Friday, April 16, 2021

Tablet Coating Process: Sugar Coating

by | July 20, 2020 0

Sugar coating has long been the traditional method of coating pharmaceutical dosage forms. The process has its origin in the confectionery industry and has been used in the pharmaceutical industry since the late 19th century. The process of sugar coating involves the successive deposition of aqueous sugar solution on the tablet cores as they are rotated and tumbled in a revolving pan by spraying sugar solution or suspensions into pans and drying off the solvent.

Sugar coating is used in immediate release applications to mask unpleasant taste and odour of some drugs or to improve aesthetic qualities of the product. It should be understood that the coating process will add some time to the overall disintegration of the tablet and may impact drug dissolution. This effect should be considered when formulating the core to ensure that the product meets disintegration and dissolution requirements stated in official compendia. For an enteric or sustained-release sugar-coated product, the formulation problem may become more complex to meet USP tablet disintegration and dissolution specifications. The selection of the core tablet and coating materials becomes more important for these applications and requires proper evaluation to assure long-term chemical and physical stability.

Sugar coatings are essentially aqueous based, unless a seal coat (e.g., alcoholic confectioners glaze) is required to protect the core tablet from water used in the sugar-coating process. Although the sugar-coating process is lengthy and labour intensive, coating materials are inexpensive and readily available.

Ideal characteristics of sugar-coated tablets

  • Sugar-coated tablets should have a smooth, rounded contour, with even colour coverage and a high degree of gloss.
  • Sugar-coated tablets must meet requirements as specified in official compendia.
  • Those tablets that have been imprinted should show distinct print with no smudging or broken print.

Raw materials used in sugar coating

Because sugar-coating process consists of various steps, a variety of additives may be incorporated into each type of formulation to achieve a particular function. These include:

i. Sucrose, other sugars, and sugar alcohols

As expected, the major ingredient used in sugar-coating process is sugar (primarily sucrose), although this may be substituted by other sugars and sugar alcohols (such as glucose, lactose, maltitol, mannitol, isomalt, sorbitol, xylitol, and sugar mixtures such as invert sugar and starch sugars) for low calorie diabetic products (typically in the candy industry) and for the fact that sucrose cause dental caries.

Sucrose is used in concentrations ranging between 50-60%, since syrups with a sugar content of less than 65% are stable at room temperature without crystallization occurring.

ii. Binders

Binders increase the strength and elasticity of the coating by forming bonds and thus a coherent matrix. Examples of binders used in sugar coating include polyvinyl acetate (PVA), polyvinyl pyrrolidone (PVP), carboxymethyl starch, dextrin, acacia gum, gelatin, agar-agar, sodium alginate, cellulose ethers, and starches.

iii. Fillers

Fillers build up the structure and adds mass to the coatings. E.g., precipitated calcium carbonate, talc, kaolin, dextrin, powdered acacia, corn starch, and calcium sulfate.

iv. Colorants

Colourants add color to the coatings and cover imperfections which may appear in the tablet core. Examples include dyes, lakes (aluminum lakes), and pigments (titanium dioxide or other inorganic coloring agents).

v. Antiadhesives, Lubricants, and Glidants

These materials reduce friction between coated tablets and thus prevent dust formation during drying. Examples include talc and colloidal silicon dioxide.

vi. Flavoring agents

Flavors improve and enhance the acceptability and palatability of the dosage form in order to maximize patient compliance. E.g., cinnamon, fruit flavors, etc.

vii. Suspension stabilizers

They prevent phase separation or sedimentation of the coating suspension while it is being applied during the coating process. Examples include surface-active agents (emulsifying agents, bentonite) or thickening agents.

viii. Smoothing agents

These materials smoothen out the surface of the coatings by lubricating and binding the fines that may be created during the coating process. Example includes a combination of syrup and acacia gum.

ix. Polishing agents

Polishing agents enhance the reflectivity and intensity of the colour of the coatings. Beeswax and carnauba wax are good examples of polishing agents used in sugar coating process.

Techniques involved in sugar coating

The three techniques commonly used in sugar-coating process include:

a. Plain sugar coating

This technique involves the application of syrup at room temperature. This coating technique includes 3 steps:

  1. Application of coating formulation onto the tablet cores
  2. Distribution of formulation on the core surfaces, and
  3. Drying to increase the strength of each coating layer.

Note that the time required for distribution and drying is critical to obtain a smooth even coating.

b. Lamination process/ Two-component coating process

Compared to the plain sugar-coating technique, the lamination coating is a more complicated technique involving two steps:

  1. Application of a syrup or binder solution first in a slight excess amount, and
  2. Dusting with a powder to bind the excess solution

In order to obtain a reasonable weight increase within a short period of time, adjustment must be made between powder and liquid quantities and process performed by skillful operators.

c. Hot sugar coating

As suggested by the name, hot sugar coating simply involves the application of heated syrup on tablet core. The syrup is heated above room temperature to reduce the viscosity of the syrup.  Attempts to prevent crystallization of sugar during processing may make this technique more complicated and more expensive since all equipment parts used must be insulated and heated.

