means of a printing process known as offset rotogravure.
Printing prior to polishing enables the ink to adhere more strongly to the tablet surface, but any legend may subsequently be removed by either friction or as a result of contact with organic solvents during the polishing process. Printing after polishing avoids the problem of print rub-off during polishing, but branding inks do not always adhere well to the waxed tablet surface. Adhesion of printing inks can be enhanced by application of a modified shellac, preprint base solution prior to printing.
Note: Alternative printing processes, such as ink-jet and pad-printing processes, have also gained acceptance.
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