Tablet Coating

Tablet coatings perform one or more of the following functions.

1. Mask the taste of unpalatable drugs,
2. Protect the drug from deterioration due to light, oxygen or moisture,
3. Separate incompatible ingredients,
4. control the release of medicaments in the gastrointestinal tract
5. Provide an elegant or distinctive finish to the tablet.

• The materials used for coating may largely comprise sucrose (sugar coating), water-soluble film-forming polymers (film coating) or substances which are soluble in the intestinal secretions but not in those of the stomach (enteric coating).
• These types of coating can all be applied by the pan or fluid-bed processes; the compression coating technique is suitable for sugar and enteric coatings, but not for film coating.

Sugar Coating.

This traditional coating technique imparts a smooth, rounded, elegant appearance to the tablet.

Advantages:
1. It gives a smooth & shining surface
2 It masks the bitterness & sweet in taste
3 It increases the elegance of the product.
Disadvantage:
1 It increases the weight of the tablet & it is hygroscopic.
2 Skilled workers are needed.
3 Time-consuming method.
 Steps Involved in Sugar Coating:
1 Sealing. 

• After, all dust has been removed from the tablet a dextrin, gelatin or acacia coat may be applied to ensure good coating adhesion.
• Where protection against the effects of water in subsequent coating solutions is required a 30 to 50 percent solution of shellac in alcohol or other suitable solvent is employed for sealing.
• Care has to be taken to avoid excess sealing as this leads to a prolongation of the disintegration time.

2 Subcoating.

• To minimise the amount of material, which must be used to round the tablet edges, the cores are made on deep concave punches.
• The sub-coat is build up in successive layers by wetting the tablets with an adhesive solution, dusting with filler and then thoroughly drying, as retained moisture may cause later cracking of the coat or deterioration of the core.
• The sub coating solution is usually an aqueous solution of sucrose to which is added acacia, gelatin or both, to impart adhesive properties.
• Talc and precipitated calcium carbonate are widely used in the sub coating filler together with some sucrose and a small proportion of acacia.
• A small proportion of inert filler, e.g., talc, may be added to the sub coating solution, which, if it contains gelatin, should be used warm to avoid gelling.
• Too heavy an application of filler must be avoided as the excess forms ‘granules’ with the coating solution and these interfere with the formation of a smooth sub coat.
• This stage of the process is continued until the tablets have a rounded appearance and the edges are well covered.
• When complete, the tablets are removed from the pan and thoroughly dried.

3 Smoothing and polishing.

• The application of a smoothing solution (60 percent sucrose in water is satisfactory) causes limited wetting of the sub-coat, which has been hardened by drying so that it is smoothed out by the tumbling action. Soluble dye or a lake colour may be added to the smoothing solution if a coloured coating is required.
• As soon as the coat has become comparatively smooth the volume of smoothing solution per application should be reduced and the tablets dried without the aid of heat.
• In the final stages, tumbling is limited by ‘inching’ rather than rotating the bowl so that the coating is not scratched or otherwise damaged.
• Polishing in a pan specially set aside for the job and coated with a beeswax, carnauba wax or similar waxy mixture.

Film Coating

• Sugar coating doubles the weight and increases the size of a tablet and this is obviously undesirable if the tablet, in the uncoated state, is already large.
• Film coating provides an alternative means of masking the taste of the medicament and providing protection against adverse climatic conditions without significantly altering the tablet weight or size.
• The obvious advantages of excluding water from the coating process may be secured with film-forming polymers, such as ethylcellulose, polyvinylpyrolidone or hydroxymethylpropyl cellulose, that are soluble in both water and anhydrous organic solvents. Some 3 to 10 percent of the foregoing materials can be dissolved in an acetone-alcohol mixture together with 5 to 10 percent of diethyl phthalate or other plasticizer to produce a film coating solution which may be applied by the pan technique.
• The plasticizer stops the film becoming brittle with age.
• Methylene chloride is often added to the solvent to reduce fire hazards, while undue absorption of expensive film forming agent is prevented by the prior application of a shellac-sealing coat to the core tablets.
  

Advantages: 
1. It does not increase the weight of the tablet
2. Masks bitter taste of the drug.
2. It protects the tablet from moisture.
3. It increases the elegance of the tablet

Enteric Coating

Tablets are made enteric-coated if,

• the medicament is decomposed in the acid secretions of the stomach,
• if it causes gastric irritation, or
• if it is intended to exert its main effect only on the intestine.

• Some official tablets coated in this way include those containing biscodyl, bismuth and emetine iodide, and erythromycin.
• Enteric coatings resist the acid conditions of the stomach but readily dissolve in the more nearly neutral fluids of the small intestine.
• They are also used in the formulation of sustained action preparations as the release of medicament is delayed by the time taken for the tablet to pass from the mouth to the intestine.
• Formalised gelatin, keratin, salol, shellac, sandarac, stearic acid and cetyl alcohol have all been used to produce enteric coatings but are either difficult to apply or erratic in their action.
• The compositions of the fluids in the gastrointestinal tract are not constant but vary with time and from person to person.
• It is clearly important that the enteric action shall largely be independent of such variations in compositions.
• Cellulose acetate phthalate is widely used for enteric coating. 
• In addition to 5 to 15 percent of cellulose acetate phthalate, the enteric coating solution may contain a plasticiser such as castor oil or butyl stearate together with a soluble or a lake dye if a coloured finish is required. The materials are dissolved or dispersed in a volatile solvent comprising alcohol, acetone, and, to reduce flammability, methylene chloride.

Advantages:
1. It gives a sustained action.
2.  It protects the tablet from moisture                       
3. It releases the medicament in the intestine.