Steps involved in sugar coating

Sugar-coating process consists of various steps, each designed to achieve a particular function. A typical sugar-coating process encompasses six stages:

  1. Sealing of the tablet core
  2. Subcoating
  3. Smoothing
  4. Colour coating`
  5. Polishing
  6. Printing

Read more on steps involved in sugar-coating of tablets

Sugar coating defects, causes, and remedies

  1. Chipping of coatings
  2. Cracking of the coatings
  3. Non-drying of the coating
  4. Twinning (buildup of multiples)
  5. Uneven coloring
  6. Blooming and Sweating
  7. Marbling

Read more on sugar-coating defects, causes, and remedies

Recovery of reject sugar-coated tablets

Owing to the amount of material applied as a coating in the sugar-coating process, it is not appropriate to grind up reject sugar-coated tablets for recompaction. One potentially viable recovery procedure (although one not without its difficulties because of handling problems) is to wash off the sugar coating by carefully dipping the coated tablets (held on a screen) into a water bath until sufficient coating is removed such that on subsequent refinishing, the desired quality is achievable. Once the requisite quantity of coating is removed, the tablets can be dried by tumbling in a coating pan under a warm air stream (50°C). Such a procedure must obviously be validated to ensure that overall product quality is not compromised.

Differences between sugar coating and film coating

Table Showing the differences between Sugar coating and Film coating

Examples of sugar-coated tablets

  • Brufen – Ibuprofen 400 mg (Mylan Products Limited)
  • Reasulf tablets – dried ferrous sulphate BP 200 mg (Reagan Remedies Ltd.)
  • Premarin – Conjugated oestrogens 625 mcg (maroon) and 1.25 mcg (yellow) (Pfizer Limited)
  • Advil – Ibuprofen tablet BP 200 mg (Pfizer Consumer Healthcare)
  • Colofac tablets – Mebeverine HCl 135 mg  (Mylan Laboratories SAS)
  • Ebu-200 – Ibuprofen tablet BP 200 mg (Me cure Industries Ltd)

Advantages of sugar coating

  • Sugar coating process utilizes inexpensive and readily available raw materials.
  • Constituent raw materials used are widely accepted with few regulatory problems (with the exception of perhaps colors)
  • The process is easily documented and controlled to meet modern GMP standards.
  • Inexpensive, simple equipment can be used.
  • Sugar coating process is generally not as critical (as film coating) and recovery (or rework) procedures are more readily accomplished.
  • Products are usually aesthetically appealing and have wide consumer acceptability.
  • Advancement in science and technology have greatly reduced the variability in coating obtained with manually operated coating pans.

Disadvantages of sugar coating

  • The process is tedious and time consuming.
  • The achievement of high esthetic quality often requires the expertise of highly skilled technicians.
  • The size and weight of the coated tablets results in increased packaging and shipping costs.
  • Finished products may vary slightly in size from batch to batch and within a batch.

Conclusion

While the popularity of sugar coating has certainly declined due to the advent of film-coated tablets (as a result of the improved mechanical properties of the latter coating), this process is still used by many companies that have invested in the complete modernization of the process.

References

  • Allen L. V and Ansel H. C. (2014). Ansel’s Pharmaceutical Dosage Forms and Drug Delivery Systems. Philadelphia: Lipincott Williams and Wilkins.
  • Aulton, M. E and Taylor, K. (2013). Aulton’s Pharmaceutics: The Design and Manufacture of Medicines, (4th ed.). Edinburgh: Churchill Livingstone.
  • Avis, K. E., Shukla, A. J. and Chang, R. (1998). Pharmaceutical Unit Operations: Coating London, Taylor & Francis Group, LLC.
  • Coating from http://cosarpharm.com/fa/wp-content/uploads/2015/12/Coating.pdf [accessed Jan 7 2019]
  • Cole, G. (2002). Pharmaceutical Coating Technology. UK,Taylor & Francis Ltd.
  • Felton, L.A. (2012). Remington Essentials of Pharmaceutics. UK: Pharmaceutical Press.
  • Lieberman, H. A., Lachman, L. and Schwartz, J. B. (1990). Pharmaceutical Dosage Forms: Tablets. New York: Marcel Dekker, Inc.
  • Ofoefule, S. I. (2002). Textbook of Pharmaceutical Technology and Industrial Pharmacy. Nigeria: Samakin (Nig.) Enterprise.

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