Sugar Coating of Tablets
The sugar coating process of tablets involves several steps, the duration of which ranges from a few hours to a few days.
A successful product greatly depends on the skill of the coating operator. This is especially true in the pan-ladling method, in which the coating solutions are poured over the tablet cores.
The operator determines
- The quantity of solution to add
- The method and rate of pouring
- When to apply drying air, and
- How long or how fast the tablets should be tumbled in the pan
Newer techniques utilize spraying systems and varying degree of automation to improve coating efficiency and product uniformity.
Regardless of the methods used a successful sugar coating process yields elegant highly glossed tablets.
The basic sugar coating tablets involves the following steps:
- Seal coating or Sealing
- Finishing or Polishing
Seal Coating or Sealing
To prevent moisture penetration into the tablet core, a seal coat is applied, in which localized over-wetting of a portion of the tablet bed occurs. Without a seal coat, the over-wetted tablets would absorb excess moisture, leading to tablet softening or disintegration and affect the physical and chemical stability of the finished product.
This adjustment thus eliminates the seal coating step i.e., in spray process, it is possible to adjust the application of the sub-coats so that over-wetting does not occur.
Shellac is an effective sealant but the disintegration and dissolution time of the tablet tends to lengthen on aging. Zein is an alcohol soluble protein derivative from corn that has also been used as effective sealant.
It is applied to round the edges and build up the tablet size. Sugar coating can increase the tablet weight by 50 to 100%. The sub-coating steps consists of applying a sticky binder solution to the tablets followed by dusting of sub-coating powder and then drying.
Subsequent sub-coats are applied in the same manner until the tablet edges have been covered and the desired thickness is achieved.
In spraying processes, a sub-coating suspension containing both the binder and insoluble powder is sprayed intermittently on the tablet bed.
Smoothing and Coloring
The purpose of this step is to cover and fill in the imperfections on the tablet surface caused by the sub-coating step and to impart the desired color to the tablet.
The first syrup coats usually contain some suspended powders and are called “Grossing Syrups”. Dilute colorants can be added to this phase to provide a tinted base that facilitates uniform coating and coloring in later steps.
In general, no color is added until the tablets are quite smooth, premature application to rough tablets can produce a molted appearance in the final coated tablets.
In subsequent smoothing steps, syrup solutions containing the dye are applied until the final size and color is achieved.
Finishing or Polishing
The desired luster is obtained in this final step of the sugar coating process.
The tablets can be polished in clean standard coating pans or canvas-lined polishing pans by carefully applying powdered wax (beeswax or carnauba) or warm solutions of these waxes in naptha or other suitable volatile solvents.