The application of a layer of sugar or sucrose over the core tablet is known as sugar coating.Sugar coating is applied over tablets for a number of reasons and most important is to mask the taste.It is the oldest technique of coating used in pharmaceutical industries. Sugar coating is a multistage process and is a lengthy process.Weight gain during sugar coating may range from 30-80% or up to 100% depending upon the formulation.
History of Coating.
The use of coating material for solid dosage form is very old and its first reference is found in islamic literature where Rhazes describes the coating of pills.
Sugar coating of pill was developed by the french in mid of 1800 and the first patent for sugar coating of pills was issued in 1837.
In 1840 the first sugar coating was applied to the pills of Cubeb and Copaiba.
The technique of sugar coating of pills was spread to the United States and many companies started using this technique.
Year 1953 was a historical year in the history of coating because in this year Abbot Laboratories launched the first sugar coated tablet.
Principle of Sugar Coating
Principle of sugar coating is very simple which is as follow,
Solution of Sucrose or sugar is applied over rotating warm tablets and solvent is evaporated by application of Heat.
Role Of Operator
Sugar coating is a complex process and it mainly depends upon the skills and experience of the operator or person who is performing the sugar coating operation.
In contrast to the film coating where most of the coating parameters are controlled by automatic system,in sugar coating mostly conventional method is used where all steps are carried out manually and following are determined by operator
- How much solution is applied in a single coat?
- How long or fast the pan speed should be?
- After how much time the coat should be applied.
- How much heat should be provided.
So for conventional sugar coating methods a skilled operator is most important.
- Although automation of sugar coating is also available in which sugar coating can be done in perforated pans using solution delivery system in form of nozzles but there are following drawbacks
- Choking of perforations
- Non uniform delivery of solution due to high viscosity.
Because of above drawbacks it is preffered to perform sugar coating in conventional coating pans.
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Steps of Sugar Coating
Sugar coating is not performed in a single step and the steps involved in sugar coating process are as follow
- Sealing
- Sub-Coating
- Smoothing
- Colouring
- Polishing
- Printing
Sealing
- Sealing is also known as waterproofing, as the name indicates it is the stage which gives protection to the core tablet against water penetration.
- Sealing or water proofing is also used to give protection against moisture penetration.
- In Manual coating or pan pouring method or pan ladling method there are chances of localized over wetting due to the water present in sucrose solution.
- This over wetting of the tablet may result in softening of the tablet or may damage the tablet surface.
- To prevent this damaging effect, a coat of sealing material in organic solvent is applied.
- On applying heat,solvent evaporates leaving behind a layer of sealing material over the core tablet.
- Seal coats in the automatic spraying process may be skipped by controlling the rate of application of sub-coat.
- So the application of seal coat is dependent on the formulation and equipment used.
- Following are commonly used sealing materials
- Shellac
- Zein
- HPC (Hydroxypropyl Cellulose)
- CAP (Cellulose Acetate Phthalate)
Note:
- When shellac is used as a sealing material it results in increase in disintegration time on storage.
- Zein is a protein obtained from corn and it does not affect disintegration time on storage.
Sub-Coating
- Sub-Coating is the first real step of sugar coating.
- For sub-Coating less concentrated sucrose syrup is used.
- This stage results in weight gain of the tablet and usually 50-100% gain in weight can be obtained.
- This stage makes the edges of the tablet round.
Methods Of Application
For application of sub-Coat following two methods are used
- Lamination Method
- Suspension Method
Lamination Method
- Lamination method may also be known as the dusting method because in this method powder materials like mixture of powdered sugar and starch or talc and acacia are used for dusting.
- A coat of binder solution is applied over hot tablets in a rotating pan.
- Then dusting is done with powder materials and allowed to rotate in the pan to spread the powder uniformly.
- After uniform distribution heat is applied to evaporate the water.
- This process is repeated until desired weight or thickness of the tablet is achieved.
Suspension Method
- In suspension method liquid sub coating solution is used in form of suspension.
- The suspension may contain
- Gum solution
- Fillers like Calcium carbonate,talc,sucrose.
Fillers are added in a binder solution to form suspension and then applied over the moving bed of the tablet.
Smoothing
- Third step in sugar coating is smoothing.
- Smoothing is also known as grossing or final rounding.
- Tablets after the second stage of sub-coating are not smooth so to make tablets smooth the third stage or smoothing process is used.
- Smoothing step is used to make the tablet surface smooth.
- In smoothing,irregularity in tablet surfaces are filled to make it smooth.
- For Smoothing concentrated sucrose solution is used.
- Usually 70% sucrose syrup is used for this stage of smoothing.
- In this stage a large volume of solution is not applied only 9 to 10 coats are enough to make the tablet surface smooth.
Coloring
- In this stage a coat of specific color is applied over the tablets.
- Color is applied by mixing it in sucrose solution.
- Color uniformity is one of the most important requirements of sugar coating.
- So the color coat application should be done with great care.
Note:
- For color coating Dyes are not used because dyes are water soluble and on drying dye migration may occur.
- Lakes are water insoluble and not migrate on drying.
Polishing
- Polishing is used to give shining luster to the tablets.
- Polishing may be done in a canvas lined coating pan or conventional coating pan.
- For polishing, carnauba wax is poured in the form of fine powder and tablets are rotated to achieve the desired level of shine or carnauba wax in organic solvent is sprayed over tablets.
- Following are used as a polishing material
- Carnauba wax
- Bee’s wax
- Candelila Wax
- Hard paraffin Wax
Imprinting
- This step is dependent on the requirement and if it is required then tablets can be passed through an imprinting machine to print a code or identification on each tablet by a printer using edible ink.
Advantages Of Sugar Coating
- Sugar coating can be done in conventional coating pans.
- Sugar and water used are cost effective.
- Sugar coated tablets look more attractive.
Dis-Advantages
- Sugar coating requires a more skilled operator.
- Sugar Coating is a multi stage process
- It is a long process.
- Due to long processing hours cost of operation may increase.
- Sugar coating results in increase in tablet size and weight .
- Large size tablets are difficult to swallow.
- Only round shape tablets are used for sugar coating.
- Embossing or logo of the core tablet is totally covered by the coating layer so embossing on core tablets is not done.
Enteric Sugar Coating
- As the name indicates this is the combination of enteric coating sugar coating.
- In the enteric sugar coating seal coat is done with enteric polymer to protect the drug from the acidic environment of the stomach.
- So only the seal coat step is changed and all remaining steps of sugar coating are performed in the same way as mentioned above.
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